Reverse Bias
When a p-n junction is reverse biased the barrier height will increase from V0 to V0+V.
Under these conditions the thermal or minority carriers diffuse through the material and cross the barrier as in the zero field case - they are unaffected.
However when the majority carriers reach the junction they face a higher barrier. As V increases under reverse bias the barrier height increases and the number of majority carriers with sufficient energy to cross the junction falls rapidly to zero.
Eventually the reverse current saturates at a value due only to the passage of minority or thermally excited carriers. The current voltage characteristic for a pn junction under reverse bias is given by
Thus as V increases under reverse bias IR>I0 , where I0 is the reverse saturation current and IR is the reverse bias current and V the reverse bias voltage on the device.
Forward Bias
When the pn junction is forward biased the barrier height at the junction will be lowered from V0 to V0-V. As before the minority carriers are unaffected by forward biasing of the junction.
When the majority carriers reach the junction they now face a reduced energy barrier V0-V. More of them can now pass over the barrier and cross the junction so the current increases exponentially as V increases.
