Diode: Reverse bias

A diode is reverse-biased when the positive terminal connects to the n-type region and the negative terminal to the p-type region. This configuration widens the depletion region and increases the barrier potential, opposing natural current flow. The result is minimal leakage current, primarily from minority charge carriers, producing a nearly flat I-V curve at lower reverse voltages.

When reverse voltage exceeds the breakdown voltage threshold, a steep current rise occurs. In heavily doped diodes, strong electric fields break valence electrons free, creating electron-hole pairs. In lightly doped diodes, accelerated minority carriers collide with atoms, triggering a cascading process that rapidly increases current with minimal voltage change.

Leakage current in reverse bias originates from thermal carrier generation within the junction and increases slightly with reverse voltage. However, these changes are too small to noticeably affect the I-V curve. Actual reverse currents often exceed predicted saturation currents; for example, small-signal diodes with femtoampere-level saturation currents may exhibit nanoampere-level reverse currents.

Heavily doped diodes exhibit abrupt breakdown due to strong electric fields that break valence electron bonds at high reverse voltages. Lightly doped diodes show gradual current increase because minority carriers accelerate more slowly across the wider depletion region, creating electron-hole pairs through collision in a cascading process rather than direct impact ionization.

Diode breakdown is not inherently damaging if reverse current remains within the safe operating area defined by maximum power dissipation capacity in the datasheet. External circuitry must limit reverse current to safe levels. Zener diodes exemplify devices engineered to operate safely within the breakdown region for voltage regulation applications.

Forward bias produces exponential current increase with voltage, while reverse bias shows negligible current increase until breakdown occurs. The reverse bias I-V curve remains nearly flat at lower voltages due to minimal leakage current from minority carriers. At breakdown voltage, a sharp knee appears on the curve, representing a steep current rise with minimal voltage variation.

Reverse bias widens the depletion region and increases the barrier potential, restricting charge carrier movement and minimizing current flow. The strong electric field across the depletion region at high reverse voltages enables impact ionization, where accelerated carriers collide with atoms to create electron-hole pairs. This field strength determines whether breakdown occurs through direct ionization or carrier collision cascades.