Table of Contents

1.1 SEMICONDUCTOR DIODES

INTRODUCTION

The aim of Electronic component is to act as switch.

The atom structure is show in fig.

Atom = Nucleus(Protons + Neutrons) + Electrons

The outermost electron in the atom is called as valance electron.

Fig: 1.1 Atom structure

Basically there are three material in nature. They are Conductor, Semiconductor and Insulator. Conduction in all this material starts when the electrons in conduction band moves to the valance band.

For conductor, the conduction and valance band already overlap, so without any external voltage it starts conducting due to normal room temperature (Noise).

For insulator, the conduction and valance band is separated with larger band gap which need very high energy to move the electron from conduction band to valance band. This will fulfill our aim to act as switch but with very high energy is need.

For Semiconductor, the conduction and valance band separated with small band gap which needs low energy to move the electron from conduction to valance band. In case of semiconductor this will act as switch with low energy which will fulfill our aim.

· Intrinsic semiconductor(Pure)

· Extrinsic semiconductor(Impure or Doped)

o N-Type semiconductor(Electrons are majority carrier)

o P- Type semiconductor(Holes are majority carrier)

Fig:1.2 Conduction and Valance band of Insulator, Semiconductor and Conductor

1.2 INTRINSIC SEMICONDUCTOR

It is a Pure semiconductor without any impurities. Fig 1.2 shows the energy band diagram of intrinsic semiconductor. For Ge(Germanium) material electrons take 0.67 eV to reach conduction band from valance band and for Si(silicon) 1.1 eV. But silicon is cheaper than germanium. That’s why silicon is used widly.

Fig: 1.3 Intrinsic Semiconductor

To reduce the energy further, should reduce the band gap. This is done by means of doping.

Doping- Process of adding impurities in intrinsic semiconductor.

1.3 EXTRINSIC SEMICONDUCTOR

All the dopped semiconductors are called as Extrinsic semiconductor.

If the dopped atom creats excess electron in the atom means then it is called as N- Type semiconductor. Electrons are majority carriers. Holes are minority carriers. (i.e.,) Pentavelant impurities are used for doping (Arsenic(As), Antimony(Sb), Phosphorous(P)).

If the dopped atom creats holes in the atom means then it is called as P-Type semiconductor. Holes are majority carriers. Electrons are minority carriers. (i.e.,) Trivalent impurities are used for doping (Boron(B), Gallium(Ga), Indium(In)).

Fig: 1.4 (a) N-Type Semiconductor, (b) P-Type Semiconductor

This P-Type and N-Type Jointly form the PN doide.

1.4 PN JUNCTION DIODE

Combination of P and N type semiconductor is called as PN junction diode. It is lightly doped diode.

Symbol

Fig: 1.5 Circuit symbol

Construction of PN Junction Diode

P-Type and N-Type semicondutor combined to form the PN junction diode.

Fig: 1.6 PN Junction Diode

In P side, each acceptor atom accepts one electron from semiconductor atom and the acceptor atom become immobile negative ion and semiconductor atom become hole. The P side has excess holes.
In N side, each donor atom donate one electron and the donor atom become immobile positive ion, there is one free electron for each positive immobile ion. The N side has excess electrons.

Depletion Region

Electrons move from N side to P side and recombine with holes in the P type material. Because of this movement and recombination, electrons in N type and holes in P type material disappear.

Fig: 1.7 Space Charge region (or) Depletion region

Near the junction there will be an array of negative and positive immobile ions, which will block the electron hole mobility from one side to another. An equilibrium condition will be reached.
This region near the junction, which consist of immobile ions, is called space charge region or transision region or depletion region(no mobile carriers).

Barrier Potential

In depletion region there are positive charges in the N side and negative charges in the P side forms electric dipole layer, giving rise to a potential difference V_o. This potential difference is called barrier Potential.
It prevent the movement of mobile carriers across the junction. V_o is 0.3 V for Ge and 0.7 V for Si.

Operation of PN Junction Diode(VI Characteristics)

There are two main operation

Forward bias
Reverse bias

· Bias is nothing but applying external voltage.

Forward Bias

(Batteries Positive terminal is connected to P type and Negative terminal is connected to N type)

Fig: 1.8 Diode in Forward Bias

When an external voltage is applied, the holes in the P type material are repelled by the positive terminal of the battery, and the electrons in the N type material are repelled by the negative terminal of the battery, Which will reduces the width of the depletion region. Further increase in external voltage above the barrier potential voltage, then the depletion region gets broken. Holes cross the junciton and move towards negative terminal of the battery and electrons moves towards positive terminal of the battery.
Due to this movement of charges, produces a high forward current, which is show in the forward VI characteristics.
When the applied voltage reaches the barrier potential then the junction break down occurs which increases the flow of electrons. The point at which diode starts conducting in forward bias is called as knee voltage or cut in voltage or threshold voltage

Reverse Bias

(Batteries Positive terminal is connected to N type and Negative terminal is connected to P type)

Fig: 1.9 Diode in Reverse Bias

When an external voltage is applied, the holes in the P type material are attracted by the negative terminal of the battery, and the electrons in the N type material are attracted by the positive terminal of the battery, Which will increases the width of the depletion region and barrier potential. The high barrier potential will not allow charge carriers to move across the junction. Therefore in reverse bias no current flow through the junction.
PN junction diode offers very low resistance in forward bias and very high resistance in reverse bias.
When the voltage across the diode is increased, the depletion layer is strengthened, therefore the current through the diode is the reverse saturation current. Further increase in reverse voltage will suddenly increases the high reverse saturation current due to the breakdown of the diode. The minimum voltage at which the breakdown occurs is called breakdown voltage.

VI Characteristic of PN Junction Diode - Notes For Engineering

Diode Current Equation or Shockley Diode Equation

The mathematical equation which describes the forward and reverse characteristic of a semiconductor diode is called diode current equation.

Where, I = forward (reverse) diode current

I_o = reverse saturation current

V = external voltage, it is positive for forward bias and negative for reverse bias

= 1 for Ge and 2 for Si

V_T = volt equivalent of temperature

Application

Diode can be used as switch, because it offers very low resistance (Closed switch) in forward bias and very high resistance (open switch) in reverse bias.
Diode can be used as rectifier to convert AC signal into DC signal.

1.5 ZENER DIODE

· Zener diode is a heavily doped PN junction diode.

· Depletion layer is very thin (due to heavy doping).

· Junction electric field is strong therefore lower applied reverse voltage is enough to cause the breakdown. Which is called as Zener breakdown.

· Zener breakdown is sharp

Symbol

Fig: 1.21 Circuit symbol

V-I Characteristics

· Forward bias

· Reverse bias

Bias- applying external voltage

Fig: 1.21 V-I Characteristics of Zener diode

Forward Bias

(Batteries Positive terminal is connected to P type and Negative terminal is connected to N type)

In forward biased condition Zener diode acts as the ordinary PN junction diode.

When an external voltage is applied, the holes in the P type material are repelled by the positive terminal of the battery, and the electrons in the N type material are repelled by the negative terminal of the battery, Which will reduces the width of the depletion region. Further increase in external voltage above the barrier potential voltage, then the depletion region gets broken. Holes cross the junciton and move towards negative terminal of the battery and electrons moves towards positive terminal of the battery.
Due to this movement of charges, produces a high forward current, which is show in the forward VI characteristics.

Reverse Bias

(Batteries Positive terminal is connected to N type and Negative terminal is connected to P type)

· At a reverse voltage the electric field in the depletion layer will be strong enough to break the covalent bonds. This produces extremely large number of electrons and holes and heavy current flow through the junction causing breakdown. The zener breakdown voltage depends on the amount of doping.

Application

· In the zener breakdown region voltage across the diode remains constant over a wide range of current, therefore zener diode can be used as Voltage regulator.

1.6 BREAKDOWNS IN DIODE

In reverse bias, breakdown of the junction occurs by two mechanisms, they are Zener Breakdown and Avalanche Breakdown.

Zener Breakdown

Zener breakdown takes place in a heavily doped diode. In a heavily doped diode the depletion layer will be thin and the electric field in the depletion layer will be high. When a small reverse bias voltage is applied, a very strong electric field (about 10⁷V/m) is set up across the thin depletion layer. This field directly breaks or ruptures the covalent bonds. Now extremely large number of electrons and holes are produced and the current through the diode increases rapidly. This mechanism is called Zener Breakdown.

Avalanche Breakdown

Avalanche breakdown takes place in lightly doped diode, whose depletion layer is large and the electric field across the depletion layer is not so strong to break covalent bond. In the depletion layer thermally generated minority carriers are accelerated by the electric field. The minority carriers move with high speed and collide with atoms. Due to the collision covalent bonds are broken and electron hole pairs are generated. These new carries so produced are also accelerated by the field and they break more covalent bonds. This forms a cumulative process is called as avalanche (or flood) multiplication and the current through the diode increases rapidly. This breakdown is called as avalanche breakdown.

2.1 BIPOLAR JUNCTION TRANSISTOR (BJT)

Transistor is a two junction, three terminal device.
Conduction due to both majority and minority charge carriers. (i.e., Bipolar).

Construction

Fig: 2.1 Transistor (a) NPN & (b) PNP

Transistor is simply a sandwich of one type semiconductor material between two layers of other type. There are two types of Transistor, they are

NPN transistor – P type material is sandwich between two layer of N type material.
PNP transistor - N type material is sandwich between two layer of P type material.

Fig: 2.2 Circuit symbol of (a) NPN & (b) PNP Transistor

Arrowhead should be at the emitter terminal. It indicates the directional of current flow when emitter base junction is forward biased.

Terminals

Emitter: The main function of this region is to supply majority charge carriers to the base. Emitter region is more heavily doped when compared with other regions.

Base: The middle section of the transistor is known as base. Base region is very lightly doped and is very thin as compared to either emitter or collector. It is made very thin to reduce recombination of charge carriers in the base region.

Collector: The main function of the collector is to collect majority charge carriers through the base. Collector region is moderately doped. The collector region is made physically larger than the emitter region. This is due to the fact that collector has to dissipate much greater power. Due to this difference, collector and emitter are not interchangeable.

Operating Modes

Transistor can be considered as two diodes connected back to back. Consider current flowing from collector to emitter of an NPN. It is operated in three regions

Fig: 2.3 Graphical representation of different regions with its voltages

Saturation – The transistor acts like a short circuit. Current freely flows from collector to emitter. i.e., both collector base and emitter base junction are forward biased
Cut-off – The transistor acts like an open circuit. No current flows from collector to emitter. i.e., both collector base and emitter base junction are Reverse biased
Active - Emitter base junction is Forward biased and collector base junction is Reverse biased
Active – Forward Active: The current from collector to emitter is proportional to the current flowing into the base.
Active – Reverse Active: Like forward active mode, the current is proportional to the base current, but it flows in reverse. Current flows from emitter to collector.

2.2 TRANSISTOR BIASING

As shown in fig 2.4, usually the emitter-base junction is forward biased (F.B) and collector-base junction is reverse biased (R.B). Due to the forward bias on the emitter-base junction, an emitter current flows through the base into the collector. Though the collector-base junction is reverse biased, almost the entire emitter current flows through the collector circuit.

Fig: 2.4 Transistor biasing (a) NPN transistor and (b) PNP transistor

2.3 OPERATION OF NPN TRANSISTOR

As shown in fig 2.5, the forward bias applied to the emitter base junction of an NPN transistor causes a lot of electrons from the emitter region to crossover to the base region. As the base is lightly doped with P-type impurity, the number of holes in the base region is very small and hence the number of electrons that combine with holes in the P-type base region is also very small. Hence a few electrons combine with holes to constitute a base current IB. The remaining electrons (more than 95%) crossover into the collector region to constitute a collector current IC. Thus the base and collector current summed up gives the emitter current, i.e.

Fig: 2.5 Current in NPN transistor

In the external circuit of the NPN bipolar junction transistor, the magnitudes of the emitter current IE, the base current IB and the collector current IC are related by

2.4 OPERATION OF PNP TRANSISTOR

As shown in fig 2.6, the forward bias applied to the emitter-base junction of a PNP transistor causes a lot of holes from the emitter region to crossover to the base region as the base is lightly doped with N-type impurity. The number of electrons in the base region is very small and hence the number of holes combined with electrons in the N-type base region is also very small. Hence a few holes combined with electrons to constitute a base current IB. The remaining holes (more than 95%) crossover into the collector region to constitute a collector current Ic. Thus the collector and base current when summed up gives the emitter current, i.e.

Fig: 2.6 Current in PNP transistor

In the external circuit of the PNP bipolar junction transistor, the magnitudes of the emitter current IE, the base current IB and the collector current Ic are related by

This equation gives the fundamental relationship between the currents in a bipolar transistor circuit. Also, this fundamental equation shows that there are current amplification factors α and β in common base transistor configuration and common emitter transistor configuration respectively for the static (D.C) currents, and for small changes in the currents.

2.5 TYPES OF CONFIGURATION

Depending upon the common terminal between the input and output of the transistor, three configurations are classified. They are: (i) Common base (CB) configuration, (ii) common emitter (CE) configuration, and (iii) common collector configuration.

(i) (CB) configuration

This is also called grounded base configuration. In this configuration, emitter is the input terminal, collector is the output terminal and base is the common terminal.

(ii) (CE) configuration

This is also called grounded emitter configuration. In this configuration, base is the input terminal, collector is the output terminal and emitter is the common terminal.

(iii) (CC) configuration

This is also called grounded collector configuration. In this configuration, base is the input terminal, emitter is the output terminal and collector is the common terminal.

Fig: 2.8 Transistor configuration: (a) Common Base, (b) Common Emitter & (c) Common Collector

2.6 CB CONFIGURATION

The circuit diagram for determining the static characteristics curves of an NPN transistor in the common base configuration is shown in fig 2.9

Fig: 2.9 Circuit to determine CB static characteristics

Input Characteristics

To determine the input characteristics, the collector-base voltage VCB is kept constant at zero volt and the emitter current IE is increased from zero in suitable equal steps by increasing VEB. This is repeated for higher fixed values of VCB. A curve is drawn between emitter current IE and emitter-base voltage VEB at constant collector-base voltage VCB. The input characteristics thus obtained are shown in fig

Fig: 2.10 CB Input Characteristics

When V_CB is equal to zero and the emitter-base junction is forward biased as shown in the fig 2.10 characteristics, the junction behaves as a forward biased diode so that emitter current I_E increases rapidly with small increase in emitter-base voltage V_EB. When V_CB is increased keeping V_EB constant, the width of the base region will decrease. This effect results in an increase of I_E. Therefore, the curves shift towards the left as V_CB is increased.

Output Characteristics

To determine the output characteristics, the emitter current I_E is kept constant at a suitable value by adjusting the emitter-base voltage V_EB. Then V_CB is increased in suitable equal steps and the collector current I_C is

Fig: 2.11 CB Output Characteristics

Noted for each value of I_E. This is repeated for different fixed values of I_E, Now the curves of Ic versus V_CB are plotted for constant values of I_E and the output characteristics thus obtained is shown in Fig. 2.11

From the characteristics, it is seen that for a constant value of I_E, I_C is independent of V_CB and the curves are parallel to the axis of V_CB. Further, Ic flows even when V_CB is equal to zero. As the emitter-base junction is forward biased, the majority carriers, i.e. electrons, from the emitter are injected into the base region. Due to the action of the internal potential barrier at the reverse biased collector-base junction, they flow to the collector region and give rise to Ic even when V_CB is equal to zero.

2.7 EARLY EFFECT OR BASE-WIDTH MODULATION

As the output voltage V_cc increased, the collector base junction is more reverse biased, therefore the depletion layer width at the collector junction increases, which reduces the effective width of the base. This dependency of base-width on collector-to-emitter voltage is known as the Early effect.

This decrease in effective base-width has three consequences:

(i) There is less chance for recombination within the base region. Hence, α increases with increasing |V_CE|.

(ii) The charge gradient is increased within the base, and consequently, the current of minority carriers injected across the emitter junction increases,

(iii) For extremely large output voltage, the effective base-width may be reduced to zero, causing voltage breakdown in the transistor. This phenomenon is called the punch through or Reach through.

For higher values of VCB, due to Early effect, the value of α increases. For example, α changes, say from 0.98 to 0.935. Hence, there is a very small positive slope in the CB output characteristics and hence the output resistance is not zero.

2.8 THERMAL RUN AWAY

Flow of collector current produces heat in the collector junction which increases the reverse saturation current I_CO,again the I_C increases. This process goes in cumulative way, the heat at the junction increases and burns the transistor. The process of self-destruction of transistor is called Thermal Run Away.

2.9 TRANSISTOR PARAMETERS

The slope of the CB characteristics will give the following four transistor parameters. Since these parameters have different dimensions, they are commonly known as common base hybrid parameters or h-parameters.

(i) Input impedance (hib). It is defined as the ratio of the change in (input) emitter voltage to the change in (input) emitter current with the (output) collector voltage V_CB kept constant. Therefore,

(ii) Output admittance (hob): It is defined as the ratio of change in the (output) collector current to the corresponding change in the (output) collector voltage with the (input) emitter current I_E kept constant. Therefore,

(iii)Forward current gain (hfb): It is defined as a ratio of the change in the (output) collector current to the corresponding change in the (input) emitter current keeping the (output) collector voltage VCB constant. Hence,

(iv) Reverse voltage gain (h_rb): It is defined as the ratio of the change in the (input) emitter voltage and the corresponding change in (output) collector voltage with constant (input) emitter current, I_E.

2.10 CE CONFIGURATION

Input characteristics

To determine the input characteristics, the collector to emitter voltage is kept constant at zero volt and base current is increased from zero in equal steps by increasing VBE in the circuit shown in Fig. 2.12.

Fig: 2.12 Circuit to determine CE static characteristics

The value of VBE is noted for each setting of IB. This procedure is repeated for higher fixed values of VCE, and the curves of IB Vs. VBE are drawn. The input characteristics thus obtained arc shown in Fig. 2.13

When VCE =0, the emitter-base junction is forward biased and the junction behaves as a forward biased diode. Hence the input characteristic for VCE = 0 is

Fig: 2.13 CE Input Characteristics

Similar to that of a forward-biased diode. When V_CE is increased, the width of the depletion region at the reverse biased collector-base junction will increase. Hence the effective width of the base will decrease. This effect causes a decrease in the base current I_B. Hence, to get the same value of I_B as that for V_CE = 0, V_BE should be increased. Therefore, the curve shifts to the right as V_CE increases.

Output characteristics

To determine the output characteristics, the base current I_B is kept constant at a suitable value by adjusting base-emitter voltage, V_BE. The magnitude of collector-emitter voltage V_CE is increased in suitable equal steps from zero and the collector current Ic is noted for each setting V_CE.

Fig: 2.14 CE Output Characteristics

and

For larger values of V_CE, due to early effect, a very small change in α is reflected in a very large change in β. For example, when α=0.98, If α increases to 0.985, then Here, a slight increase in α by about 0.5 results in an increases in by β about 34%. Hence, the output characteristics of CE configuration show a larger slope when compared with CB Configuration.

The output characteristics have three regions, namely, saturation region, cutoff region and active region. The region of curves to the left of the line OA is called the saturation region (hatched), and the line OA is called the saturation line. In this region, both junctions are forward biased and an increase in the base current does not cause a corresponding large change in Ic. The ratio of V_CE(sat) to I_C region is called saturation resistance,

The region below the curve for I_B = 0 is called the cut-off region (hatched). In this region, both junctions are reverse biased. When the operating point for the transistor enters the cut-off region, the transistor is OFF. Hence, the collector current becomes almost zero and the collector voltage almost equals Vcc, the collector supply voltage. The transistor is virtually an open circuit between collector and emitter

The central region where the curves are uniform in spacing and slope is called the active region (unhatched). In this region, emitter-base junction is forward biased and the collector-base junction is reverse biased. If the transistor is to be used as a linear amplifier, it should be operated in the active region,

If the base current is subsequently driven large and positive, the transistor switches into the saturation region via the active region, which is traversed at a rate that is dependent on factors such as gain and frequency response. In this ON condition, large collector current flows and collector voltage falls to a very low value, called V_CEsat, typically around 0.2 V for a silicon transistor, the transistor is virtually a short circuit in this state.

High speed switching circuits are designed in such a way that transistors are not allowed to saturate, thus reducing switching times between ON and OFF times.

Transistor parameters

The slope of the CE characteristics will give the following four transistor parameters. Since these parameters have different dimensions, they are commonly known as common emitter hybrid parameters or h-parameters,

(i) Input impedance (hie): It is defined as the ratio of the change in (input) base voltage to the change in (input) base current with the (output) collector voltage V_CE kept constant. Therefore,

(ii) Output admittance (hoe): It is defined as the ratio of change in the (output) collector current to the corresponding change in the (output) collector voltage with the (input) base current I_B kept constant. Therefore,

(iii) Forward current gain (hfe). It is defined as a ratio of the change in the (output) collector current to the corresponding change in the (input) base current keeping the (output) collector voltage V_CEconstant. Hence,

(iv) Reverse voltage gain (hre): It is defined as the ratio of the change in the (input) base voltage and the corresponding change in (output) collector voltage with constant (input) base current, I_B. Hence,

2.11 CC CONFIGURATION

The circuit diagram for determining the static characteristics of an NPN transistor in the common collector configuration is shown in fig. 2.15.

Fig: 2.15 Circuit to determine CC static characteristics

Input characteristics

To determine the input characteristics, V_EC is kept at a suitable fixed value. The base-collector voltage V_BC is increased in equal steps and the corresponding increase in I_B is noted. This is repeated for different fixed values of V_EC. Plots of V_BC versus I_Bfor different values of V_EC shown in Fig. 2.16 are the input characteristics.

Fig: 2.16 Input Characteristics

Output characteristics

The output characteristics are the same as those of the common emitter configuration.

2.12 COMPARISON

Property	CB	CE	CC
Input resistance	Low (about 100 Ω)	Moderate (about 750 Ω)	High (about 750 KΩ)
Output resistance	High (about 450 KΩ)	Moderate (about 45 KΩ)	Low (about 25 Ω)
Current gain	1	High	High
Voltage gain	About 150	About 500	Less than 1
Phase shift between input & output voltages	0 or 360^o	180^o	0 or 360^o
Applications	For high frequency circuits	For audio frequency circuits	For impedance matching
Current amplification factor=(output current)/(input current)

RELATIONSHIP BETWEEN α AND β

We know that

By definition,

Therefore,

i.e.

Dividing both sides by , we get

Therefore,

and , or

From this relationship, it is clear that as α approaches unity, β approaches infinity. The CE configuration is used for almost all transistor applications because of its high current gain, β

3.1 METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR (MOSFET or IGFET)

There are two types of MOSFET, they are

· Enhancement MOSFET

· Depletion MOSFET

3.2 ENHANCEMENT MOSFET

In enhancement MOSFET, the drain current is increased by the gate voltage.

Construction

The N channel MOSFET consist of a lightly doped P type substrate. Two heavily doped N+ regions are formed into the substrate.

Fig: 2.24 N channel Enhancement MOSFET

One N+ region acts as source S and another N+ region acts as drain D. A thin layer of Silicon dioxide (SiO₂) is grown over the surface, and holes are made in the oxide layer to form metal contacts with source and drain. The region between the N+ source and drain is called Channel. The gate metal contact G is made on the SiO₂ layer, above the channel. The gate metal and the channel act as the two plates of a parallel plate capacitor and the SiO₂ layer acts as the dielectric.

OPERATION

The channel is lightly doped P type semiconductor, its resistance is high therefore electrons cannot move freely from source to drain. When a Positive voltage is applied at the gate, it induces negative charges in the channel.

These induced negative charge are called inversion layer. Now the channel has electrons as carriers, therefore the conductivity of the channel increases and electrons flow from source to drain. Thus the drain current is increased (enhanced) by the gate voltage.

Fig: 2.25 Operation of N channel Enhancement MOSFET

Symbol

Fig: 2.26 Symbol of Enhancement N channel and P channel MOSFET

Drain characteristics

The graph between drain source voltage V_DS and drain current I_D for a constant gate source voltage V_GS is called the drain characteristics of MOSFET. For plotting drain characteristics, gate source voltage V_GS is kept constant, the change in drain current I_D is noted for change in drain source voltage V_DS. The variation of I_D with V_DS for different constant value of V_GS can be obtained and plotted on a graph.

Fig: 2.27 Drain characteristics

When V_GS is zero, the drain current is zero. When V_GS is made positive, drain current increases.

Transfer characteristics

The graph between gate source voltage V_GS and drain current I_D for a constant drain and source voltage V_DS is called the transfer characteristics of MOSFET.

For a fixed drain source voltage V_DS voltage, when V_GS is zero or negative, very small saturation current flow through the MOSFET, called I_DSS. When V_GS is increased above zero, the drain current, remain at I_DSS for small values of V_GS. Above the gate source threshold voltage V_GST the current increases rapidly.

N channel enhancement MOSFET cannot be operated with gate voltage negative (depletion mode).

Fig: 2.28 Transfer characteristics

3.3 DEPLETION MOSFET

In depletion MOSFET, the drain current can be increased or decreased by the gate voltage.

Construction

The N channel depletion MOSFET consist of a lightly doped P type substrate. Two heavily doped N+ regions are formed into the substrate.

Fig: 2.29 N channel Depletion MOSFET

One N+ region acts as source S and another N+ region acts as drain D. A lightly doped N type channel is formed between source and drain. A thin layer of silicon dioxide (SiO₂) is grown over the surface, and holes are made in the oxide layer to form metal contacts with source and drain. The gate metal contact G is made on the SiO₂ layer, above the N type channel. The gate metal and the channel act as the two plates of a parallel plate capacitor and the SiO₂ layer act as the dielectric.

Operation

The channel is lightly doped N type semiconductor, therefore electrons can move from source to drain.

Enhancement mode

When a positive voltage is applied at the gate, it induces negative charges in the channel, therefore the conductivity of the channel increases and more electrons flow from source to drain. Thus the drain current is increases (enhanced) by the positive gate voltage.

Fig: 2.30 N channel depletion MOSFET with Negative Gate Voltage

Depletion Mode

When the gate voltage is negative, it induces positive charges in the channel. The induced positive charges reduce the conductivity of the channel (prevent the movement of electrons from source to drain); therefore the drain current decreases for negative gate voltage.

Symbol

Fig: 2.31 Symbol of Depletion N channel and P channel MOSFET

Drain Characteristics

The graph between drain source voltage V_DS and drain current I_D for a constant gate source voltage V_GS is called the drain characteristics of MOSFET.

For plotting drain characteristics, gate source voltage V_GS is kept constant, the change in drain current I_D is noted for change in drain source voltage V_DS. The variation of I_D with V_DS for different constant values of V_GScan be obtained and plotted on a graph.

Fig: 2.32 Drain characteristics

When V_GS is zero, small drain current flow through the MOSFET. When V_GS is made positive, drain current is increases. When V_GS is made negative, the drain current is decreases.

Transfer Characteristics

The graph between gate source voltage V_GS and drain current I_D for a constant drain and source voltage V_DS is called the transfer characteristics of MOSFET.

For a fixed V_DS voltage, when V_GS is zero small drain current flow through the MOSFET. When V_GS is increased above zero, the drain current is also increased. When V_GS is reduced below zero, the drain current decreases and reaches zero. The gate source voltage V_GS at which the drain current become zero is called V_GS(off), at this voltage the current is completely occupied by the positive charges, therefore electrons cannot move from source to drain.

Fig: 2.33 Transfer Characteristics

N channel depletion MOSFET can be operated with gate voltage negative (depletion mode) and with gate voltage positive (enhancement mode).

3.4 SILICON CONTROLLED RECTIFIER (SCR)

Diode and transistor cannot be used as switch in high current. Thyristors (SCR, DIAC & TRIAC) are semiconductor switch, made to operate in high current application.

Construction

SCR is a four layer semiconductor device, which consist of alternate P type and N type silicon. SCR consists of three junctions J₁, J₂ and J₃ and three terminals known as anode A, cathode K and Gate G.

The function of gate is to control the firing of SCR. SCR conducts only in one direction i.e., from anode to cathode and hence it is called unidirectional switch.

Symbol & Structure

Fig: 3.16 (a) Symbol of SCR, (b) structure of SCR

Two Transistor analogy of SCR

The operation of SCR may be explained by dividing it into two transistors: a PNP transistor T₁ and another NPN transistor T_2. Collector of each transistor is coupled to the base of the other transistor.

Fig: 3.17 Two transistor model of SCR

When gate is open and V_AK is positive and less than the break over voltage VBO, transistors T₁ and T₂ remain in cutoff. Therefore, no current flows through the SCR. When the gate is made positive, a small gate current flow through the base of T₂. This increase its collector current. As the collector current of T₂is the base current of T₁, T₁ is switched ON and its collector current increases. The collector current of T₁ is base current of T₂. Therefore an increase in current of one transistor causes an increase in current of the other transistor. This process goes in an accumulative way and both transistors are driven into saturation. Now a heavy current flow through the load. The SCR is in ON condition.

Operation of SCR

SCR can be operated with gate open or with positive voltage at the gate. In SCR a load is connected in series with anode and anode is kept at positive potential with respect to cathode with the help of a battery.

I_G=0 and V_AK is positive

When no voltage is applied at the gate(I_G=0) and V_AK is positive, junctions J₁ and J₂ are forward biased while the junction J₂ is reverse biased. Due to the reverse bias of the junction J₂, no current flows through the load R_L. The current flowing through the SCR is very small reverse saturation current. SCR is said to be in cut-off condition.

Fig: 3.18 SCR with open Gate

When positive voltage at the anode is increased, the reverse bias at the junction J₂ is also increases and at a particular voltage the junction J₂ breaks down. Now the SCR conducts heavily and is said to be in ON state. The resistance of SCR become low (0.01Ω to 1Ω) therefore the voltage across the SCR decreases.

The anode voltage at which the SCR start conducting is called break over voltage. The current at which SCR is switched ON is called the holding current I_H. Holding current is the minimum current required to keep the SCR in ON condition.

SCR can be used as a switch with V_AK positive.

I_G>0 and V_AK is positive

When V_AK is positive, junction J₁ and J₃ are forward biased while the junction J₂ is reverse biased. When the gate is made positive with respect to cathode, the gate voltage forward biases junction J₃. Electrons from N type material cross the junction J₃ and move into P type material, holes from P type material cross the junction J₃ and move in to the N type material. Electrons are the majority carriers in the P type material, which can cross the reverse biased junction J₂. More electrons that get energy from the positive voltage can create more carriers and break the junction J₂ for a low V_AK, and SCR conducts heavily. Once SCR starts conducting, the gate loses its control, SCR cannot be switched off by the gate voltage.

Fig: 3.19 VI characteristics of SCR

V_AK is negative

When V_AK is negative, junction J₁ and J₃ are reverse biased while the junction J₂ is forward biased. Due to the reverse bias of junction J₁ and J₃, no current flows through the load R_L. The current flowing through the SCR is very small reverse saturation current. When the negative voltage at the anode is increased, break down of junctions happens similar to zener breakdown which damages SCR. SCR cannot be used as switch with negative V_AK.

SCR Properties

SCR can be switched ON by positive anode voltage.
Positive gate voltage makes SCR to switch ON at lower positive anode voltages.
When SCR is in ON condition it cannot be switched OFF by gate voltage.
The anode voltage is to be reduced, to reduce the current below holding current in order to switch OFF the SCR.
SCR cannot be used with negative anode voltage.
SCR is unidirectional switch.

4.1 PHOTOCONDUCTIVITY

When light in the form of photons strikes the semiconductor, each photon delivers energy to the electrons. If the photon energy is greater than the energy hand gap of the semiconductor, free mobile charge carriers are liberated and, as a result, resistivity of the semiconductor is decreased, so conduction starts. This Process of generating electric current from incident light is called photoconductivity.

4.2 LIGHT DEPENDENT RESISTOR(LDR) OR PHOTORESISTOR:

It is a semiconductor device whose resistance varies inversely with the intensity of light falls upon it. It is also known as photoresistive cell or photoresistor because it operates on the principle of photoresistivity.

when light in the form of photons strikes the semiconductor, each photon delivers energy to the electrons. If the photon energy is greater than the energy hand gap of the semiconductor, free mobile charge carriers are liberated and, as a result, resistivity of the semiconductor is decreased, so conduction starts.

Photoconductive cells are generally made of cadmium compounds such as cadmium sulphide {CdS) and cadmium selenide (CdSe).

Fig: 3.25 circuit Symbol

A photoconductive cell is an inexpensive and simple detector which is widely used in OFF/ON circuits, light-measurement and light-detecting circuits.

Fig: 3.26 (a) Characteristics curve of LDR, (b) Basic structure of LDR

There are two types of LDR

· Intrinsic photo resistor(Pure semiconductors are used for construction)

· Extrinsic photo resistor(Doped semiconductors are used for construction)

Application

Light detector

4.3 PHOTODIODES

A photodiode is a type of photodetector capable of converting light into either current or voltage.

A photodiode is a two terminal PN junction device, which operates in a reverse bias. It has a small transparent window, which allows light to strike the PN junction.

Fig: 3.27 Photodiode

Photodiode produces reverse current in the reverse bias by thermally generated electron-hole pairs in the depletion layer, which are swept across the junction by the electric field created by the reverse voltage.

Its reverse current increases with the light intensity at the PN junction When there is no incident light, the reverse current is almost negligible and is called the dark current. An increase in the amount of light energy produces an increase in the reverse current for a given value of reverse-bias voltage.

Fig: 3.28 Characteristic curve of a photodiode

Applications

1. Used in consumer electronic device such as compact disc (CD) players, smoke detectors, and the receivers for infrared remote control equipments from televisions to air-conditioners,

2. Used or accurate measurement of light intensity in science and industry. The photodiodes have more linear response than photo-conductors.

3. Widely used in medical applications such as detectors for computer tomography, instruments to analyze samples pulse oximeters.

4. Because of their fast switching speed, used for optical communication and in lighting regulation.

5. Optical communication systems

6. Character recognition

7. Encoders etc.

4.4 AVALANCHE PHOTO DIODE (APD)

ADP is used in optical communication for detection of light at the receiving end. It converts the input light energy into electrical energy.

It essentially consist of reverse biased PN junction. The depletion region in the reverse biased PN junction is formed by immobile positively charged donor atoms in the N-type semiconductor material and immobile negative charged acceptor atoms in the P-type material. The electric field in this depletion region is very high where most of the photons are absorbed and primary charge carriers (electron-hole pair) generated. There charge carriers acquire sufficient energy from the electric field to excite new electron hole pair by this process known as impact ionization.

These new carriers created by impact ionization can themselves produce additional carriers by the same mechanism. For this process, APD requires a high reverse bias voltage in the order of 100- 400 V. carrier multiplication factors as greater as 104 may be obtained using defect free materials. Electron-hole pair thus generated separate and drift under the influence of the electric field in the depletion region and diffuse outside the depletion region so that they are finally collected in the detector terminals. This leads to a flow of current in the external circuit whose magnitude is proportional to the intensity of light incident on APD.

Fig: 3.29 Structure of APD

Due to the internal gain mechanism in an APD, a large electrical response is obtained even for a weak input signal. Quantum efficiency closer to 100% in the working region can be obtained.

4.5 PHOTOTRANSISTOR

It is light-sensitive transistor and is similar to an ordinary bipolar junction transistor (BJT) except that it has no connection to the base terminal. Its operation is based on the photodiode that exists at the CB junction. Instead of the base current, the input to the transistor is provided in the form of light.

Silicon NPN are mostly used as phototransistor. The device is usually packed with a lens on top although it is sometimes encapsulated in clear plastic. When there is no incident light on the CB junction, there is a small thermally-generated collector-to-emitter leakage current I_CEO which, in this case, is called dark current and is in the nA range

When light is incident on the CB junction, a base current I_λis produced which is directly proportional to the light intensity. Hence, collector current I_C=βI_λ

The photo-transistor has the advantages of greater sensitivity and current capacity than photodiodes, However, photodiodes are faster of the two switching in less than a nanosecond.

Fig: 3.30 (a) structure of phototransistor, (b) characteristic curve of Phototransistor

4.6 OPTOCOUPLER

An optocoupler is a solid-slate component in which the Light emitter, the light path and the light detector are all enclosed within the component and cannot be changed externally. As the optocoupler provides electrical isolation between circuits, it is also called optoisolator. An optoisolator allows signal transfer without coupling wires, capacitors or transformers. It can couple digital (ON/OFF) or analog (variable) signals.

Fig: 3.31 Schematic representation of the optocoupler

Optoisolator consists of an infrared LED and a photodetector such as PIN photodiode for fast switching.

Optoisolators transduce input voltage to proportional light intensity by using LEDs. The light is transduced back to output voltage using light sensitive devices, GaAs.

The wavelength response of each device is made to be as identical as possible to permit the highest measure of coupling possible. There is a transparent insulating cap between each set' of elements embedded in the structure (not visible) permit the passage of light. They are designed with very small response times in such a way that they can be used to transmit data in the MHz range.

Optoisolator is used as an interface between high voltage and low voltage systems. Application for this device includes the interfacing of different types of logic circuits and their use in level-and- position-sensing circuits.

The switching time of an optoisolator decreases with increased current, while for many devices it is exactly the reverse.

5.1 RECTIFIERS

Rectifier is defined as an electronic device used for converting (A.C) voltage into unidirectional voltage (D.C). i.e. A.C. voltage into a pulsating D.C. voltage

· Half wave Rectifier

· Full wave Rectifier

5.2 HALF-WAVE RECTIFIER (A.C. voltage into a pulsating D.C. voltage)

Fig: 5.1 (a) Basic structure of half-wave rectifier, and (b) input output waveforms of half wave rectifier

Let V_i be the voltage to the primary of the transformer and given by the equation

where V_γ is the cut-in voltage of the diode.

During positive half cycle, the diode D is in Forward bias condition. Diode conduct.

During negative half cycle, the diode D is in reverse bias condition. Diode does not conduct.

The rms voltage at the load resistance can be calculated as

Peak Factor

5.3 FULL-WAVE RECTIFIER (A.C. voltage into a pulsating D.C. voltage)

It uses two diodes of which one conducts during one half-cycle while the other diode conducts during the other half-cycle of the applied ac voltage. There are two types of full-wave rectifiers viz. (i) center tapped transformer full-wave rectifier and (ii) bridge rectifier.

During positive half-cycle, diode D₁ forward bias, conducts current and diode D₂ reverse bias, does not conduct.

During negative half-cycle, diode D₂ forward bias, conducts current and diode D₁ reverse bias, does not conduct.

Fig: 5.2 (a) Full-wave Rectifier, (b) input and output waveform

RIPPLE FACTOR (Г)

The average voltage or dc voltage available across the load resistance is

Therefore,

Efficiency (η): The ratio of dc output power to ac input power is known as rectifier efficiency (η)

The maximum efficiency of a full-wave rectifier is 81.2%

and

If the diode forward resistance (r_f) and the transformer secondary winding resistance (r_s) are included in the analysis, then

Transformer Utilisation Factor (TUF) The average TUF in a full-wave rectifying circuit is determined by considering the primary and secondary winding separately and it gives a value of 0.693

Form factor

Form factor =

Peak factor

Peak factor=

Peak inverse voltage for full-wave rectifier is 2V_mbecause the entire secondary voltage appears across the non-conducting diode.

5.4 BRIDGE RECTIFIER (A.C. voltage into a pulsating D.C. voltage)

The need for a center tapped transformer in a full-wave rectifier is eliminated in the bridge rectifier.

For the positive half-cycle of the input ac voltage, diodes D₁, and D₃ conduct, whereas diodes D₂ and D₄ do not conduct. The conducting diodes will be in series through the load resistance R_L. So the load current flows through R_L.

Fig: 5.3 (a) Bridge rectifier, (b) input and output waveforms

During the negative half-cycle of the input ac voltage, diodes D₂ and D₄conduct, whereas diodes D₁ and D₃ do not conduct. The conducting diode D₂and D₄ will be in series through the load R_L and the current flows through R_Lin the same direction as in the previous half-cycle. Thus a bidirectional wave is converted into a unidirectional one.

The average values of output voltage and load current for bridge rectifier are the same as for a center-tapped full wave rectifier. Hence,

If the values of the transformer secondary winding resistance (r_s) and diode forward resistance (r_f) are considered in the analysis, then

The maximum efficiency of a bridge rectifier is 81.2% and the ripple factor is 0.48. The PIV is V_m

Advantages of the bridge rectifier

The ripple factor and efficiency of the rectification are the same as the full-wave rectifier. The bulky center tapped transformer is not required. Transformer utilization factor is considerably high. Since the current flowing in the transformer secondary is purely alternating, the TUF increases to 0.812.

Disadvantage is it requires four. But the diodes are cheaper. Apart from this, the PIV rating required for the diodes in a bridge rectifier is only half of that for a center tapped full-wave rectifier, this is a great advantage.

5.5 FILTERS

The ripple in the rectified (contains dc and ac component) wave being very high, the factor being 48% in the full-wave rectifier; majority of the applications which cannot tolerate this. Filters are used to minimise the undesirable ac, i.e. ripple leaving only the dc component to appear at the output.

The full wave rectified output voltage is applied at filters input. The output of a filter is not exactly a constant dc level. But it also contains a small amount of ac component. Some important filters arc:

(a) Inductor filter

(b) Capacitor filter

(d) and CLC or π-type filter

Fig: 5.4 concept of filter

5.6 CAPACITOR FILTER

An inexpensive filter for light loads is found in the capacitor filter which is connected directly across the load, as shown in Fig. 5.6 (a). The property of a capacitor is that it allows ac component and blocks the dc component.

During the positive half-cycle, the capacitor charges up to the peak value of the transformer secondary voltage, Vm, and will try to maintain this value as the full-wave input drops to zero. The capacitor will discharge through RL slowly until the transformer secondary voltage again increases to a value greater than the capacitor voltage. The diode conducts for a period which depends on the capacitor voltage (equal to the load voltage). The diode will conduct when the transformer secondary voltage becomes more than the 'cut-in’ voltage of the diode. The diode stops conducting when the transformer voltage becomes less than the diode voltage. This is called cut-out voltage.

From the cut-in point to the cut-out point, what-ever charge the capacitor acquires is equal to the charge the capacitor has lost during the period of non-conduction.

Fig: 5.6 (a) Capacitor filter, (b) Ripple voltage triangular waveform

If the value of the capacitor is fairly large, or the value of the load resistance is very large, then it can be assumed that the time T₂ is equal to half the periodic time of the waveform .i.e.

With the assumptions made above, the ripple waveform will be triangular in nature and the rms value of the ripple is given by

6.1 VOLTAGE REGULATION USING ZENER DIODE (Gives Constant Output Voltage)

A voltage regulator is an electronic circuit that provides a stable DC voltage independent of the load current, temperature and ac line voltage variations.

The quality of the regulation specified by (i) Line regulation and (ii) Load regulation. For a good regulator the line and load regulation should be minimum value.

6.2 ZENER DIODE SHUNT REGULATOR

The zener diode is selected with V_z equal to the voltage desired across the load. The zener diode has a characteristic that under reverse bias condition, the voltage across it practically remains constant, even if the current through it changes by a large extent. Under normal conditions, the input current I_i=I_L+I_Z flows through resistor R. The input voltage V_i can be written as

V_i=I_iR+V_Z= (I_L+I_Z)R+V_Z

Fig: 5.11 Zener voltage Regulator

When the input voltage V_i increases (say due to supply voltage variations), as the voltage across zener diode remains constant, the drop across resistor R will increase with a corresponding increase in I_L + I_z. As V_z is a constant, the voltage across the load will also remain constant and hence, I_L will be a constant. Therefore, an increase in I_L + I_Z will result in an increase in I_z which will not alter the voltage across the load.

It must be ensured that the reverse voltage applied to the zener diode never exceeds PIV of the diode and at the same time, the applied input voltage must be greater than the breakdown voltage of the zener diode for its operation. The zener diodes can be used as 'stand-alone' regulator circuits and also as reference voltage sources.

LINE REGULATION

A change in input voltage to a regulator will cause a change in its output of load voltage. Line regulation is defined as the change in output voltage for a change in line supply voltage keeping the load current and temperature constant. Line regulation is given by

LOAD REGULATION

Voltage regulator will experience a slight change in output voltage when there is a change in load current demand (i.e., Full load voltage).

7.1 SENSOR

A sensor is a device that detects the change in the environment and responds to some output on the other system. A sensor converts a physical phenomenon into a measurable analog voltage (or sometimes a digital signal) converted into a human-readable display or transmitted for reading or further processing.

One of the best-known sensors is the microphone, which converts sound energy to an electrical signal that can be amplified, transmitted, recorded, and reproduced.

7.2 TYPES OF SENSORS

· Passive Sensor

· Active Sensor

7.2.1 PASSIVE SENSOR

A passive sensor is a microwave instrument designed to receive and to measure natural emissions produced by constituents of the Earth's surface and its atmosphere.

7.2.2 ACTIVE SENSOR

An active sensor is a radar instrument used for measuring signals transmitted by the sensor that were reflected, refracted or scattered by the Earth's surface or its atmosphere.

Electronic Components and Devices

1.1 SEMICONDUCTOR DIODES

5.2 HALF-WAVE RECTIFIER (A.C. voltage into a pulsating D.C. voltage)

5.3 FULL-WAVE RECTIFIER (A.C. voltage into a pulsating D.C. voltage)

5.4 BRIDGE RECTIFIER (A.C. voltage into a pulsating D.C. voltage)

7.2.1 PASSIVE SENSOR

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