BJT Fundamentals

BJT Fundamentals

Sung June Kim

kimsj@snu.ac.kr

http://helios.snu.ac.kr

Chapter 10.

Contents

q Drift

q Diffusion

q Generation-Recombination q Equations of State

2

q Terminology

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

SMDL Semiconductor Device Fundamentals BJT Fundamentals

heavy doping heavy doping

ü All terminal currents are positive when the transistor is operated in the standard amplifying mode

ü 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

EB BC CE

0 (

CE EC

)

V + V + V = V = - V

ü The basic circuit configuration in which the device is connected and the biasing mode

The most widely employed

configuration

Seldom used

Biasing Mode

Biasing Polarity E-B Junction

Biasing Polarity C-B Junction

Saturation Forward Forward

Active

Inverted Cutoff

Forward

Reverse Reverse

Forward Reverse

Reverse

ü 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

§ Electrostatics

üTwo independent pn junctions

üAssuming the pnp transistor regions to be uniformly doped and taking N_AE (E) >> N_DB (B) > N_AC (C)

W=quasineutral base width

q Introductory Operational Considerations

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

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

Carrier activity in a pnp BJT under active mode biasing

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

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

ü 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, I_Cn: a current from the minority carrier electrons in the collector that wander into the C-B depletion region and are swept into the base

ü Very few of the injected holes are lost by recombination in the base à I_Cp » I_Ep

E Ep En

I = I + I

C Cp Cn

I = I + I

B1 En

I = I

B3 Cn

I = I

B2

recombination current in B

I =

E C

Cn C

En E

I I

I I

I I

P P

@

\

>>

>>

ü 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

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

q Performance Parameters

• Emitter Efficiency

0 £ g £ 1

ü Current gain is maximized by making g as close as possible to unity

• Base Transport Factor

ü The fraction of the minority carriers injected into the base that

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

0 £ a

T

£ 1

ü Maximum amplification occurs when a_T is as close as possible to unity

P n P P

E E

E E

E

I I

I I

I

= + g =

p p

E C

T

I

= I

a

• Common Base d.c. Current Gain

ü When connected in the common base configuration,

Cp T Ep T E

I = a I = ga I

C Cp Cn T E Cn

I = I + I = ga I + I

dc T

a = ga

CBO Cn

I = I

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

0 £ a

dc

£ 1

• Common Emitter d.c. Current Gain

ü When connected in the common emitter configuration,

0 CB E

dc

C

I I

I = a +

0 CE B

dc

C

I I

I = b +

dc dc

1

dc

b a

= a -

C dc

B

I b = I

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

ü Rearranging and solving for I_C,

If I_CE0 is negligible compared to I_C

)

0

(

_{C E CE}

dc

C

I I I

I = a + +

B C

E

CB E

dc C

I I

I

I I

I

+

=

+

= a

₀

,

Q

dc CB B

dc dc C

I I

I a a

a

+ -

= -

1 1

0

1

>>

b

dc

dc CB CE

I I

a

= - 1

0 0

17

Summary

18