The MOSFET

Untitled
At A, pictorial diagram of an N-channel MOSFET. At B, the schematic symbol. Electrodes are S = source, G = gate, and D = drain.
The acronym MOSFET (pronounced “moss-fet”) stands for metal-oxide-semiconductor f ield e ffect t ransistor. A simplified cross-sectional drawing of an N-channel MOSFET, along with the schematic symbol, is shown in above figure. The P-channel device is shown in the drawings of below figure.
Untitled
At A, pictorial diagram of a P-channel MOSFET. At B, the schematic symbol. Electrodes are S = source, G = gate, and D = drain.

Extremely High Input Impedance

When the MOSFET was first developed, it was called an insulated-gate FET or IGFET. That’s still a good name for it. The gate electrode is insulated from the channel by a thin layer of dielectric material. As a result, the input impedance is even higher than that of a JFET. The gate-to-source resistance of a typical MOSFET is, as a matter of fact, comparable to that of a typical capacitor! This means that a MOSFET draws essentially no current, and therefore no power, from the signal source. This makes the device ideal for low-level and weak-signal amplifiers. But MOS devices aren’t quite perfect. They have an Achilles heel. They are electrically fragile.

Beware of Static!

The trouble with MOSFETs is that they can be easily damaged by electrostatic discharges. When building or servicing circuits containing MOS devices, technicians must use special equipment to ensure that their hands don’t carry electrostatic charges that might ruin the components. If a discharge occurs through the dielectric of a MOS device, the component is permanently destroyed. Warm and humid climates do not offer total protection against the hazard. This author learned that fact by ruining several MOSFETs while designing circuits in the summertime—in Miami, Florida!

Flexibility in Biasing

Untitled
A family of characteristic curves for a hypothetical N-channel MOSFET.
In electronic circuits, an N-channel JFET can sometimes be replaced directly with an N-channel MOSFET, and P-channel devices can be similarly interchanged. But the characteristic curves for MOSFETs are not the same as those for JFETs. The main difference is that the S-G junction in a MOSFET is not a P-N junction. Therefore, forward breakover cannot occur. A gate bias voltage, EG, more positive than +0.6 V can be applied to an N-channel MOSFET, or an EG more negative than −0.6 V to a P-channel device, without a current leak taking place. A family of characteristic curves for a hypothetical N-channel MOSFET is shown in the graph of above figure.

Depletion Mode versus Enhancement Mode

Normally the channel in a JFET is wide open; as the depletion region gets wider and wider, choking off the channel, the charge carriers are forced to pass through a narrower and narrower path. This is known as the depletion mode of operation for a field effect transistor. A MOSFET can also
 
be made to work in the depletion mode. The drawings and schematic symbols of top figure 1st and top figure 2nd show depletion-mode MOSFETs.
 
Metal-oxide semiconductor technology also allows an entirely different means of operation. An enhancement-mode MOSFET normally has a pinched-off channel. It is necessary to apply a bias voltage, EG, to the gate so that a channel will form. If EG = 0 in an enhancement-mode MOSFET, then ID = 0 when there is no signal input. The schematic symbols for N-channel and P-channel enhancement-mode devices are shown in following figure. Note that the vertical line is broken. This is how you can recognize an enhancement-mode device in circuit diagrams.
Untitled
Schematic symbols for enhancement mode MOSFETs. At A, the N-channel device; at B, the P-channel device.