ETE444 :: Lec15

Dr. Mashiur Rahman

Copyright: Semiconductor Physics And Devices 3rd ed. - J. Neamen

Forward-active mode

Injection and collection of electrons

E

c

i

i

α = Common-Base current gainβ = Common-Emitter gain

iE1= due to the flow of electrons injected into the base = ic

B

c

i

i

Four operation mode of bipolar transistor

n

Cuttoff mode

VCB> 0VBE < 0

Minority carrier distribution

B-E = reverse biased Mejority carrier electrons from the emitter will not injected into the bias

Forward active mode

VCB> 0VBE > 0

Minority carrier distribution

The B-E junction becomes forward biased, an emitter current will be generated and the injection of electrons into the base results in a collector current

Inverse active

VCB < 0VBE < 0

Minority carrier distribution

The B-E forward biased and C-B

Saturation mode

VCB< 0VBE > 0

Minority carrier distribution

Both B-E and B-C junctions are forward biased and the collector current no longer controlled by the B-E voltage.

Application

• Voltage amplifier by Bipolar Transistors

• Switching

Voltage amplifier by Bipolar Transistors

Input sinusoidal signal voltage

Sinusoidal collector currents

Sinusoidal base currents

Sinusoidal voltage across the Rc resistor

Switching

Circuit used for transistor switching

Delay time

Raise time Storage time

Fall time

Switching :: Important parameter

• Current gain

– lower doping in the base region is used.

• Switching time

– The transistor can be doped with gold to introduce midgap recombination centers and thus reduce the switching time.