Metal Oxide Semiconductor Field Effect Transistors
The BJT and JFET have a diode in their input circuit which controls their mode of operation. The metal oxide semiconductor field effect transistor (MOSFET) works on a similar principle, but the diode is buried within the MOSFET. The MOSFET input diode is controlled by an electric field in the gate region, thus the input impedance is always extremely high because there is no forward biased diode to lower the input impedance. The input impedance of MOSFETs is so high that
there is no mechanism that readily bleeds off the accumulated charge except for humidity, thus they are often packaged with lead shorting wires to drain the charge. The lead shorting devices protect the MOSFETs from charge buildup and the subsequent catastrophic discharge current. All semiconductor devices should be protected from static discharge, but MOSFETs are the
most liable to build up a killing charge. Do not be lax with static protection because some sensitive BJTs are affected by only a few hundred volts static discharge.
The MOSFET is a majority carrier device, and because majority carriers have no recombination delays, the MOSFET achieves extremely high bandwidths and switching times. The gate is electrically isolated from the source, and while this provides the MOSFET with its high input impedance, it also forms a good capacitor. Driving the gate with a dc or a low frequency signal is a snap because ZIN is so high, but driving the gate with a step signal is much harder because the gate capacitance must be charged at the signal rate. This situation leads to a paradox; the high input impedance MOSFET must be driven with a low impedance driver to obtain high switching speeds and low bandwidth.
MOSFETs do not have a secondary breakdown area, and their drain-source resistance has a positive temperature coefficient, so they tend to be self protective. These features, coupled with the very low on resistance and no junction voltage drop when forward biased, make the MOSFET an extremely attractive power supply-switching transistor.
The MOSFET (see Figure 5 for a description) can be visualized as a bar of doped silicon that contains a capacitively coupled diode junction in the middle of the bar. If the silicon bar is doped N, then the MOSFET is called an N-channel device. When the n-channel gate is charged negative with respect to the source the internal gate diode is biased off, the bar is depleted of carriers, and the source to drain resistance is quite high (several hundred M). When the n-channel gate is charged positive with respect to the source, the internal gate diode is biased on, and the bar is flooded with carriers thus causing a low source to drain resistance (in the low mrange). The converse is true for a P-channel MOSFE