Early effect (or) Base-width modulation:-
In Common Base configuration in the Reverse Bias, As the voltage increases, the space-charge width between collector and base tends to increase, with the result that the effective width of the base decreases. This dependency of Base-width on the Collector to emitter voltage is known as the early effect.
The early effect has three consequences:-
- There is less chance for recombination with in the base region. Hence increases with increasing .
- The charge gradient is increased with in the base and consequently, the current of minority carriers injected across the emitter junction increases.
- For extremely large voltages, the effective Base-width may be reduced to zero, causing voltage break-down in the transistor. This phenomenon is called the Punch-through.
For higher values of , due to early effect the value of increases, for example changes say from 0.98 to 0.985. Hence there is a very small positive slope in the CB output characteristics and hence the output resistance is not zero.