Download Prospectus

The operation of metal oxide semiconductors

In our previous article, we discussed how semiconductors operate.  We’re now going to get a bit more specific and look in detail at metal oxide semiconductors.

Junction Field-effect Transistors (JFET)

We first need to understand Junction Field-effect Transistors before focusing on a metal oxide semiconductor.

A transistor is a linear semiconductor device that controls current with the application of a lower-power electrical signal.  Transistors can be grouped into two major groups: bipolar and field-effect.

All field-effect transistors are unipolar devices and the main current through them is either  electrons through an N-type semiconductor or holes through a P-type semiconductor. This becomes more evident when a physical diagram of the device is seen.  The diagram below shows the n-channel JFET:

n-channel JFET

In a JFET, the controlled current passes from source to drain, or from drain to source.  The controlling voltage is applied between the gate and source.  The current doesn’t have to cross through a PN junction on its way between source and drain: the path, known as a channel, is an uninterrupted block of semiconductor material.

The diagram below shows the p-channel JFET:

p-channel JFET

Normally, n-channel JFETs are more commonly used than the p-channel JFETs.

Operational characteristics

The main difference between n-channel and p-channel JFETs is biasing of the PN junction formed between the gate material and the channel.   With no voltage applied, the channel is a wide-open path for current to flow.

However, if a voltage is applied between gate and source of such polarity that it reverse-biases the PN junction, the flow between source and drain connections becomes limited or regulated.

Maximum gate-source voltage “pinches off” all current through source and drain, thus forcing the JFET into cut-off mode.

This behaviour is due to the depletion region of the PN junction expanding under the influence of a reverse-bias voltage, eventually occupying the entire width of the channel if the voltage is great enough.

, The operation of metal oxide semiconductors

Another way to think about it is by picturing reducing the flow of a liquid through a flexible hose by squeezing it: with enough force, the hose will be constricted enough to completely block the flow.

Metal Oxide semiconductor field effect transistor (MOSFET)

The most common type of Insulated Gate Field Effect Transistor (IGFET) is the metal oxide semiconductor field transistor (MOSFET).

The MOSFET or IGFET is a voltage-controlled field effect transistor that differs from a JFEET.  The MOSFET has a metal oxide gate electrode which is electrically insulated from the main semiconductor n-channel or p-channel by a very thin layer of insulating material, usually glass.

This ultra-thin insulated metal gate electrode can be thought of as one plate of a capacitor. The isolation of the controlling Gate makes the input resistance of the MOSFET extremely high (MΩ) region thereby making it almost infinite.

As the Gate terminal is electrically isolated from the main current carrying channel between the drain and source, no current flows into the gate, and just like the JFET, the MOSFET also acts like a voltage-controlled resistor where the current flowing through the main channel between the Drain and Source is proportional to the input voltage. 

Both  the JFET and the MOSFETs are susceptible to large amounts of static charge, so easily damaged. 

MOSFETs are three terminal devices with a Gate, Drain and Source and both P-channel (PMOS) and N-channel (NMOS) MOSFETs are available. The main difference this time is that MOSFETs are available in two basic forms:

  • Depletion Type   –   the transistor requires the Gate-Source voltage, (VGS) to switch the device “OFF”. The depletion mode MOSFET is equivalent to a “Normally Closed” switch.
  • Enhancement Type   –   the transistor requires a Gate-Source voltage, (VGS) to switch the device “ON”. The enhancement mode MOSFET is equivalent to a “Normally Open” switch.

The image below shows the symbols and construction for both configurations of MOSFETs:

MOSFET diagrams

Basic MOSFET Structure 

Both the Depletion and Enhancement type MOSFETs use an electric field produced by a gate voltage to alter the flow of charge carriers, electrons for n-channel or holes for P-channel, through the semiconductive drain-source channel.  The image below shows this:

a basic MOSFET

The gate electrode is placed on top of a very thin insulating layer and there are a pair of small n-type regions just under the drain and source electrodes.

With an insulated gate MOSFET device no such limitations apply so it is possible to bias the gate of a MOSFET in either polarity, positive (+ve) or negative (-ve). 

This makes the MOSFET device especially valuable as electronic switches or to make logic gates because with no bias they are normally non-conducting and this high gate input resistance means that very little or no control current is needed as MOSFETs are voltage controlled devices. 

Both the p-channel and the n-channel MOSFETs are available in two basic forms, the Enhancement type and the Depletion type.

Keep an eye out for our next article looking at another type of semiconductor

Interested in our courses?

You can read more about our selection of accredited online electrical and electronic engineering courses here.

Check out individual courses pages below:

Diploma in Electrical and Electronic Engineering

Higher International Certificate in Electrical and Electronic Engineering

Diploma in Electrical Technology

Diploma in Electronics

Diploma in Renewable Energy (Electrical)

Higher International Diploma in Electrical and Electronic Engineering

Alternatively, you can view all our online engineering courses here.

Recent Posts

Understanding the mechanisms involved in a voltage amplifier

Understanding the mechanisms involved in a voltage amplifier In our previous articles, we discussed analogue signals and their characteristics.  We’re going to dive a little deeper into analogue signals and see how a voltage amplifier works. What is an operational amplifier? Operational amplifiers, or op-amps, are one of the basic building blocks of analogue electrical […]

Characteristics of analogue electronics

Characteristics of analogue electronics In our previous articles, we’ve discussed subjects such as the different types of semiconductors and how they work.  We’re now going to focus on analogue electronics—what it is and its characteristics. What is an analogue signal? An analogue signal is continuous; in other words, it can have an infinite number of […]

The structure and mechanism of a Bipolar transistor

The structure and mechanism of a Bipolar transistor In our previous article, we discussed the operation of metal oxide transistors, and now we’re going to have a look at bipolar transistors.  We’ll look at their mechanism and how they work. What is a bipolar transistor? A bipolar junction transistor is a semiconductor device which can […]