http://helios.snu.ac.kr p

Chapter 10.

BJT Fundamentals

Sung June Kim g

kimsj@snu.ac.kr

http://helios.snu.ac.kr p

Contents

‰ Terminology

‰ Electrostatics

‰ Introductory Operational Considerations

‰ Introductory Operational Considerations

‰ Performance Parameters

2

‰ Terminology

9 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

SMDL

Semiconductor Device Fundamentals BJT Fundamentals

9 All t i l t iti h th t i t i t d i th 9 All terminal currents are positive when the transistor is operated in the standard amplifying mode

9 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 C

I = I + I

0 ( )

V

+ V

+ V

= 0 ( V

= − V

) V + V + V = V = V

9 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 Forward

Saturation Forward Forward

Active Inverted

Forward

Reverse Forward

Reverse

Cutoff Reverse Reverse

9 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 concepts

for establishing operational principles and concepts

BJT fabrication

BJT fabrication

El t t ti

ƒ Electrostatics

9Two independent pn junctions

9Assuming the pnp transistor regions to be uniformly doped and taking g p p g y p g N_AE (E) >> N_DB (B) > N_AC (C)

W=quasineutral base width

‰ Introductory Operational Considerations

Carrier activity in a pnp BJT under active mode biasing

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

9The p⁺-n nature of the junction leads to many more holes being injected than electrons being injected

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

9The accelerating electric field in the C-B depletion region rapidly sweeps these carriers into the collector

9 I th h l t i j t d i t th b I th l t t 9 I_Ep: the hole current injected into the base, I_En: the electron current injected into the emitter, I_Cp: a current almost exclusively resulting from the injected holes that successfully cross the base, Ij y _CnCn: a current from the minority carrier electrons in the collector that wander into the C-B depletion region and are swept into the base

9 Very few of the injected holes are lost by recombination in the base Æ 9 Very few of the injected holes are lost by recombination in the base Æ I_Cp ≈ I_Ep

I

I

+ I

I = I + I

C Cp Cn

I = I

+ I

I = I

En

E

I

I

>>

B1 En

I = I

B3 Cn

I = I

E C

Cn C

I I

I I

≅

∴

>>

B2

recombination current in B

I =

E C

9 d t i I /I h I i l t t i BJT 9 d.c. current gain: I_C/I_B, where I_B is an electron current in a pnp BJT and I_C is predominantly a hole current

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

9 Control of the larger I_C by the smaller I_B is made possible

Bipolar Junction Transistor (BJT) p ( )

• Current components

• Current components

‰ P f P t

‰ Performance Parameters

• Emitter Efficiency

0 ≤ ≤ γ 1

P P

E E E E

E

I I

I I

I

= + γ =

9 Current gain is maximized by making γ as close as possible to unity

P En

E E

• Base Transport Factor

9 The 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

0 ≤ α

≤ 1

p p

E C

T

I

= I α

9 Maximum amplification occurs when α_T is as close as possible to unity

• Common Base d.c. Current Gain

9 When connected in the common base configuration,

I I

I α +

where is α_dc the common base d.c. current gain and I_CB0 is the collector current that flows when I_E=0

0 CB E

dc

C

I I

I = α +

Cp T Ep T E

I = α I = γα I

I I + I γα I + I

E

C Cp Cn T E Cn

I = I + I = γα I + I

dc T

α = γα 0 ≤ α

≤ 1

CBO Cn

I = I

• Common Emitter d.c. Current Gain

9 When connected in the common emitter configuration,

I I

I

β

I

I

I = β +

where is β_dc the common emitter d.c. current gain and I_CE0 is the collector current that flows when I_B=0

) ( I I I

I + + _Q I

= α

I

+ I

,

9 Rearranging and solving for I_C,

)

(

dc

C

I I I

I = α + +

B C

E

CB E

dc C

I I

I = +

0

,

dc CB B

dc dc C

I I

I α α

α

+ −

= −

1 1

0

dc dc

1

β α

= α

−

dc dc

>> 1 β

dc

CB CE

I I

α

= − 1

0 0

C d

β = I

α

− 1

dc

I

β

(ref)Evaluation of the terminal currents

(ref)Evaluation of the terminal currents