1-1 Reverse breakdown voltage

The reverse breakdown voltage is determined by Zener breakdown or avalanche breakdown.

Zener breakdown

When a pn junction is reverse-biased, a depletion layer extends across the pn junction. An electric field causes a gap between the valence band of the p-type region and the conduction band of the n-type region to decrease inside the depletion layer. As a result, electrons tunnel from the valence band of the p-type region to the conduction band of the n-type region because of quantum tunneling. Zener breakdown is a phenomenon in which the tunneling of electrons across the depletion region causes a sudden increase in reverse current. Figure 1.3 illustrates Zener breakdown.

Avalanche breakdown

When a pn junction is reverse-biased, a small quantity of electrons passes through the pn junction. These electrons are accelerated in the depletion layer by an electric field, acquiring large kinetic energy. The accelerated electrons collide with the atoms in a crystal lattice, ionizing them and creating electron holes. The electrons of these atoms are excited to the conduction band and knocked out, becoming free electrons. The free electrons are also accelerated and collide with other atoms, creating more electron-hole pairs and leading to further knocking-out processes. This phenomenon is called avalanche breakdown.

Comparison of avalanche breakdown and Zener breakdown

Since diodes with high breakdown voltage are lightly doped, they form a wide depletion layer (forbidden band). Conversely, since diodes with low breakdown voltage are heavily doped, they form a thin depletion layer (forbidden band). When a diode has a wide depletion layer, electron tunneling (Zener breakdown) is less likely to occur, causing avalanche breakdown to become more predominant. In the case of a heavily doped diode with a thin depletion layer, Zener breakdown is more like to occur. As temperature increases, the width of the forbidden band (E_g) decreases, contributing to the Zener effect. In addition, as temperature increases, the lattice vibration of a semiconductor increases, causing its carrier mobility to decrease. As a result, avalanche breakdown becomes less likely to occur. Zener breakdown voltage decreases with temperature whereas avalanche breakdown voltage increases with temperature. Generally, Zener breakdown is predominant below roughly 6 V whereas avalanche breakdown is predominant above roughly 6 V. It should be noted that even diodes of the same product series exhibit different temperature characteristics.