The history of field effect transistors (FETs) dates back to 1925, when Julius Lillenfield invented both the junction FET and the insulated gate FET. Both of these devices currently dominate electronics technology. This chapter is an introduction to the theory of junction and insulated gate FETs.

What you are expected to learn

At the end of this module, you should be able to:

• Describe the difference between transistors, JFETs, and MOSFETs.
• Draw schematic symbols for both P-channel and N-channel JFETs, depletion MOSFETs, and enhancement MOSFETs.
• Describe how a JFET, depletion MOSFET, and enhancement MOSFET operate.
• Identify the parts of JFETs and MOSFETs.
• Describe the safety precautions that must be observed when handling MOSFETs.
• Describe the procedure for testing JFETs and MOSFETs with an ohmmeter.

How to learn from this module

Going through this module can be both a fun and a meaningful learning experience. All you need to do is make use of your time and resources efficiently. To do this, here are some

tips for you:

1. Take time in reading and understanding each lesson. It is better to be slow but sure than to hurry finishing the module only to find out that you missed the concepts you are supposed to learn.

2. Do not jump from one chapter to another. Usually, the lessons are arranged such that one is built upon another, hence an understanding of the first is essential in comprehending the succeeding lessons.

3. Be honest. When answering the test items, do not turn to the key to correction page unless you are done. Likewise, when performing experiments, record only what you have really observed.

4. Safety first. Perform the experiments with extra precaution. Wear safety gears whenever necessary.

5. Don’t hesitate to ask. If you need to clarify something, approach your teacher or any knowledgeable person.

What to do before (Pretest)

Junction Field Effect Transistor

• A JFET uses a channel instead of junctions (as in transistors) for controlling a signal.
• The three leads of a JFET are attached to the gate, source, and drain.
• The input signal is applied between the gate and the source for controlling a JFET.
• JFETs have extremely high input resistance.
• The schematic symbols for JFETs are:

The MOSFET

• MOSFETs (insulated gate FETs) isolate the metal gate from the channel with a thin oxide layer.
• Depletion mode MOSFETs are usually N-channel devices and are classified as normally on.
• Enhancement mode MOSFETs are usually P-channel devices and are normally off.
• One difference between JFETs and MOSFETs is that the gate can be made positive or negative on MOSFETs.
• The schematic symbol for a depletion MOSFET is:

• The source and drain leads can be interchanged on most JFETs and MOSFETs because the devices are symmetrical.
• A JFET is a normally on device. It is turned off with gate voltage. This is called the depletion mode.
• A MOSFET uses an insulated gate structure. Manufacturers make both depletion-type and enhancement-type MOSFETs.
• MOSFETs must be handled carefully to avoid rupture of the thin oxide layer separating the metal gate from the channel.
• Electrostatic charges from fingers can damage a MOSFET.
• Prior to use, keep the leads of a MOSFET shorted together.
• Wear a grounded metallic wrist strap when handling MOSFETs.
• Use a grounded soldering iron when soldering MOSFETs into a circuit and make sure the power to the circuit is off.
• JFETs and MOSFETs can be tested using a commercial transistor tester or an ohmmeter.

Self Check