CHAPTER 8

MOSFETS

OBJECTIVESDescribe and Analyze:

• Theory of MOSFETS

• MOSFET Amplifiers

• E-MOSFET Switches

• Troubleshooting

Introduction• MOSFET stand for Metal-Oxide-Semiconductor (or

Metal-Oxide-Silicon) Field-Effect-Transistor• Like JFETs, MOSFETs come in N-channel and

P-Channel types• Unlike JFETs, MOSFETs can be manufactured as

enhancement-mode (E-MOSFETs) as well as depletion-mode (D-MOSFETs).

• There is no PN junction. The metal gate of a MOSFET is isolated from the silicon channel by a thin layer of silicon oxide (Si O2, commonly known as glass)

• MOSFETs can be damaged by static electricity

D-MOSFETs

Similar to a JFET, but Zin of device is almost infinite

D-MOSFETs

Unlike JFETs, D-MOSFETS can work with zero bias

D-MOSFETs

The same bias circuits used with JFETs can be used with E-MOSFETs. In addition, a class-A MOSFET amplifier can work with VGS = 0.

D-MOSFET AmplifiersThe equations for Zin, Zout and Av developed for JFET

amplifiers can be used with D-MOSFET amplifiers.

Like JFETs, D-MOSFETs are used in the front ends of radio receivers because of their inherently low internal noise.

D-MOSFET AGC Amplifiers

Since gm depends on the Q-point, MOSFETs are used for Automatic Gain Control in radio receivers

D-MOSFET Mixers

Unlike JFETs, D-MOSFETs can be built with two gates. That allows them to be used as radio “mixers” to

multiply one signal by another.

E-MOSFETs

In an enhancement-mode MOSFET, the drain is isolated from the source because the substrate is doped opposite the source and drain. Voltage applied to the gate causes the substrate under the gate to “flip polarity”. P-material becomes N-material as charge carriers are attracted into the region by the gate.

E-MOSFETs

The key parameter for an E-MOSFET is the threshold voltage (VGS(TH)) required to turn it on

E-MOSFET Switches

The most common use for an E-MOSFET is switching

E-MOSFET Switches The IRF510 E-MOSFET is a typical power switch. Its

key specifications are:

• VGS(TH) = 4 Volts max• RDS(on) = 0.54 Ohms max• ID(MAX) = 5.6 Amps• IDSS = 25 A (remember: it’s off)• BVDSS = VDS(MAX) = 100 Volts• PD(MAX) = 43 Watts• Rise-time tR = 63 ns

E-MOSFET Switches

• The most common application of E-MOSFETs like the IRF510 is to drive the transformer in switch-mode power supplies and DC to DC converters.

E-MOSFET Switches

E-MOSFETs can also switch analog signals

E-MOSFET Switches

Choppers convert DC or low-frequency AC into

higher-frequency AC suitable for processing

Digital MOSFET Switch

Because of their small size, low power, and speed, digital ICs such as microprocessors use MOSFET switches

E-MOSFET Amplifiers

When biased on, E-MOSFETs can have a high gm

CMOS

• CMOS stands for:

Complementary Metal-Oxide-Semiconductor.

• They combine N-channel and P-channel MOSFETs.

• They are primarily used in low-power digital ICs.• Are sometimes used in “mixed signal” ICs which

combine analog and digital signals on one chip.

Troubleshooting• MOSFETs can not be checked with an Ohm-meter.

• As usual, check the DC bias levels.

• Check the input and output levels of signals to see if they are approximately what you expected.

• If it’s necessary to replace a MOSFET, use the same part number. If that’s not an option, pick a device suitable for the application: switch, RF mixer, AGC amplifier, etc.