Lecture7 bipolar junction transistor

– Emitter current is comprised almost entirely of carriers injected into the base rather than carriers injected into the emitter... Carrier Transport in the Base Region• Since the width

Lecture 7: Bipolar Junction Transistor

+

– Emitter current is comprised almost entirely of

carriers injected into the base (rather than carriers injected into the emitter).

Carrier Transport in the Base Region

• Since the width of the

quasi-neutral base region (WB = x2-x1) is

much smaller than the

minority-carrier diffusion length, very few

of the carriers injected (from the

emitter) into the base recombine

before they reach the

collector-junction depletion region.

 Minority-carrier diffusion current is

~constant in the quasi-neutral base

• The minority-carrier

concentration at the edges of the

collector-junction depletion

region are ~0

The B-E junction is forward-biased so electrons from the emitter are injected across the

B-E junction into the base These injected

electrons create an excess concentration

of minority carriers in the base The B-C

junction is reverse biased, so the minority

carrier electron concentration at the edge of the B-C junction is ideally zero We expect

the electron concentration in the base to be like that shown in beside figure

The large gradient in the electron

concentration means that electrons injected from the emitter will diffuse across the base region into the B-C space charge region

where the electric field will sweep the

electrons into the collector We want as manyelectrons as possible to reach the collector without recombining with any majority

carrier holes in the base For this reason, the width of the base needs to be small

compared with the minority carrier diffusion length

Diffusion current reminder

Collector Current

• The equation above shows that the BJT is

indeed a voltage-dependent current source; thus it can be used as an amplifier.

B 2

Emitter Current

• Applying Kirchhoff’s Current Law to the BJT,

we can easily find the emitter current

C B

C

E I I I I

Summary of BJT Currents

1

exp 1

exp 1

T

BE S

E

T

BE S

B

T

BE S

C

V

V I

I

V

V I

I

V

V I

I

decreases