A semiconductor diode conducts very differently under forward and reverse bias. An idealized exponential model is

$$I_D=I_S\left(e^{V_D/(nV_T)}-1\right),$$

where $I_S$ is a small scale current, $V_T$ is the thermal voltage and $n$ accounts for nonideal behavior.

For circuit analysis, simpler models are often more useful. The ideal diode is an open circuit when reverse biased and a short circuit when forward biased. A constant-voltage model treats a conducting silicon diode as having an approximately fixed forward drop.

The appropriate model depends on the question. Switching logic may need only on/off behavior, while precision analog analysis may require the nonlinear curve or a local small-signal approximation.

Reverse bias normally gives little current, but sufficiently large reverse voltage can cause breakdown. Ordinary diodes must be kept below destructive limits unless the device is designed to operate there, as in a Zener diode.