MOSFET Symbol

A metal-oxide-semiconductor field-effect transistor (MOSFET) is a voltage-controlled electronic component invented by a South Korean Engineer Dawon Kahng, and his colleague Martin M. Atalla in 1959 whilst working at Bell Labs. It was the first insulated gate FET, consisting of a metal gate (M), oxide insulation (O), and silicone semiconductor material (S). The three connections of a field effect transistor are source, drain, and gate. In this device, a voltage at the gate junction produces an electric field, which controls the current flowing between the source and drain junctions. This MOS technology was later utilised to develop integrated circuits (IC) which Kahng foresaw.

Voltage Controlled Device

The differences in the operation between a MOSFET and a bipolar junction transistor (BJT) are huge, but the main ones are that a BJT is a current controlled device because current at the base junction controls the flow of current between the collector and emitter junctions. However, a MOSFET is a voltage-controlled device, where a voltage at the gate junction creates an electric field, which controls the flow of current between the source and drain junctions.

Unipolar

Another huge difference is that in a BJT, the electrons and holes are both responsible for carrying the current between the collector-emitter junctions, hence its name is "bipolar".

However, a MOSFET is a "unipolar" device because only one charge carrier, either electrons, or holes carry the charge. In an n-channel MOSFET the majority carriers are electrons, however in p-channel MOSFET, the majority carriers are holes.

Symbol

IEEE defines the electrical symbol for use in circuit diagrams; however, there are many variations to the FET, JFET, and MOSFET symbols. As technology improves, manufacturers often make their own symbols to represent the component better. The American standard is of course far simpler and easier to remember.

A schematic showing the symmetrical complimentary output stage of an amplifier is easier to read with mirrored transistor symbols. Consequently, they mirror the p-channel symbol along the horizontal axis, and as a result, the source terminal ends up at the top.

If there were no labelling on the symbol, then it is very easy to identify the source terminal, because the gate junction is closer to it.

Arrow

For n-channel, the arrow points inward, however for p-channel the arrow points outward, and there is a very good reason for the meaning of this.

In p-channel MOSFET, conduction occurs through holes, which are the majority carriers for that device. Holes are positive and they attract to the negative side, hence the arrow indicates movement from drain to source.

In n-channel MOSFET, conduction occurs through electrons, which are the majority carriers for that device. They emerge from the source terminal and are attracted to drain which is positive; hence, the arrow indicates movement from source to drain.

Usually a circuit symbol contains a substrate terminal, however in practice it is not utilised. Instead, it connects internally to the source junction, thereby minimising the number of terminals to three.

Depletion Types

N-Channel Depletion

P-Channel Depletion

The depletion MOSFET, or d-MOSFET, is a device, which usually remains in a conducting mode unless a voltage at the gate junction is applied. The voltage at the gate has the effect of switching the device OFF because the electric field produced by the gate voltage reduces the majority charge carriers.

Enhancement Types

N-Channel Enhancement

P-Channel Enhancement

An enhancement MOSFET or e-MOSFET is a device, which remains in a non-conducting state unless a voltage at the gate is present. A voltage at the gate has the effect of switching ON the device, because the electric field produced by the gate voltage increases the charge carriers.

Input Impedance

A MOSFET is a common term for insulated gate field-effect transistor (IGFET). Since a layer of oxide insulates the gate junction, a MOSFET usually has very high input impedance in the order of a few mega ohms. This makes it a very useful component for the input stage of an audio amplifier, where impedance matching issues usually occur.

Temperature Sensitive

MOSFETS are also very hardy transistors because as their temperature increases, the resistance of the drain junction also increases thus choking the current flow. Therefore, they are ideal for use in the power output stage of an amplifier.

IC Application

The reason why integrated circuits (IC) utilise MOSFETS is that they occupy a very small footprint, and are much simpler to fabricate using lithographic technologies.

For IC construction n-channel, MOS is highly favoured because it is 2.5 times faster than p-channel MOS. This is because n-channel conduction occurs through electrons, as opposed to p-channel, which occurs through holes.

This Article Continues...