302 Chapter 9
control the drain current and the device is defunct. Because it is theoretically possible for
an inadvertent electrostatic charge, such as might arise with respect to the ground if a user
were to wear nylon or polyester fabric clothing and well-insulated shoes, it is common
practice in the case of small-signal MOSFETs for protective diodes to be formed on the
chip at the time of manufacture. These could be either zener diodes or simple junction
diodes connected between the gate and the source or the source and drain, as shown in
Figure 9.19.
In power MOSFETs, these protective devices are seldom incorporated into the chip.
There are two reasons for this: (1) that the effective gate/channel area is so large that the
associated capacitance is high, which would then require a relatively large inadvertently
applied static charge to generate a destructive gate/channel voltage (typically 40 V), and
(2) that such protective diodes could, if they were triggered into conduction, cause the
MOSFET to act as a four-layer thyristor and become an effective electrical short circuit.
However, there are usually no performance penalties that will be incurred by connecting
some external protective zener diode in the circuit to prevent the gate/source or gate/drain
voltage exceeding some safe value; this is a common feature in the output stages of audio
power amplifi ers using MOSFETs.
Apart from the possibility of gate breakdown, which, in power MOSFETs, always occurs
at less than the maker’s quoted voltage, except at zero drain current, MOSFETs are quite
robust devices, and the safe operating area rating (SOAR) curve of these devices, shown
for a typical MOSFET in Figure 9.20 , is free from the threat of secondary breakdown
whose limits are shown, for a power BJT, in Figure 9.12. The reason for this freedom
DQ 1SGFigure 9.19 : Diode gate protection.