A diode typically has a high resistance in the reverse direction and does not allow electrons to flow in this reverse-biased mode. When the reverse voltage reaches a certain value, this resistance drops and the diode conducts in reverse-biased mode. The potential at which this occurs is called the breakdown voltage. The threshold voltage at which this occurs is known as the breakdown voltage or dielectric strength. In a gas, the breakdown voltage is determined by Paschen’s Law.
Paschen’s Law is an equation that gives the breakdown voltage as a function of atmospheric pressure and gap length.
Vb = Bpd/[ln(Apd) − ln[ln(1 + 1/γse)]]
Vb is the DC breakdown voltage
p is the pressure of the gas,
d is the gap distance in meters
A and B are constants that depend on the surrounding gas
γse is the secondary electron emission coefficient.