Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

A metal-oxide-semiconductor field-effect transistor (MOSFET) is a component used in the field of electronic engineering. Based on newer technology, it gained popularity over silicon transistors due to its superior performance. The electronics industry has always had a need for greater switching speeds and power-handling capability from a transistor, and this technology opened new possibilities of increasing the switching speeds of power transistors, thus changing the industry overnight.

Interestingly, the MOS field-effect transistor evolved out of MOS integrated circuit (IC) technology. Unlike the silicon transistor, which is a current controlled device, a MOSFET is a voltage-controlled device. This means that a voltage at the gate junction causes the majority carriers (holes) to move. Holes are positive because they lack an electron, therefore they move to the negative terminal, whilst the electrons, which are negative, move to the positive terminal.

Symbol

The diagram above shows the symbol of an n-channel enhancement MOSFET. These transistors usually have a terminal called "Source", which is the source for electrons. Electrons are negatively charged particles, which emerge from the negative potential and move towards the positive potential. This terminal therefore connects to ground. The "Drain" terminal is where the negatively charged electrons attract to and drain, and therefore it connects to the positive rail.

The Gate junction, insulated by a layer of oxide, prevents the flow of electrons into the gate. The oxide layer also provides a small amount of capacitance at the gate and consequently the input impedance of the gate is very high.

Applications

MOS technology was widely used in the construction of microprocessor IC and CMOS digital logic circuits. As digital switches, they had low power consumption and therefore used in calculators, watches, and portable battery powered applications.

Power transistors made from this technology are highly efficient due to their fast switching capability, and low power dissipation. Hence, they are widely used in audio power amplifier applications.

Performance as a Switch

A MOSFET transistor is more efficient than a bipolar junction transistor in switching applications. When there is no voltage across the Gate junction, the MOSFET is completely OFF and there is a very high resistance across the Source-Drain junctions. When fully switched ON the voltage across the Source-Drain junctions is zero because the transistor conducts fully.

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