Bipolar Junction Transistors

(BJT)

SUT25772

Spring 2011

Dr.Fardmanehsh

Prepared by: Siavash Kananian

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Transistors

Two main categories of transistors: bipolar junction transistors (BJTs) and

field effect transistors (FETs).

Transistors have 3 terminals where the application of current (BJT) or voltage (FET) to the input terminal increases the amount of charge in the active region.

The physics of "transistor action" is quite different for the BJT and FET.

In analog circuits, transistors are used in amplifiers and linear regulated power supplies.

In digital circuits they function as electrical switches, including logic gates, random access memory (RAM), and microprocessors.

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The First Transistor: Point-contact transistor

A point-contact transistor

was the first type of solid

state electronic transistor

ever constructed.

It was made by researchers

John Bardeen & Walter

Houser Brattain at Bell

Laboratories in December

1947.

The point-contact transistor was

commercialized and sold by Western

Electric and others but was rather

quickly superseded by the junction

transistor.

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The Junction Transistor

First BJT was invented early in 1948, only weeks after the point contact transistor.

Initially known simply as the junction transistor.

It did not become practical until the early 1950s.

The term bipolar was tagged onto the name to distinguish the fact that both carrier types play important roles in the operation.

Field Effect Transistors (FETs) are unipolar transistors since their operation depends primarily on a single carrier type.

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Bipolar Junction Transistors (BJT)

A bipolar transistor essentially consists of a pair of PN Junction diodes that are joined back-to-back.

There are therefore two kinds of BJT, the NPN and PNP varieties.

The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.

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The First BJT

Transistor Size (3/8L X 5/32W X 7/32H) No Date Codes. No Packaging.

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Modern Transistors

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BJT Fabrication

BJT can be made either as discrete devices or in planar integrated form.

In discrete, the substrate can be used for one connection, typically the collector.

In integrated version, all 3 contacts appear on the top surface.

The E-B diode is closer to the surface than the B-C junction because it is easier make the havier doping at the top.

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BJT Structure - Discrete

Early BJTs were fabricated using alloying - an complicated

and unreliable process.

The structure contains two p-n diodes, one between the

base and the emitter, and one between the base and the

collector.

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BJT Structure - Planar

In the planar process, all steps are performed

from the surface of the wafer

The Planar Structure developed by Fairchild in the late 50s shaped the basic

structure of the BJT, even up to the present

day.

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BJTs are usually constructed vertically

Controlling depth of the emitters n doping sets the base width

np

n

E B C

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Advanced BJT Structures

The original BJT structure survived, practically unchanged, since the mid 60s.

As the advances in MOS development appears, some of the fabrication technology are also applied to the BJT. Low defect epitaxy

Ion implant

Plasma etching (dry etch)

LOCOS (local oxidation of Si)

Polysilicon layers

Improved lithography

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Isolation Methods

The most significant advances in reducing overall device size and packing density have come from improved isolation methods.

The traditional junction isolation technique requires the p+ deep diffusion to be aligned to the n+ buried layer that is covered by a thick epitaxial layer.

The area (and hence junction capacitance) is determined by alignment tolerance, area for side diffusion, and allowance for the spread of the depletion region.

Modern isolation techniques: oxide isolation, and trench isolation.

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Oxide & Trench Isolation

Oxide isolation processes were intorduced in the late 70s. They utilize wet anisotropic etch (KOH) of the Si wafer with Si3N4 as mask.

The KOH etch will erode the plane. Oxide is either deposited or grown to fill the V-grooves.

The base and emitter are formed on the large mesa and the collector on the small mesa.

To further reduce the area between adjacent mesa, trench isolation can be used, making use of trench etching.

The trench is typically 2m wide and 5m deep. The trench walls are oxidized and the remaining volume is filled with polysilicon.

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Double Poly Transistors

A further extension of the self-aligned BJT structure is to use

double polysilicon (n+ for emitter, p+ for base) to reduce the

area required for contacts.

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Example of BJT Specification Sheet

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How the BJT works Figure shows the energy

levels in an NPN transistor under no externally applying voltages.

In each of the N-type layers conduction can take place by the free movement of electrons in the conduction band.

In the P-type (filling) layer conduction can take place by the movement of the free holes in the valence band.

However, in the absence of any externally applied electric field, we find that depletion zones form at both PN-Junctions, so no charge wants to move from one layer to another.

NPN Bipolar Transistor

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How the BJT works What happens when we

apply a moderate voltage between the collector and base parts.

The polarity of the applied voltage is chosen to increase the force pulling the N-type electrons and P-type holes apart.

This widens the depletion zone between the collector and base and so no current will flow.

In effect we have reverse-biassed the Base-Collector diode junction.

Apply a Collector-Base voltage

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Charge Flow What happens when we apply a

relatively small Emitter-Base voltage whose polarity is designed to forward-bias the Emitter-Base junction.

This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary.

Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively-biassed Collector region.

As a result the electrons which get into the Base move swiftly towards the Collector and cross into the Collector region.

Hence a Emitter-Collector current magnitude is set by the chosen Emitter-Base voltage applied.

Hence an external current flowing in the circuit.

Apply an Emitter-Base voltage

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Charge Flow Some of free electrons crossing

the Base encounter a hole and

'drop into it'.

As a result, the Base region

loses one of its positive

charges (holes).

The Base potential would

become more negative

(because of the removal of the

holes) until it was negative

enough to repel any more

electrons from crossing the

Emitter-Base junction.

The current flow would then

stop. Some electron fall into a hole

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Charge Flow To prevent this happening we

use the applied E-B voltage to remove the captured electrons from the base and maintain the number of holes.

The effect, some of the electrons which enter the transistor via the Emitter emerging again from the Base rather than the Collector.

For most practical BJT only about 1% of the free electrons which try to cross Base region get caught in this way.

Hence a Base current, IB, which is typically around one hundred times smaller than the Emitter current, IE.

Some electron fall into a hole

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Terminals & Operations

Three terminals: Base (B): very thin and lightly doped central region (little

recombination).

Emitter (E) and collector (C) are two outer regions sandwiching B.

Normal operation (linear or active region): B-E junction forward biased; B-C junction reverse biased.

The emitter emits (injects) majority charge into base region and because the base very thin, most will ultimately reach the collector.

The emitter is highly doped while the collector is lightly doped.

The collector is usually at higher voltage than the emitter.

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Terminals & Operations

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Operation Mode

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Operation Mode

Active: Most importance mode, e.g. for amplifier operation.

The region where current curves are practically flat.

Saturation: Barrier potential of the junctions cancel each other out

causing a virtual short.

Ideal transistor behaves like a closed switch.

Cutoff: Current reduced to zero

Ideal transistor behaves like an open switch.

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Operation Mode

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BJT in Active Mode

Operation Forward bias of EBJ injects electrons from emitter into base

(small number of holes injected from base into emitter)

Most electrons shoot through the base into the collector across the reverse bias junction (think about band diagram)

Some electrons recombine with majority carrier in (P-type) base region

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Circuit Symbols

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Circuit Configuration

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Band Diagrams (In equilibrium)

No current flow

Back-to-back PN diodes

Ec

Ev

Ef

Emitter Base Collector

N P N

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Band Diagrams (Active Mode)

EBJ forward biased

Barrier reduced and so electrons diffuse into the base

Electrons get swept across the base into the collector

CBJ reverse biased

Electrons roll down the hill (high E-field)

Ec

Ev

Ef

Emitter Base Collector

N P N

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Minority Carrier Concentration Profiles

Current dominated by electrons from emitter to base (by design) b/c of the forward bias and minority carrier concentration gradient (diffusion) through the base

some recombination causes bowing of electron concentration (in the base)

base is designed to be fairly short (minimize recombination)

emitter is heavily (sometimes degenerately) doped and base is lightly doped

Drift currents are usually small and neglected

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Diffusion Current Through the Base

Diffusion of electrons through the base is set by concentration profile at the EBJ

Diffusion current of electrons through the base is (assuming an ideal straight line case):

Due to recombination in the base, the current at the EBJ and current at the CBJ are not equal and differ by a base current

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Collector Current

Electrons that diffuse across the base to the CBJ junction are swept across

the CBJ depletion region to the collector b/c of the higher potential applied

to the collector.

Note that iC is independent of vCB (potential bias across CBJ) ideally

Saturation current is

inversely proportional to W and directly proportional to AE

Want short base and large emitter area for high currents

dependent on temperature due to ni2 term

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Collector Current

Electrons that diffuse across the base to the CBJ junction are swept across

the CBJ depletion region to the collector b/c of the higher potential applied

to the collector.

Note that iC is independent of vCB (potential bias across CBJ) ideally

Saturation current is

inversely proportional to W and directly proportional to AE

Want short base and large emitter area for high currents

dependent on temperature due to ni2 term

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Collector Current

Electrons that diffuse across the base to the CBJ junction are swept across

the CBJ depletion region to the collector b/c of the higher potential applied

to the collector.

Note that iC is independent of vCB (potential bias across CBJ) ideally

Saturation current is

inversely proportional to W and directly proportional to AE

Want short base and large emitter area for high currents

dependent on temperature due to ni2 term

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Base Current

Base current iB composed of two components:

holes injected from the base region into the emitter region

holes supplied due to recombination in the base with diffusing electrons

and depends on minority carrier lifetime tb in the base

And the Q in the base is

So, current is

Total base current is

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Beta

Can relate iB and iC by the following equation

and b is

Beta is constant for a particular transistor

On the order of 100-200 in modern devices (but can be higher)

Called the common-emitter current gain

For high current gain, want small W, low NA, high ND

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Emitter Current

Emitter current is the sum of iC and iB

a is called the common-base current gain

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I-V Characteristics

Collector current vs. vCB shows the BJT looks like a

current source (ideally)

Plot only shows values where BCJ is reverse biased and so BJT

in active region

However, real BJTs have non-ideal effects

VCE

IC

VBE1

VBE2

VBE3

VBE3

> VBE2

> VBE1

VBE

IC

VCE

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I-V Characteristics

Base-emitter junction looks

like a forward biased diode

Collector-emitter is a family of

curves which are a function of

base current.

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I-V Characteristics

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Example:

Calculate the

values of and from the transistor

shown in the

previous

graphs.

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Early Effect

Early Effect

Current in active region depends (slightly) on vCE VA is a parameter for the BJT (50 to 100) and called the Early voltage

Due to a decrease in effective base width W as reverse bias increases

Account for Early effect with additional term in collector current equation

Nonzero slope means the output resistance is NOT infinite, but IC is collector current at the boundary of active region

VCE

VBE1

VBE2

VBE3

Active region

Saturation region

-VA

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Early Effect

What causes the Early Effect? Increasing VCB causes depletion region of CBJ to grow and

so the effective base width decreases (base-width modulation)

Shorter effective base width higher dn/dx

EBJ CBJ

dn/dxV

CB > V

CB

Wbase

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Common-emitter

It is called the common-emitter configuration because (ignoring the

power supply battery) both the signal source and the load share the

emitter lead as a common connection point.

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Common-collector

It is called the common-collector configuration because both the signal

source and the load share the collector lead as a common connection

point. Also called an emitter follower since its output is taken from the emitter resistor, is useful as an impedance matching device since its input impedance is

much higher than its output impedance.

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Common-base

This configuration is more complex than the other two, and is less

common due to its strange operating characteristics.

Used for high frequency applications because the base separates the

input and output, minimizing oscillations at high frequency. It has a high

voltage gain, relatively low input impedance and high output impedance

compared to the common collector.

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Collector Resistance, rC

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Emitter Resistance, rE

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Base Resistance, rB

Mainly effects small-signal and transient

responses.

Difficult to measure since it depends on bias

condition and is influenced by rE.

In the Ebers-Moll model (SPICEs default model for BJTs), rB is assumed to be

constant.

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Breakdown Voltages

The basic limitation of the max. voltage in a transistor is the same as that in a pn junction diode.

However, the voltage breakdown depends not only on the nature of the junction involved but also on the external circuit arrangement.

In Common Base configuration, the maximum voltage between the collector and base with the emitter open, BVCBO is determined by the avalanche breakdown voltage of the CBJ.

In Common Emitter configuration, the maximum voltage between the collect and emitter with the base open, BVCEO can be much smaller than BVCBO.

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Breakdown Voltages

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Breakdown Voltages

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Breakdown Voltages

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BJT Analysis

Here is a

common

emitter BJT

amplifier:

What are the

steps?

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Input & Output

We would want to know the collector current (iC), collector-emitter voltage (VCE), and the voltage across RC.

To get this we need to fine the base current (iB) and the base-emitter voltage (VBE).

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Input Equation

To start, lets write Kirchoffs voltage law (KVL) around the base circuit.

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Output Equation

Likewise, we can write KVL around the collector

circuit.

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Use Superposition:

DC & AC sources Note that both equations are written so as to calculate the

transistor parameters (i.e., base current, base-emitter voltage, collector current, and the collector-emitter voltage) for both the DC signal and the AC signal sources.

Use superposition, calculate the parameters for each separately, and add up the results:

First, the DC analysis to calculate the DC Q-point

Short Circuit any AC voltage sources

Open Circuit any AC current sources

Next, the AC analysis to calculate gains of the amplifier.

Depends on how we perform AC analysis

Graphical Method

Equivalent circuit method for small AC signals

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BJT - DC Analysis

Using KVL for the input and output circuits and the transistor characteristics, the following steps apply: 1. Draw the load lines on the transistor characteristics 2. For the input characteristics determine the Q point for

the input circuit from the intersection of the load line and the characteristic curve (Note that some transistor do not need an input characteristic curve.)

3. From the output characteristics, find the intersection of the load line and characteristic curve determined from the Q point found in step 2, determine the Q point for the output circuit.

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Base-Emitter Circuit Q point

The Load Line

intersects the

Base-emitter

characteristics

at VBEQ = 0.6 V

and IBQ = 20 A

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Collector-Emitter Circuit Q point

Now that we have

the Q-point for the

base circuit, lets proceed to the

collector circuit.

The Load Line intersects the Collector-emitter characteristic, iB = 20 A at

VCEQ = 5.9 V and ICQ = 2.5mA, then = 2.5m/20 = 125

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BJT DC Analysis - Summary

Calculating the Q-point for BJT is the first step in analyzing the circuit

To summarize: We ignored the AC (variable) source

Short circuit the voltage sources

Open Circuit the current sources

We applied KVL to the base-emitter circuit and using load line analysis on the base-emitter characteristics, we obtained the base current Q-point

We then applied KVL to the collector-emitter circuit and using load line analysis on the collector-emitter characteristics, we obtained the collector current and voltage Q-point

This process is also called DC Analysis

We now proceed to perform AC Analysis

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BJT - AC Analysis

How do we handle the variable source Vin(t) ?

When the variations of Vin(t) are large we will

use the base-emitter and collector-emitter

characteristics using a similar graphical

technique as we did for obtaining the Q-point.

When the variations of Vin(t) are small we will

shortly use a linear approach using the BJT

small signal equivalent circuit.

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BJT - AC Analysis

Lets assume that Vin(t) = 0.2 sin(t).

Then the voltage sources at the base vary from a maximum of 1.6 + 0.2 = 1.8 V to a minimum of 1.6 -0.2 = 1.4 V

We can then draw two load lines corresponding the maximum and minimum values of the input sources

The current intercepts then become for the: Maximum value: 1.8 / 50k = 36 A

Minimum value: 1.4 / 50k = 28 A

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AC Analysis Base-Emitter Circuit

From this graph, we find:

At Maximum Input Voltage:

VBE = 0.63 V, iB = 24 A

At Minimum Input Voltage:

VBE = 0.59 V, iB = 15 A

Recall: At Q-point:

VBE = 0.6 V, iB = 20 A

Note the asymmetry around the Q-

point of the Max and Min Values for

the base current and voltage which

is due to the non-linearity of the

base-emitter characteristics

imax = 24-20 = 4 A; iBmin = 20-15 = 5 A

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AC Analysis Base-Emitter Circuit

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AC Characteristics-Collector Circuit

Using these max and min values for the base current on the collect

circuit load line, we find:

At Max Input Voltage: VCE = 5 V, iC = 2.7mA

At Min Input Voltage: VCE = 7 V, iC = 1.9mA

Recall: At Q-point: VCE = 5.9 V, iB = 2.5ma

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AC Characteristics-Collector Circuit

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BJT AC Analysis - Amplifier Gains

From the values calculated from the base and

collector circuits we can calculate the amplifier gains:

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BJT AC Analysis - Summary

Once we complete DC analysis, we analyze the

circuit from an AC point of view.

AC analysis can be performed via a graphical

processes

Find the maximum and minimum values of the input

parameters (e.g., base current for a BJT)

Use the transistor characteristics to calculate the output

parameters (e.g., collector current for a BJT).

Calculate the gains for the amplifier

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The pnp Transistor

Basically, the pnp transistor is similar to the npn except the parameters have the opposite sign.

The collector and base currents flows out of the transistor; while the emitter current flows into the transistor

The base-emitter and collector-emitter voltages are negative

Otherwise the analysis is identical to the npn transistor.

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Current flow in a pnp transistor biased to operate in the active

mode.

The PNP Transistor

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The pnp Transistor

Two junctions

Collector-Base and Emitter-Base

Biasing

vBE Forward Biased

vCB Reverse Biased

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pn(0)

pn(x)

pno

WEB WBC

WB

npo

np(0)

E

IC

IE

IB

x

B

VCBVEB

np(x)

E

(b)

C

p+ n p

(a)

Emitter Base Collector

H o l edri f t

Recobi nat ion

Elec trons

ElectronDiffusion

Leakage current

IB

(d)

IC

IE

BE C

Hole diffusion

E

IC

IE

IB

VCB

VEB

Output

circuitInput

circuit

pnpE C

B(c)

(a) A schematic illustration of pnp BJT with 3 differently doped regions. (b)

The pnp bipolar operated under normal and active conditions. (c) The CB

configuration with input and output circuits identified. (d) The illustration of

various current component under normal and active conditions.

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Current flow in an pnp transistor biased to operate in the

active mode.

The pnp Transistor

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Two large-signal models for the pnp transistor operating in

the active mode.

The pnp Transistor

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