1

Bipolar JunctionTransistors (BJTs)

Electronics Concept

THE PAST• 1904 Flemming

– invented a value-Diode– Cathode & Anode – Positive voltages at Anode; current flows– Negative voltages at Anode; No current flows– Acted as Detector

• 1906 De Forset– Put a third electrode in between and small change in voltage on

the grid resulted in a large plate voltage change.– Acted as an amplifier

• Vacuum Tubes not reliable

Solid State Components• December 1947

– Closely space two gold-wires probes were pressed into the surface of a germanium crystal and amplification of input voltages experienced.

– Low gain, low bandwidth and Noisy

• 1947 - 1950– Junction Transistor invented where the operation depended upon

diffusion instead of conduction current.– Had charged carrier of both polarities -- Electron and Holes

• 1951 – Solid state transistors were produced commercially.

• Transistor characteristics vary greatly with changes in temperature. Germanium had excessive variations above 750 C, thus Silicon transistor were invented as Silicon Transistors could be used up to 2000 C

Semi Conductor Concept

Solid State Physics

The Integrated Circuit• 1958

– Kilby conceived the monolithic idea – Building entire circuit out of Germanium or Silicon

– Phase-Shift Oscillator, Multivibrator were build from Germanium with thermally bonded gold connecting wires.

– Noyce manufactured multiple devices on a single piece of Silicon and was able to reduced the size, weight and cost per active element.

Technological Advances

• 1960 - Small Scale Integration (SSI) – less than100 components per chip

• 1966 - Medium Scale Integration (MSI) – More than 100 less than 1000 components per chip

• 1969 - Large Scale Integration (LSI) – More than 1000 less than 10,000 components per chip

• 1975 - Very Large Scale Integration (VLSI)– More than 10,000 components per chip

INTRODUCTION - BJT• Three terminal device

• Basic Principle– Voltage between two terminals controls current flowing in the third

terminal.

• Device is used in discrete and integrated circuits and can act as :– Amplifier

– Logic Gates

– Memory Circuits

– Switches

• Invented in 1948 at Bell Telephone Industries

• MOSFET has taken over BJT since 1970’s for designing of integrated circuits but still BJT performance under sever environment is much better than MOSFET e.g. Automotive Electronics

• BJT is used in – Very high frequency applications (Wireless Comm)– Very high speed digital logics circuit (Emitter Coupled

Logic)

• Innovative circuit combine MOSFET being high-input resistance and low power operating devices with BJT merits of being high current handling capacity and very high frequency operation – known as BiMOS or BiCMOS

INTRODUCTION

• Study would include

– Physical operation of BJT– Terminal Characteristics– Circuit Models– Analysis and design of transistor circuits

INTRODUCTION

A simplified structure of the npn transistor.

Device Structure & Physical Operation• npn & pnp Transistor

• Three terminal ---- Emitter, Base, Collector

• Consists of two pn junctions– np-pn -------- npn– pn-np -------- pnp

• Modes– Cutoff, Active, Saturation, Reverse Active

• Junctions – Emitter Base Junction (EBJ)– Collector-Base Junction (CBJ)

TWO EXAMPLES OF DIFFERENT SHAPES OF TRANSISTOR

A simplified structure of the npn transistor.

Current flow in an npn transistor biased to operate in the active mode.

Notation Summarized

Notation

Base (collector) Voltage with respect to Emitter

Base (collector) Current toward electrode from external circuit

Instantaneous Total Value (DC + AC) vB (vC) iB (iC)

Quiescent Value (DC) VB (VC) IB (IC)Instantaneous Value of varying component (AC) vb (vc) ib (ic)Effective Value of varying components Vb (Vc) Ib (Ic)

Supply Voltage (Magnitude) VBB (VCC)  

npn Transistor

• Current in Forward biased junction in active mode:

– Emitter Current (IE) flows out of the emitter

– Base Current (IB) flows into the Base

– Collector Current (IC) flows into the Collector

• Majority carriers are electrons as emitter junction is heavily doped and is wider than the base junction, and base junction being lightly doped and has smaller area.

Current Flow• EBJ – Forward Biased, CBJ – Reversed Biased

• Only diffusion current is considered as drift current due to thermally generated minority carriers is very small.

• Current components across the EBJ are due to:

– Electrons injected from the emitter into the base• This current component is at higher level due to heavily doped emitter• High density of electrons in emitter

– Holes injected from the base into the emitter• This current component is small due to lightly doped base• Low density of holes in base

Profiles of minority-carrier concentrations in

the base and in the emitter of an npn transistor

operating in the active mode: vBE > 0 and vCB ³ 0.

IC is independent of VCE, as long as the CBJ is reversed biased – collector is positive w.r.t base

• IS (Saturation Current) is– Inversely proportional to the base width– Directly proportional to the area of the EBJ– Typical range 10 -12 ---- 10 -18 A – Varies with changes in temperature (Doubles @

every 50 C rise in temperature)

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iB=iC/β• β (Beta) Common emitter current gain

– Ranges from 50 –200– Constant for a particular transistor– Influenced by :

• Width of the base junction (W)• Relative doping of base region w.r.t. emitter region

• α (Common – Base Current Gain) is – constant for a particular transistor– Less or close to unity

• Small change in α corresponds to a very large change in β

npn Transistor : Emitter Current

Active Mode Parameters npn Transistor

Large-signal equivalent-circuit models of the npn BJT

operating in the forward active mode.

Common parametersnpn & pnp BJT

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Cross-section of an npn BJT.

Model for the npn transistor when operated in the reverse active mode (i.e., with the CBJ forward biased and the EBJ reverse biased).

Equivalent Circuit model

•vBE – forward biased EBJ causing an exponentially related current iC to flow

• iC is independent of value of the collector voltage as long as CBJ is reversed biased.

vCB ≥ 0 V

• Collector terminal behaves as an ideal constant current

source and its value is determined by vBE

iC = αiE

The iC –vCB characteristic of an npn transistor fed with a constant emitter current IE. The transistor enters the saturation mode of operation for vCB –0.4 V, and the collector current diminishes.

Current flow in a pnp transistor biased

to operate in the active mode.

Large-signal model for the pnp transistor

operating in the active mode.

Current flow in a pnp & npn transistor biased to operate in the active mode.

Large-signal model for the pnp & npn transistor

operating in the active mode.

Circuit symbols for BJTs.

Active Mode Parameters pnp Transistor

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Figure 5.17 Effect of temperature on the iC–vBE characteristic. At a constant emitter current (broken line), vBE changes by –2 mV/C.

The iC–vCB characteristics of an npn transistor.

Common Base Characteristics • iC – vCB for various iE

• Base at constant voltage – grounded thus acts a common terminal potential

• Curve is not horizontal straight line but has a small positive slop.

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The Early Effect

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– (b) Characteristics are not perfectly horizontal line iC - vCE

Figure 5.19 (a) Conceptual circuit for measuring the iC –vCE characteristics of the BJT. (b) The iC –vCE characteristics of a practical BJT.

Common Emitter Configuration

• Emitter serves as a common terminal between input and output terminal

• Common Emitter Characteristics (ic-vCE)

can be obtained at different value of vBE

and varying vCE (dc), Collector current can be measured

• vCE < - 0.4 V CBJ become forward biased & BJT leaves active mode & enters saturation mode

• Characteristics is still a straight line but with a finite slope

• when extra-polated, the characteristics lines meet at a

point on the negative vCE axis @ vCE = -VA

• Typical value of VA ranges 50-100v & called early voltage, after the name of english scientist JM Early

The Early Effect

• At given vBE , increasing vCE increases reverse biased voltage on CBJ & thus depletion region increases, Resulting in a decrease in the effective base width W

– Is is inversely proportional to the base width

– Is increases , and Ic also increases proportionally

– called Early Effect

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Figure 5.20 Large-signal equivalent-circuit models of an npn BJT operating in the active mode in the common-emitter configuration.

Table 5.3 Symbols & Large Signal Model

Table 5.3 Symbols & Large Signal Model

npn Transistor

pnp Transistor

Common Emitter Configuration

Figure 5.27 Circuit whose operation is to be analyzed graphically.

Little Practical Value

Figure 5.28 Graphical construction for the determination of the dc base current

Figure 5.29 Graphical construction for determining the dc collector current IC and the collector-to-emitter voltage VCE

Figure 5.30 Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB

Figure 5.30 Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB

Figure 5.32 A simple circuit used to illustrate the different modes of operation of the BJT.

Operation as a Switch

• Cutoff and saturation modes of operation

• vi less than 0.5 V the transistor is in cutoff mode,

iB =o, iC=o, vC= VCC

• vi greater than 0.5 V (≈ 0.7V), the transistor conducts

iB=(vi-VBE)/RB

iC= βiB

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Biasing of BJT Amplifier circuit

• Biasing to establish constant DC Collector current Ic & should be

• Calculatable• Predictable• Insensitive to temp. variations• Insensitive to large variations in β

– To allow max. output signal swing with no distortion

Figure 5.43 Two obvious schemes for biasing the BJT: (a) by fixing VBE; (b) by fixing IB.

Figure 5.44 Classical biasing for BJTs using a single power supply:

• Typical Biasing – Single power supply– Voltage Divider Network

– RE in Emitter Circuit

Typical Biasing

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A BJT biased using a constant-current source I.

Biasing using a constant current source

• Current in Emitter means

– Constant IC IC =α IE

– Independent of RB & β value thus RB can be made large to

• Increase Input resistance• Large signal swing at collector

•Q1 acts as Diode CBJ is short circuits

Biasing using a constant current source

Q1 acts as Diode CBJ is short circuits

VCC-IREFR-VBE+VEE=0

I = IREF=(VCC-VBE+VEE)/R

Since Q1 & Q2 have VBE is same

I constant till Q2 in Active Mode (Region) & can be guaranteed by

–Voltage at collector V > (-VEE+VBE)

Current Mirror

• IE is independent of β & RB

• RB can be made large thus increasing input resistance

• Simple Design

• Q1 & Q2 are matched pair

• Q1 is Diode collector- Base connected

• β high IB can be neglected α = 1 IC = IE

Biasing using a constant current source

I = IREF=(VCC-VBE+VEE)/R