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Chapter 10. BJT Fundamentals

Chapter 10.

BJT Fundamentals

Sung June Kimgkimsj@snu.ac.kr

http://helios.snu.ac.krp

Contents

Terminology

Electrostatics

Introductory Operational ConsiderationsIntroductory Operational Considerations

Performance Parameters

Terminology

The BJT is a device containing three adjoining, alternately doped regions, with the middle region being very narrow compared to the diffusion length

heavy doping heavy doping

SMDLSemiconductor Device Fundamentals BJT Fundamentals

All t i l t iti h th t i t i t d i thAll terminal currents are positive when the transistor is operated in the standard amplifying mode

The current flowing into a device must be equal to the current flowing out, and voltage drop around a closed loop must be equal to zero

E B CI I I= +

0 ( )V V V V V+ + = = −EB BC CE CE EC0 ( )V V V V V+ + = =

The basic circuit configurations in which the device is connected

The most widely employed configuration

Seldom used

co gu at o

Biasing Mode

Biasing Polarity E-B Junction

Biasing Polarity C-B Junction

Saturation Forward ForwardSaturation Forward Forward

ActiveInverted

ForwardReverse Forward

Reverse

Cutoff Reverse Reverse

Although the npn BJT is used in a far greater number of circuit applications and IC designs, the pnp BJT is a more convenient vehicle for establishing operational principles and conceptsfor establishing operational principles and concepts

BJT fabrication

El t t tiElectrostaticsTwo independent pn junctions Assuming the pnp transistor regions to be uniformly doped and taking g p p g y p g

NAE (E) >> NDB (B) > NAC (C)

W=quasineutral base width

Introductory Operational Considerations

Carrier activity in a pnp BJT under active mode biasing

The primary carrier activity in the vicinity of the forward-biased E-B junction is majority carrier injection across the junction

The p+-n nature of the junction leads to many more holes being injected than electrons being injected

The vast majority of holes diffuse completely through the quasineutral base and enter the C-B depletion regionp g

The accelerating electric field in the C-B depletion region rapidly sweeps these carriers into the collector

I th h l t i j t d i t th b I th l t tIEp: the hole current injected into the base, IEn: the electron current injected into the emitter, ICp: a current almost exclusively resulting from the injected holes that successfully cross the base, ICn: a current from j y Cn

the minority carrier electrons in the collector that wander into the C-B depletion region and are swept into the base

Very few of the injected holes are lost by recombination in the baseVery few of the injected holes are lost by recombination in the base ICp ≈ IEp

I I I+E Ep EnI I I= +

C Cp CnI I I= +p

I I=EnE II

B1 EnI I=

B3 CnI I=EC

≅∴

B2 recombination current in BI =EC

d t i I /I h I i l t t i BJTd.c. current gain: IC/IB, where IB is an electron current in a pnp BJT and IC is predominantly a hole current

Schematic visualization of amplification in a pnp BJT under active mode biasing

Control of the larger IC by the smaller IB is made possible

Bipolar Junction Transistor (BJT)p ( )

• Current components• Current components

P f P tPerformance Parameters

• Emitter Efficiency

0 1γ≤ ≤PP

Current gain is maximized by making γ as close as possible to unitynP EEE

• Base Transport FactorThe fraction of the minority carriers injected into the base thatThe fraction of the minority carriers injected into the base that

successfully diffuse across the quasineutral width of the base and enter the collector

T0 1α≤ ≤p

Maximum amplification occurs when αT is as close as possible to unity

• Common Base d.c. Current GainWhen connected in the common base configuration,

III +αwhere is αdc the common base d.c. current gain and ICB0 is the collector current that flows when IE=0

0CBEdcC III += α

Cp T Ep T EI I Iα γα= =

I I I I Iγα+ +

C Cp Cn T E CnI I I I Iγα= + = +

dc Tα γα= dc0 1α≤ ≤

CBO CnI I=

• Common Emitter d.c. Current GainWhen connected in the common emitter configuration,

III β 0CEBdcC III += β

where is βdc the common emitter d.c. current gain and ICE0 is the collector current that flows when IB=0

)( IIII ++ CBEdcC III += ,0αQ

Rearranging and solving for IC,

0)( CBBCdcC IIII ++= αBCE

CBEdcC

III +=,0

IIIαα

dc1αβ

1>>dcβdc

IIα−

Iβ = If I is negligible compared to I

dcα−1

β If ICE0 is negligible compared to IC

(ref)Evaluation of the terminal currents(ref)Evaluation of the terminal currents

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