1. Field Effect Transistors (FETs)
Chapter 23
Field Effect Transistors (FETs)

2. Objectives After completing this chapter, you will 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

3. Objectives (cont’d.)
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

4. Junction FETs
Figure Cross section of an N-channel JFET.

5. Junction FETs (cont’d.)
Figure Lead connections for an N-channel JFET.

6. Junction FETs (cont’d.)
Figure Properly biased N-channel JFET.

7. Junction FETs (cont’d.)
Figure Schematic symbols for JFETs.

8. Figure 23-5. The polarities required to bias an N-channel JFET.
Figure The polarities required to bias a P-channel JFET.

9. Depletion Insulated Gate FETs (MOSFETs)
Figure N-channel depletion MOSFET.

10. Depletion Insulated Gate FETs (MOSFETs) (cont’d.)
Figure N-channel depletion MOSFET with bias supply.

11. Depletion Insulated Gate FETs (MOSFETs) (cont’d.)
Figure Schematic symbol for an N-channel depletion MOSFET.

12. Depletion Insulated Gate FETs (MOSFETs) (cont’d.)
Figure Properly biased N-channel depletion MOSFET.

13. Depletion Insulated Gate FETs (MOSFETs) (cont’d.)
Figure Schematic symbol for a P-channel depletion MOSFET.

14. Enhancement Insulated Gate FETs (MOSFETs)
Figure P-channel enhancement MOSFET.

15. Enhancement Insulated Gate FETs (MOSFETs)
Figure Schematic symbol for a P-channel enhancement MOSFET.

16. Enhancement Insulated Gate FETs (MOSFETs)
Figure Properly biased P-channel enhancement MOSFET.

17. Enhancement Insulated Gate FETs (MOSFETs)
Figure Schematic symbol for an N-channel enhancement MOSFET.

18. Enhancement Insulated Gate FETs (MOSFETs)
Figure Properly biased N-channel enhancement MOSFET.

19. MOSFET Safety Precautions
Keep the leads shorted together
Wear a grounded metallic wristband
Use a grounded soldering iron tip
Make sure power is off

20. Testing FETs
Commercial transistor test equipment
Ohmmeter

21. Summary
The three leads of a JFET are attached to the gate, source, and drain
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

22. Summary (cont’d.)
Enhancement mode MOSFETs are usually P-channel devices and are normally off
Electrostatic charges from fingers can damage a MOSFET
JFETs and MOSFETs can be tested using a commercial transistor tester or an ohmmeter