Biasing the junction

Forward bias:

• the p-region is connected to the positive terminal of a battery and the n-region to the negative terminal.
• The field due to the battery opposes the internal field; it narrows and weakens the barrier.
• Majority carriers can cross the junction in large numbers, and a large current flows (it rises rapidly once the applied voltage exceeds about 0.7 V for silicon).

Reverse bias:

• the battery is reversed (p to negative, n to positive).
• The battery field adds to the internal field, widens the transition region and strengthens the barrier.
• Only a tiny current of minority carriers flows (almost zero, the reverse saturation current).
• Beyond a high reverse voltage the junction breaks down.

The behaviour is shown by the current-voltage (I-V) characteristic: practically zero current for negative voltage, a very small current until about 0.7 V, then rapid increase for positive voltage.