Forward characteristic of rectifier diode varies according to current level and temperature.
At low-current region, VF is low at high temperature, and the opposite is true at high-current region.
In general, the diode should be used with a sufficient temperature margin below the Q point, which is the cross point of the above two conditions.
(1)Blue area where carrier mobility is dominant: VF decreases as temperature increases.
Since the carrier moves easily when it gets hot, VF is lower than at low temperature.
(2)Red area where carrier collision dominates: VF increases as temperature increases.
When a large current flows, a lot of carriers move. In the case of high temperature, the probability of collision between carriers increases and VF becomes higher than at low temperature.