Page 60 - Power Electronics Handbook

Voltage-reference. diodes 53

I Anode

Gate
(optional)
(a) I Cathode
Cathode

rse biased
ion

(b) Anobe
Figure 1.30 The photothyristor: (a) symbol; (b) construction

1.13 Voltage-reference diodes

Voltage-reference diodes are two-layer devices which are used primarily
for providing a stable voltage reference, or for overvoltage surge
suppression. The device is capable of passing a large current whilst
maintaining a high voltage across it, hence it dissipates considerable
amount of power, placing it in the category of a power semiconductor.
Figure 1.31(a) shows the symbol for a voltage-reference diode. In the
forward direction it behaves like an ordinary diode, whilst in the reverse
direction it functions as a reference diode.
The operation of the voltage-reference diode is based on two distinct
effects, zener and avalanche, although these devices are often referred to
collectively as zener diodes. If the p and n regions of the diode are heavily
doped, then the depletion region between them will be narrow. Applying a
low voltage in the reverse direction across the device will now caw a high
electric field to be formed across the junction, given by the ratio of this
voltage to the depth of the depktion layer. When this field exceeds a
critid value, equal to 3 x 10-~ ~cm, electrons will gain sufficient energy
to break away from their bonds (see figure 1.5) causing a large cumnt to
flow. This is known as zener breakdown and the characteristic is shown in
Figure 1.31@). The zener breakdown occurs below about 5 V and has a
fairly gentle knee in its characteristic curve.
If the p and n layers of the diode are lightly doped, then they will have a
wider depletion layer between them, so that the critical field strength for
the zener effect cannot be reached, even under relatively large reverse