FET Circuit Design Tutorial

- essentials of designing field effect transistor or FET circuits: circuit types, design methodologies, equations and techniques.

Field effect circuits are used in many different areas of electronics. FET circuits are able to provide characteristics that are not possible when using the more traditional bipolar transistors.

Accordingly FET circuit design techniques are often used in overall circuit design.

With the number of different types of FET, there are several differ types of FET circuit that can be used both as discrete circuits and within integrated circuits as well.

FET circuit technology

When considering the use of a FET circuit, it is necessary to consider FET technology and the type of field effect transistor will be the most applicable.

Note on Field Effect Transistor Technology:

There are several different types of FET ranging from the Junction FET (JFET) through the Metal Oxide Silicon FET or MOSFET through to more exotic variants including V-FETs and more. All these different types of FET are based around the same basic technology where an electric field alters the flow of current through a semiconductor channel.

Click on the link for further information about the Field Effect Transistor, FET

The FET has three electrodes:

  • Source:   The Source, S is the electrode on the FET through which the majority carriers enter the channel, i.e. at acts as the source of carriers for the device. Current entering the channel through the source is designated by IS
  • Drain:   The Drain, D is the FET electrode through which the majority carriers leave the channel, i.e. they are drained from the channel. Conventional current entering the channel at D is designated by the letters ID. Also Drain to Source voltage is often designated by the letters VDS
  • Gate:   The Gate G), is the terminal that controls the channel conductivity. By applying voltage to G, one can control ID

FET Circuit Suymbols
Circuit symbols for the basic JFET components

FET types for circuit design

As there are several different types of field effect transistor that can be used, it is necessary to define at least some of the FETs that can be used within the circuit design process.

The table below defines some of the different types and characteristics that can be encountered.

FETs for Use in Circuit Design
Characteristic Details
J-FET The J-FET or junction FET is a form of FET where the gate is formed by using a diode junction onto the channel. The isolation is maintained by ensuring that the diode junction remains reverse biased when operated within the circuit. IT is a key requirement of the FET circuit design to ensure the junction remains reverse biased for satisfactory operation.
MOSFET This type of field effect transistor relies on a metal oxide later between the gate and channel. It offers a very high input resistance.
Dual-gate MOSFET As the name implies, this form of MOSFET has two gates. In FET circuit design, this gives additional options.
Enhancement mode Enhancement mode FETs are OFF at zero gate-source voltage. They are turned on by pulling the gate voltage in the direction of the drain voltage, i.e. towards the supply rail, which is positive for N-channel devices and negative for P-channel devices. In other words by pulling the gate voltage towards the drain voltage, the number of carriers in the active layer of the channel is enhanced.
Depletion mode In a depletion-mode MOSFET, the device is normally ON at zero gate-source voltage. Any gate voltage in the direction of the drain voltage will tend to deplete the active area of channel of carriers and reduce the current flowing.
N-channel An N channel FET has a channel made from N-type semiconductor in which the majority carriers are electrons.
P-channel An P channel FET has a channel made from P-type semiconductor in which the majority carriers are holes.

When designing an FET circuit, one of the first steps is to determine what type of FET will be suitable for the application required. Channel type, mode type and the basic type of FET will all need to be determined to enable the FET circuit design to proceed.

By Ian Poole

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