There are four diode ratings that apply in one way or another to all types of diodes and
applications:
1. Forward voltage drop VF : is the forward-conducting junction level ( 0.7 V for Si diodes
and 0.3 V for Ge diodes)1.
2. Average forward current IF : is the maximum amount of forward current that the diode
can carry for an indefinite period. If the average current exceeds this value, the diode
will overheat and, eventually, will be destroyed.
3. Peak reverse voltage VR, or reverse breakdown voltage. This is the largest amount of
reverse-bias voltage the diodes’s junction can withstand for an indefinite period of time.
If a reverse voltage exceeds this level, the voltage will punch through the depletion layer
and allow current to flow backwards through the diode, which is a destructive operation
(except for the case of a Zener diode).
4. Maximum power dissipation P. The actual diode power dissipation is calculated multiplying
the forward voltage drop and the forward current. Exceeding the maximum
power dissipation will result in thermal breakdown of the diode.
Excessive forward current and reverse breakdown voltage are the most common causes
of diode failure. In both cases the diode gets very hot, what destroys the pn junction. Occasional
peaks of voltage or current exceeding these rates for very short times (few milliseconds)
may not overheat the junction, but repeated peaks may fatigue the junction. By
design, diodes are selected with ratings that exceed two or three times the expected peaks
in the circuit.
