Differentiate between three segments of a transistor on the basis of their size and level of doping.

Question

(i) Differentiate between three segments of a transistor on the basis of their size and level of doping.

(ii) How is a transistor biased to be in active state ?

(iii) With the help of necessary circuit diagram, describe briefly how n-p-n transistor in CE configuration amplifies a small sinusoidal input voltage. Write the expression for the ac current gain.

Answer 1

(i) Emitter : It is of moderate size and heavily doped. 

Base : It is very thin and lightly doped.

Collector : It is moderately doped and larger in size.

(ii) Transistor is said to be in active state when its emitter-base junction is (suitably) reverse biased.

When a small sinusoidal voltage is superposed on the dc base biased, the base current will have sinusoidal variation superimposed on the value of I_E 

As a consequence, the collector current also will have sinusoidal variations, superimposed on the value of I_C , producing corresponding (amplified) changes in the value of V₀ 

ac current gain, β_ac = (ΔI_C/ΔI_B)_VCB