What is a MOSFET?

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Editorial Team - everything PE

Sep 12, 2021

MOSFET is a voltage-controlled semiconductor device that has three terminals – gate, source, and drain. It offers high switching speed and good efficiency at low voltages as compared to a thyristor or an insulated-gate bipolar transistor (IGBT). MOSFETs are ideal for use in speed control of d.c and a.c motors, switching, battery protection circuits, automotive industry, stepper motor controllers, switching of linear power supplies, relays, lighting control, solenoid drivers, induction-heating, appliance control, robotics, and instrumentation applications.

Types of MOSFETs

Based on the operation mode, the MOSFETs are classified into two types - Enhancement MOSFET and Depletion MOSFET.

Based on the doped substrate (n-type or p-type) used, the enhancement MOSFET is classified into two types - N-channel enhancement mode, and P-channel enhancement mode.

Based on the doped substrate (n-type or p-type) used, the depletion MOSFET is classified into two types - N-channel depletion-mode and P-channel depletion mode.

Enhancement Mode MOSFET

In enhancement mode, MOSFET is equivalent to the “Normally Open” switch. This means that when there is no voltage applied across the gate terminal, the MOSFET does not conduct. The mode indicates that the transistor is at an OFF state at zero gate-source voltage. It is classified into two categories – N channel enhancement mode and P channel enhancement mode.

N-channel enhancement-mode: An enhancement mode MOSFET can be turned on by applying high gate voltage than the source voltage. Such kinds of MOSFET that are turned on through this process are called N channel enhancement-mode MOSFET.

When applying the positive gate voltage, the negatively induced charges (electrons) increase between source and drain. This leads to the formation of an N-channel between the source and drain.

P-channel enhancement modeAn enhancement mode MOSFET can be turned on by lowering the gate voltage than the source voltage. Such kinds of MOSFET that are turned on through this process are called P channel enhancement-mode MOSFET.

When applying the negative gate voltage, the positively induced charges (holes) increase between source and drain. It leads to the formation of conducting P-channel between the source and drain.

Depletion Mode MOSFET

In depletion mode, MOSFET is equivalent to the “Normally Closed” switch. The mode indicates that the transistor is at an ON state at zero gate-source voltage. It is classified into two categories – N channel depletion mode and P channel depletion mode. In this mode, when voltage is applied across the gate terminal either positive (in P-channel depletion mode) or negative (an N-channel depletion mode), the conductivity and drain current decrease.

N-channel depletion-mode: When no voltage is applied across the gate terminal, the MOSFET normally is in ON condition. When applying positive gate voltage and the drain at a positive potential, the channel conductivity increases due to negative charges induced in the N-channel. When applying a negative gate voltage, the channel conductivity decreases (drain current decreases) due to positive charges induced in the N-channel.

P-channel depletion-mode: When no voltage is applied across the gate terminal, the MOSFET normally is in ON condition. When applying positive gate voltage and the drain at a negative potential, the channel conductivity decrease due to negative charges induced in the P-channel. When applying a negative gate voltage, the channel conductivity increase due to positive charges induced in the P-channel.

Key Specifications of a MOSFET

Types of MOSFET: It represents the mode of MOSFET. The basic modes of MOSFETs are the N or P channel depletion MOSFET and N or P channel enhancement MOSFET.

Transistor polarity: It represents the MOSFET channel polarity. It can be either be P channel or an N channel.

Number of channels: It represents the number of channels in the MOSFET.

Continuous drain current: It is the maximum continuous drain current that the MOSFET can handle.

Drain source breakdown voltage: It refers to the maximum voltage that can be applied across drain and source terminals after which the MOSFET enters the breakdown region.

Drain source resistance: It represents the drain to source on-state resistance and is usually measured in the milli-ohm range.

Gate source voltage:  It represents the voltage that can be applied across the gate and source terminal.

Gate Source Threshold Voltage: It represents the minimum voltage that is applied between the gate and source terminal to make the MOSFET turn ON.

Gate Charge: It represents the total charge that is accumulated at the gate terminal. The gate charge value is used to find how fast a MOSFET switches from ON to OFF state, and vice-versa.

Power dissipation: It represents the maximum power dissipation across the output terminal of a MOSFET.