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The MOSFET Help

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By — McGraw-Hill Professional
Updated on Sep 11, 2011

Introduction

The acronym MOSFET (pronounced “MOSS-fet”) stands for metal-oxide-semiconductor field-effect transistor . A simplified cross-sectional drawing of an n -channel MOSFET and its schematic symbol are shown in Fig. 16-14 a and b . The p -channel device is shown at Fig. 16-14 c and d .

Semiconductors The MOSFET The Main Problem

Fig. 16-14 . Pictorial diagram of an n-channel MOSFET ( a ), schematic symbol for an n-channel MOSFET ( b ), pictorial diagram of a p -channel MOSFET ( c ), and schematic symbol for a p -channel MOSFET ( d ).

When the MOSFET was first developed, it was called an insulated-gate FET or IGFET . This is perhaps more descriptive of the device than the currently accepted name. The gate electrode is actually insulated, by a thin layer of dielectric, from the channel. As a result of this, the input resistance (and hence the impedance) is extremely high. The MOSFET draws essentially no current from the input signal source. This is an asset in weak-signal amplifiers.

The Main Problem

The trouble with MOSFETs is that they can be damaged easily by electrostatic discharge. When building or servicing circuits containing MOS devices, technicians must use special equipment to ensure that their hands do not carry electrostatic charges that might ruin the components. If a static discharge occurs through the dielectric of a MOS device, the component will be destroyed permanently. A humid environment does not offer significant protection against this hazard.

Flexibility

In practical circuits, an n -channel JFET sometimes can be replaced with an n -channel MOSFET; p -channel devices similarly can be interchanged. However, the characteristic curves for MOSFETs are not the same as those for JFETs. The source-gate ( S-G ) junction in a MOSFET is not a p-n junction. Forward breakover does not occur under any circumstances. If this junction conducts, it is because the S-G voltage is so great that arcing takes place, damaging the MOSFET permanently. A family of characteristic curves for a hypothetical n -channel MOSFET is shown in Fig. 16-15.

Semiconductors The MOSFET Flexibility

Fig. 16-15 . A family of characteristic curves for a hypothetical n-channel MOSFET.

Depletion Versus Enhancement

In a JFET, the channel conducts with zero bias, that is, when the potential difference between the gate and the source is zero. As the depletion region grows, charge carriers pass through a narrowed channel. This is known as depletion mode . A MOSFET can work in the depletion mode, too. The drawings and schematic symbols of Fig. 16-14 show depletion-mode MOSFETs.

Semiconductors The MOSFET The Main Problem

Fig. 16-14 . Pictorial diagram of an n-channel MOSFET ( a ), schematic symbol for an n-channel MOSFET ( b ), pictorial diagram of a p -channel MOSFET ( c ), and schematic symbol for a p -channel MOSFET ( d ).

Metal-oxide-semiconductor technology allows a second mode of operation. An enhancement-mode MOSFET has a pinched-off channel at zero bias. It is necessary to apply a gate bias voltage E G to create a channel. If E G = 0, the drain current I D is zero when there is no signal input. The schematic symbols for n -channel and p -channel enhancement-mode devices are shown in Fig. 16-16. In schematic diagrams, they can be differentiated from depletion-mode devices by looking at the vertical lines inside the circles. Depletion-mode MOSFETs have solid vertical lines; enhancement-mode devices have broken vertical lines.

Semiconductors Integrated Circuits Compactness

Fig. 16-16 . ( a ) The symbol for an n -channel enhancement-mode MOSFET. ( b ) The symbol for a p -channel enhancement-mode MOSFET.

Practice problems of these concepts can be found at: Semiconductors Practice Test

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