Reverse bias breakdown
We have found that a p-n junction biased in the reverse direction exhibits a small, essentially voltage-independent saturation current. This is true until a critical reverse bias is reached, for which reverse breakdown occurs.
Reverse biased junction:
- This is true until a critical reverse bias is reached, for which reverse breakdown occurs (Fig. given below).
At this critical voltage (Vbr) the reverse current through the diode increases sharply, and relatively large currents can flow with little further increase in
- The existence of a critical breakdown voltage introduces almost a right-angle appearance to the reverse characteristic of most diodes.
- There is nothing inherently destructive about reverse breakdown. If the current is limited to a reasonable value by the external circuit, the p-n junction can be operated in reverse breakdown as safely as in the forward-bias condition.
- For example, the maximum reverse current which can flow in the device of Fig. is (E — Vbr)/R; the series resistance R can be chosen to limit the current to a safe level for the particular diode used.
- If the current is not limited externally, the junction can be damaged by excessive reverse current, which overheats the device as the maximum power rating is exceeded.
- It is important to remember, however, that such destruction of the device is not necessarily due to mechanisms unique to reverse breakdown; similar results occur if the device passes excessive current in the forward direction.
- As we shall see, useful devices called breakdown diodes are designed to operate in the reverse breakdown regime of their characteristics.
- Reverse breakdown can occur by two mechanisms, each of which requires a critical electric field in the junction transition region.
- The first mechanism, called the Zener effect, is operative at low voltages (up to a few volts reverse bias).
- If the breakdown occurs at higher voltages (from a few volts to thousands of volts), the mechanism is avalanche breakdown.