bjt (first lecture)

Electronics-I (EEE 231)

Syed Bilal JavedLecturer, Department of Electrical EngineeringCIIT, Islamabad.

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Note: Some of the material used in these slides is copyright of the respective authors and is used solely for teaching purpose.

Lecture No. 15 (November 01, 2012)

Chapter 5

BIPOLAR JUNCTION TRANSISTOR

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Learning Outcomes

At the end of this chapter, the students will be able to:

Understand the construction and operation of Bipolar Junction Transistors.

Understand the DC analysis and design techniques of BJT circuits.

Examine three basic applications of BJT.

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Introduction

The basic of electronic systems is a semiconductor device. The famous and commonly use of this device is Transistors (TRANSfer-resISTOR).

There are two types of Transistors

1) BJTs (Bipolar Junction Transistor) Current in the transistor is due to the flow of both electrons and holes, hence the name bipolar

2) FET (Field Effect Transistor)FETs are unipolar because the conduction is only due

to one type of charge carrier.

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BJTs StructureThere are two types of BJTs: (a) pnp and (b) npn-type.

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BJT consists of three terminal:

Base (B): This region is very thin and lightly doped.

Emitter (E): It is heavily doped

Collector (C): It is also heavily doped but its doping level is smaller than emitter.

Collector region area is slightly more than that of emitter. This is because the collector region has to handle more power than the emitter and more surface area is required for heat dissipation.

• By imaging the analogy of diode, transistor can be construct like two diodes that connected together as in figure below.

• However, we cannot replace transistor by back to back

connected diodes. (Why????? )

B

E C

B

E C

Fig. 4.2: (a) pnp transistor (b) npn transistor

(a)

(b)

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BJTs Structure (Cont....)

• A biased transistor means a transistor with external voltage (DC) is applied. • A single pn junction has two different types of bias:- Forward Bias- Reverse Bias

• Thus, a two-pn-jnuction device has four types of bias.

SaturationForwardForward

ActiveForwardReverse

Reverse ActiveReverseForward

Cut-offReverseReverse

Mode of operationBE junction

(BEJ)

BC junction

(BCJ)

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Biased Transistor

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Operation of npn Transistor in Active mode

BE Junction F.B. BC Junction R.B.

o The base to emitter junction is forward biased by the dc source VEE..

Thus, the depletion region at this junction is reduced.

o The collector to base junction is reverse biased, increasing depletion region at collector to base junction shown in fig.

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Operation of npn Transistor in Active mode (Cont....)

- VEE +

The forward biased EB junction causes the electrons in the n-type emitter to flow towards the base, creating an excess minority carrier concentration in the base. This constitutes the emitter current.

The base region is very narrow so that, in the ideal case, the injected electrons will not recombine with any of the majority carrier holes in the base shown by the straight line in Figure 5.4 (on next slide).

However, if some carrier recombination does occur in the base, the electron concentration will deviate from the ideal linear curve, as shown in the Figure 5.4 (on next slide).

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Since the B–C junction is reverse biased, the electron concentration at the edge of that junction is approximately zero.

The reason for the zero concentration at the collector side of the base is that the positive collector voltage VCB causes the electrons at that end to be swept across the CBJ depletion region.

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Emitter Current: Since the B–E junction is forward biased, the current passing through this junction is an exponential function of B–E voltage, just as we saw in pn junction diode.

Therefore, the current at emitter terminal is

The parameter VT is the usual thermal voltage. The emission coefficient “n” that multiplies VT is assumed to be 1, as we discussed in Chapter 1 in considering the ideal diode equation.

The multiplying constant, IEO, contains electrical parameters of the junction, but in addition is directly proportional to the active B–E cross-sectional area. Typical values of IEO are in the range of 10−12 to 10−16 A, but may, for special transistors, vary outside of this range.

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Direction of Emitter Current:

The flow of the negatively charged electrons is through the emitter into the base and is opposite to the conventional current direction.

The conventional emitter current direction is therefore out of the emitter terminal.

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Collector Current:.

Most of the diffusing electrons will reach the boundary of the collector-base depletion region. Because the collector is more positive than the base (by VCB volts), these successful electrons will be swept across the CBJ depletion region into the collector. They will thus get "collected" to constitute the collector current ic.

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Collector Current:.

The number of electrons reaching the collector per unit time is proportional to the number of electrons injected into the base, which in turn is a function of the B–E voltage.

To a first approximation, the collector current is proportional to eVBE/VT and is independent of the reverse-biased B–C voltage. The device therefore looks like a constant-current source.

The collector current is

The collector current is slightly smaller than the emitter current.

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Base Current:.

The base current iB is composed of two components.

1) The first component iB1 is due to the holes injected from the base region into the emitter region. This current component is proportional to eVBE/ VT.

2) A few electrons recombine with majority carrier holes in the base. The holes that are lost must be replaced through the base terminal. This “recombination current” is directly proportional to the number of electrons being injected from the emitter, which in turn is an exponential function of the B–E voltage.

The total base current is the sum of the two components:

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Try to do analysispnp transistor

Transistor Currents

o The direction of conventional currents in an npn and pnp transistors are shown below.

• Note:- The arrow is always drawn on the emitter

- The arrow indicates the direction of the emitter current- pnp: E-B- npn: B-E

C

B

E

pnp

IC

IE

IB

C

E

npn

B

IC

IE

IB

IC=the collector current

IB= the base current

IE= the emitter current

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Transistor Currents (Cont....)

There are 3 types of transistor configuration in electric circuit:

a) CB (common base) b) CE (common emitter)c) CC (common collector)

This configuration is base on which the terminal is connected to the input signal and output signal.

Table below shows the relationship between input terminal and output terminal with the transistor configuration.

EBCC

CBCE

CECB

Output terminalInput terminalConfiguration

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Transistor Configuration

CB is derived from the fact that the :Input is applied between emitter and base and output is taken from the collector and base. Here, base of the transistor is common to both input and output circuits and hence the name common base (CB) configuration.

CB for npn Transistor CB for pnp Transistor

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Common Base Configuration

To understand complete electrical behaviour of a transistor it is necessary to study the interrelation of the various currents and voltages.

These relationships can be plotted graphically which are commonly known as the characteristics of transistor.

The most important characteristics of transistor in any configuration are input and output characteristics.

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Characteristics of Transistor

Input Characteristics: It is the curve between input current (along Y-axis) and input voltage (along X-axis) at different values of output voltage.

Output Characteristics: It is the curve between output current (along Y-axis) and output voltage (along X-axis) at different values of input current.

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Characteristics of Transistor (Cont....)

It is the curve between input current IE (along

Y-axis) and input voltage VBE

(along X-axis) at different values of output voltage

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Common Base Input Characteristics

Input characteristics for acommon-base npn transistor

VCB = 25 V

VCB = 10 VVCB = 0 V

From the input characteristics of CB, we can observe the following important points:

1. After the cut-in voltage (barrier potential), the emitter current (IE)

increases rapidly with small increase in base-emitter voltage (VBE). It means that input resistance is very small.

2. It can be observed that there is slight increase in emitter current (IE) with increase in VCB for a given value of VBE. This is because

reverse biasing voltage (VCB) enhances such current.

Note: In figure (on previous slide) each curve resembles a forward biased diode characteristic, as expected.

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Common Base Input Characteristics (Cont....)

It is the curve between output current IC (along

Y-axis) and output voltage VCB

(along X-axis) at different values of input current IE.

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Common Base Output Characteristics

Output characteristics for acommon-base npn ideal transistor

ICBO

From the output characteristics of CB, we can observe the following important points:

1. The output characteristics has three basic regions: active, cutoff and saturation

2. Active Mode:

For the operation in the active region, the BE junction is forward biased while CB junction is reverse biased. In this region collector current IC is

approximately equal to the emitter current IE and transistor works as an

amplifier. 28

Common Base Output Characteristics (Cont....)

2. Active Mode (Cont....)

a) In the active region, IC is almost constant, and the graph is almost parallel to

x-axis. The collector current IC is almost independent on collector base

voltage VCB and the transistor can be said to work as constant-current

source. This provides very high output resistance.

b) As IE increases IC also increases. Thus, IC depends upon input current IE but

not on collector voltage. Hence, input current controls output current. Since transistor requires some current to drive it, it is called current operating device.

c) In the active region, the collector-base junction is reverse biased. For every transistor there is limit on the maximum value for this reverse bias voltage.

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Note that when CB Junction of npn transistor is reverse biased for VCB = -0.2V to -0.3V because VCB is less than the minimum voltage (Vγ) required to forward bias the CB junction.

Therefore, the transistor is still basically biased in the forward-active mode. The collector current iC is still essentially equal to the emitter current iE .


3. Saturation Mode

a) The region of the characteristics where VCB is negative is called the

saturation region.

b) In this saturation region, the emitter-base and collector-base junctions are both forward biased.

c) However, as the forward-bias C–B voltage increases, the linear relationship between the collector and emitter currents is no longer valid, and the collector current very quickly drops to zero.

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4. Cuttoff Mode

a) The region of the output characteristics lying below the IE=0 line is called the

cutoff region, where the collector current is nearly zero and the collector-base and emitter-base junctions of a transistor are reverse biased.

b) If the emitter current is zero, the collector current is simply ICBO (collector-base

leakage current when the emitter is an open circuit)as shown in fig. This current is so small in magnitude and called leakage current this current corresponds to the reverse-bias saturation current in a diode, as described in Chapter 1.

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Breakdown Voltage for CB configuration

As is the case in a reverse-biased diode, the current through the collector-base junction of a transistor may increase suddenly if the reverse biasing voltage (VCB) is made sufficiently large.

This increase in current is typically caused by the avalanching mechanism. However, in a transistor it can also be the result of a phenomenon called punch through or reach throgh.

To avoid this punch through effect VCB should always be kept below the maximum safe limit specified by the manufacturer.

Breakdown Voltage for CB configuration (Cont....)The curves shown at the right side of dotted line (VCBmax is exceeded) represent the breakdown condition.

When collector to base voltage increases, width of the depletion region at the junction increases. Therefore, when VCB increases above the VCBmax, increase in depletion region is such that it penetrates into the base until it makes contact with emitter-base depletion region. This condition is called “punch through” or “reach through” effect.

When this situation occurs, breakdown occurs i.e. large collector current flows which destroys the transistor.

Base current IB (A) is small compare to emitter current IE (mA)

and collector current IC (mA).

The relationship among these current can be analyse with KCL : IE =IB + IC

Emitter current that flows through collector known as IE . The

value is big compare to leakage current.

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Common Base Configuration

IC = IC(majority) + IC(minority)

IC = IE + IC(minority)

IC = IE

Where IC (minority) is ignored due to small value.

= IC / IE

Ideally = 1, but in reality it is between 0.9 and 0.998.

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Common Base Configuration (Cont...)

The emitter and collector currents are related by iC = αiE . We can also relate the coefficients by IS = αIEO. The parameter α is called the common-base current gain whose value is always slightly less than unity.

The ratio of collector current (IC) to the base current (IB) is called beta ( = IC / IB) which is common emitter current gain. This is known as beta for

the transistor.

Where is a constant for a particular transistor. For modern npn transistors, is in the range 50 to 200, but it can be as high as 1000 for special devices. The constant is called the common-emitter current gain.

The ratio of collector current (IC) to the emitter current (IE) is called alpha

(α = IC / IE) which is common base current gain. This is known as beta for

the transistor

Finally, we should note that because α and characterize the operation of the BJT in the "forward-active" mode they are often denoted αF and F. We

shall use α and αF interchangeably and, similarly, and F.

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Transistor Parameters (α and )

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Relationship between α and

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Transistor Parameters (α and ) (Cont....)

CE is derived from the fact that the :Input is applied between base and emitter and output is taken from collector and emitter. Here, emitter of the transistor is common to both, input and output circuits and hence the name common emitter (CE) configuration.

CE for npn Transistor CE for pnp Transistor

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Common Emitter Configuration

As shown in figures the bias voltage VBB forward biases the

base-emitter junction and VCC is used to reverse bias the

collector-base junction.

CE for npn Transistor .

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Common Emitter Configuration (Cont....)

The fig. Shows the input and output voltages and currents for the common-emitter configuration.

• Input voltage is the base-emitter voltage (VBE)

• Output voltage is the collector-emitter voltage (VCE)

• Input Current is the base current (IB)

• Output current is the collector current (IC)

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It is the curve between input current IB (along

Y-axis) and input voltage VBE (along X-

axis) at different values of output voltage VCE.

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Common Emitter Input Characteristics

Input characteristics for aCommon-emitter npn transistor

From the input characteristics of CE, we can observe the following important points:

1. After the cut-in voltage (barrier potential), base current (IB) increases rapidly

with small increase in base-emitter voltage (VBE). It means that input

resistance is very small.

2. It can be observed that for a fixed value of VBE, IB increase as VCE

decreases. This is because a large value of VCE results in a large reverse bias

of the collector-base junction, which widens the depletion region and makes the base smaller. Hence, there are fewer recombinations in the base region, reducing the base current.

Note: In figure (on previous slide) each curve resembles a forward biased diode characteristic, as expected.

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Common Emitter Input Characteristics (Cont....)

It is the curve between output current IC (along Y-axis) and output voltage

VCE (along X-axis) at different values of input current IB.

This characteristics is often called collector characteristics.

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Common Emitter Output Characteristics

From the output characteristics of CE, we can observe the following important points:

1. The output characteristics has three basic regions: active, cutoff and saturation

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Common Emitter Output Characteristics (Cont....)

2. Active Mode

a) For the operation in the active region, the BE junction is forward biased while CB junction is reverse biased. The region where the curves are approximately horizontal is the “active region” of the CE configuration.

b) As VCE is increased, reverse bias increases. This causes depletion region to

spread more in base than in collector (Why?), reducing the chance of recombination in the base. This increases the value of α. This early effect causes collector current to rise more sharply with increasing VCE in the

linear region of output characteristics of CE transistor.

Ans: Depletion region spreads more in the lightly doped region. Base is lightly doped as compared to collector.

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2. Saturation Mode

a) If VCE is reduced to a small value such as 0.2 V, then collector-base junction

becomes forward biased.

b) When both the junctions are forward biased, the transistor operates in the saturation region, which is indicated on the output characteristics. The saturation value of VCE usually ranges between 0.1 V to 0.3 V.

3. Cutoff Mode

c) In this region, both the junctions of the transistor are reverse biased.

d) When the input base current is made equal to zero, the collector current is the reverse leakage current ICEO. The region below IB=0 is the cutoff

region.

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Breakdown Voltage for CE configuration

In the active region, the collector-base junction is reverse biased. For every transistor there is limit on the maximum value for this reverse bias voltage. If this limit is exceeded as shown in figure, the breakdown occurs in the transistor. This effect is commonly known as punch through or reach throgh.

To avoid this punch through effect VCB should always be kept below the maximum safe limit specified by the manufacturer.

• It is called common-emitter configuration since : - emitter is common or reference to both i/p and o/p

terminals.- emitter is usually the terminal at ground potential.

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Common Emitter (CE) Configuration


In the transistor, the rate of flow of electrons and the resulting collector current are an exponential function of the B–E voltage, as is the resulting base current. This means that the collector current and the base current are linearly related. Therefore, we can write

Or

The parameter β is the common-emitter current gain and is a key parameter of the bipolar transistor. In this idealized situation, β is considered to be a constant for any given transistor. The value of β is usually in the range of 50 < β < 300, but it can be smaller or larger for special devices.

The value of β is highly dependent upon transistor fabrication techniques and process tolerances.

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When B–E junction is forward biased and the B–C junction is reverse biased. Then1) Using the piecewise linear model of a pn junction, we assume that the

B–E voltage is equal to VBE(on), the junction turn on voltage.

2) Since VCC = VCE + iC RC , the power supply voltage must be sufficiently large to keep the B–C junction reverse biased.

3) The base current is established by VBB and RB, and the resulting collector current is iC = βiB .

If we set VBB = 0, the B–E junction will have zero applied volts; therefore,iB = 0, which implies that iC = 0. This condition is called cutoff.

npn Transistor: Forward Active Mode Operation ( Cont....)

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The set of curves shown in figure are also called output characteristic curves (output current “iC ” versus output voltage “vCE” at different values of input current “iB” ) for CE transistor.

Current Voltage Characteristics for CE Configuration

In the npn device, in order for the transistor to be biased in the forward-active mode, the B–C junction must be zero or reverse biased, which means that VCE must be greater than approximately VBE(on).

For VCE > VBE(on), There is a finite slope to the curves. If, however, VCE < VBE(on), the B–C junction becomes forward biased, the transistor is no longer in the forward-active mode, and the collector current very quickly drops to zero.

Current Voltage Characteristics for CE Configuration (Cont...)

Figure 5.14 shows an exaggerated view of the current–voltage characteristics plotted for constant values of the B–E voltage.

When extrapolated, the characteristic lines meet at a point on the negative vCE axis, at vCE = -VA, The voltage VA, a positive number, is a parameter for the particular BJT, with typical values in the range of 50 V to 100 V. It is called the Early voltage, after J. M. Early, the engineering scientist who first studied this phenomenon.

Current Voltage Characteristics for CE Configuration (Cont...)

Circuit Symbols and Conventions

Summary of Transistor Operation

When the npn bipolar transistor biased in the forward-active region. Then:

The forward-biased B–E voltage, VBE, causes an exponentially related flow of electrons from the emitter into the base where they diffuse across the base region and are collected in the collector region.

The collector current, iC , is independent of the B–C voltage as long as the B–C junction is reverse biased. The collector, then, behaves as an ideal current source.

The collector current is a fraction α of the emitter current, and the base current is a fraction 1/β of the collector current. If β >> 1, then α ≈ 1 and iC ≈ iE .

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Important Points

The voltage notation vBE , with the dual subscript, denotes the voltage between the B (base) and E (emitter) terminals. Implicit in the notation is that the first subscript (the base terminal) is positive with respect to the second subscript (the emitter terminal).

Since we are considering the case of a transistor biased in the forward-active mode, the common–base current gain and common-emitter current gain parameters are often denoted as αF and βF , respectively.

For ease of notation, we will simply define these parameters as α and β.

Leakage Current for CB configuration

In the common-base circuits in Figure 5.11, if we set the current source iE = 0, transistors will be cut off, but the B–C junctions will still be reverse biased.

A reverse bias leakage current exists in these junctions, and this current corresponds to the reverse-bias saturation current in a diode, as described in Chapter 1.

Leakage Current for CB Configuration (Cont....)

The direction of these reverse-bias leakage currents is the same as that of the collector currents.

The term ICBO is the collector leakage current in the common-base configuration, and is the collector-base leakage current when the emitter is an open circuit. This leakage current is shown in Figure 5.15(a).

The cutoff region for the common-emitter configuration is not as well defined as for the common-base configuration. Note on the output characteristics for CE shown in Fig. 2 that IC is not equal to zero when IB is zero. For the common-base configuration, when the input current IE was equal to zero, the collector current was equal only to the reverse saturation current ICEO

Leakage Current for CE Configuration

Leakage Current for CE Configuration (Cont....)

Another leakage current can exist between the emitter and collector with the base terminal an open circuit.

Figure 5.15(b) is a block diagram of an npn transistor in which the base is an open circuit (iB = 0).

If we consider the case discussed in previous slide, where IB = 0 A, and substitute a typical value of such as α = 0.996, the resulting collector current is the following:

If ICBO were 1µA, the resulting collector current with IB = 0 A would be 250(1µA) = 0.25 mA,

For future reference, the collector current defined by the condition IB = 0 A will be assigned the notation ICE0 as indicated by following Eq.

Leakage Current for CE Configuration (Cont....)

The common-base current–voltage characteristics shown in Fig. (1) are ideal in that breakdown is not shown. Fig. (2) shows the same iC versus vCB characteristics with the breakdown voltage.

Fig. (1) Ideal Fig. (2) Practical

Breakdown Voltage for CB configuration

Relationship between Breakdown Voltages of CB & CE configuration

The breakdown voltage characteristics for the two configurations are also different.The breakdown voltage for the open-base case is given by

where n is an empirical constant usually in the range of 3 to 6.

Comment: The breakdown voltage of the open-base configuration is substantially less than that of the C–B junction. This represents a worst-case condition, which must be considered in any circuit design.

Design Pointer: The designer must be aware of the breakdown voltage of the specific transistors used in a circuit, since this will be a limiting factor in the size of the dc bias voltages that can be used.

bjt (first lecture)

Documents

base junction

emitter junction

base region

operation of npn transistor

active modebe junction

forward biased eb junction

base b

single pn junction