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n-channel MOSFET

General Introduction

Transistors are devices that are used to drive output transducers. They deal with electrical power in response to electrical signals.

The above is a fairly concise description of what transistors do. By considering a systems approach to electronics, processes such as counters and logic circuits often need to drive larger output devices such as bulbs, motors and heaters etc. The devices used to perform the various processes are often small low power ICs that can only source or sink relatively small currents (several mA) and work at fairly low voltages. The output transducers may, on the other hand, require larger currents (several Amps) and work at higher voltages. Something needs to allow the process devices to work with the output transducers and this is where transistors are used.

Transistors are devices that require only small input currents or voltages but can handle large output currents.

There are two distinct types of transistor, the bipolar transistor and the MOSFET, and for each type of transistor there are two varieties. Bipolar transistors can be either npn type or pnp type. Bipolar transistors are devices that are operated by current. MOSFETs are operated by voltages and can be either n-channel or p-channel MOSFETs. As transducer drivers, MOSFETs tend to be better at handling high currents than bipolar transistors but bipolar transistors are better suited to low voltage circuits.

Overview

MOSFETAn n-channel MOSFET is an active circuit component with three legs and usually made from silicon. The n-channel MOSFET is usually just referred to as a MOSFET. A MOSFET is just one example of a Field Effect Transistor (FET) and all these devices rely on the electric field due to a voltage. MOSFETs are voltage operated devices.

An n-channel MOSFET can be thought of as a simple electronic switch or transducer driver. As a transducer driver the MOSFET can be used to control powerful devices requiring large currents.


n-channel MOSFET basics

MOSFET symbolAn n-channel MOSFET has three terminals or, in other words, three legs.

The legs are called the Gate, Drain and Source. The Source is identified by the arrow although this is not always shown.

Basic action: When the Gate-Source voltage exceeds the threshold voltage of the MOSFET the Drain-Source resistance falls to a (very) low value and allows current to flow into the Drain and out of the Source. No current flows in to the Gate as the Gate has a very high resistance.


Types of MOSFET

MOSFETsSmall signal MOSFETs can be used to make amplifiers and other analogue circuits as well as making all types of digital circuits. The very high input (Gate) resistance of the MOSFET makes them ideal as input amplifiers to many devices. For example, a MOSFET is used as the input amplifier in a digital voltmeter (such as a multimeter) meaning the voltmeter has an almost infinite input resistance.


MOSFETsMOSFETs designed to control large currents have a very low Drain-Source resistance when they are turned on so that, even when large currents flow, very little heat is generated through ohmic heating. A typical power MOSFET can take 30A or more and still only get warm in use. The metal tag or metal case is usually connected to the Source. Very high power applications, such as power amplifiers, use MOSFETs bolted to a heatsink (as shown in the picture).


Used as a Transducer Driver

MOSFET driverA MOSFET is very easy to use as a transducer driver because there are no other components needed and no calculations to perform. The electrical signal from a control circuit is used to control the MOSFET and the MOSFET controls the current through the output transducer.

The circuit shows an n-channel MOSFET being used to control a bulb. When the push button is pressed the Gate is connected to 12V. This is well above the minimum voltage needed to "turn on" the MOSFET and so the Drain-Source resistance reduces to almost zero and current flows through the bulb. The bulb is ON.

When the push button is released the high value resistor (nominally 10kΩ but could be much higher if necessary) ensures the Gate is connected to 0V. The MOSFET is now "turned off" and the Drain-Source resistance rises to a very high value and no current flows through the bulb. The bulb is OFF.


MOSFET and diodeIf an inductive load, such as a motor or relay, is used then a protection diode is needed to protect the MOSFET from the back e.m.f. produced when the transducer is turned off. When the motor is turned off a large voltage can be generated between the 12V rail and the Drain of the MOSFET. This large voltage can easily damage the MOSFET. The diode restricts the maximum voltage at the Drain to a safe 12.7V in the circuit shown.


Gate Resistance

One of the main advantages of a MOSFET is the very very high Gate resistance which can exceed 10MΩ. This means that effectively no current flows in to the Gate and MOSFETs have no effect of the circuit they are connected to.

The very high Gate resistance means that MOSFETs are sensitive to static electricity. Stray static can charge the Gate to very high voltages and damage the MOSFET and they must therefore be handled carefully. It is is also important to ensure the Gate is connected to ground using a pull-down resistor if necessary otherwise the MOSFET can behave unexpectedly.

Gate-Source Voltage

When the voltage between the Gate and the Source (called the Gate-Source voltage, Vgs) is below the threshold voltage (Vth) for the MOSFET then the Drain-Source resistance is very high and no current flows. When the Gate-Source voltage is above the threshold voltage, the Drain-Source resistance falls to a few ohms or less and the MOSFET conducts.

The threshold voltage for many MOSFETs is around 3V and so MOSFETs cannot be used in devices where the supply voltage is relatively low. For example, a portable piece of equipment powered from 2xAA batteries, such as the TV remote, is operating at 3V and so standard MOSFETs cannot reliably be used without special circuitry to provide the necessary Gate-Source voltage.

MOSFET Equation

The current that flows into the Drain (ID) is related to the Gate-Source Voltage (Vgs), the threshold voltage (Vth) and a property of the MOSFET, called the transconductance (gM), that describes how well it conducts.

MOSFET test circuitThe simple circuit shown can be used to investigate the MOSFET equation. The potentiometer allows the Vgs to be varied and the ammeter measures the Drain current.

When the Gate-Source voltage is less than the threshold voltage the MOSFET does not conduct and the Drain current is zero.

When the Gate-Source voltage is above the threshold voltage, the Drain current increases as the Gate-Source voltage increases. The rate at which the Drain current increases is determined by the Transconductance of the MOSFET. The Transconductance (from Transfer Conductance) is a property of the individual MOSFET and is measured in Siemens (S).


The equation to determine the Drain current when the MOSFET is conducting is:

ID=gM(Vgs−Vth)

Example: If the threshold voltage of a MOSFET is 3.0V and the Transconductance is 0.4S (400mS) what is the Drain current when the Gate-Source voltage is 4.5V?

Solution: ID= 0.4 × (4.5 − 3.0) = 0.4 × 1.5 = 0.6A
If no other components are limiting the current, the Drain current flowing through the MOSFET will be 0.6 Amps.

A quite advanced video demonstrating the MOSFET equation and numerical examples is available HERE (10 minutes) or on the YouTube Channel.

End Note

The most basic application of a MOSFET is when it is configured as a transducer driver. Although it is acceptable to think of the MOSFET as a simple electronic switch in this case, there are many more complex applications where this simplistic approach is not appropriate.