Chapter 7
7.5 Power MOSFETs
7.5.1 Introduction
Compared to BJTs, which are current controlled devices, fi eld effect transistors are
voltage controlled devices. There are two basic fi eld-effect transistors (FETs): the
junction FET (JFET) and the metal-oxide semiconductor FET (MOSFET). Both have
played important roles in modern electronics. The JFET has found wide application in
such cases as high-impedance transducers (scope probes, smoke detectors, etc.) and the
MOSFET in an ever-expanding role in integrated circuits, where CMOS (complementary
MOS) is perhaps the most well-known.
Power MOSFETs differ from bipolar transistors in operating principles, specifi cations,
and performance. In fact, the performance characteristics of MOSFETs are generally
superior to those of bipolar transistors: signifi cantly faster switching time, simpler
drive circuitry, the absence of a second breakdown failure mechanism, the ability to
be paralleled, and stable gain and response time over a wide temperature range. The
MOSFET was developed out of the need for a power device that could work beyond
the 20-kHz frequency spectrum, anywhere from 100 kHz to above 1 MHz, without
experiencing the limitations of the bipolar power transistor.
7.5.2 General Characteristics
Bipolar transistors are described as minority-carrier devices in which injected minority
carriers recombine with majority carriers. A drawback of recombination is that it limits
the device s operating speed. Current-driven base-emitter input of a bipolar transistor
presents a low-impedance load to its driving circuit. In most power circuits, this low-
impedance input requires somewhat complex drive circuitry.
By contrast, a power MOSFET is a voltage-driven device whose gate terminal is
electrically isolated from its silicon body by a thin layer of silicon dioxide (SiO
). As
a majority-carrier semiconductor, the MOSFET operates at much higher speed than its
bipolar counterpart because there is no charge-storage mechanism. A positive voltage
applied to the gate of an n-type MOSFET creates an electric fi eld in the channel region
beneath the gate; that is, the electric charge on the gate causes the p-region beneath the
gate to convert to an n-type region, as shown in Figure 7.25(a) .
This conversion, called the surface-inversion phenomenon, allows current to fl ow
between the drain and source through an n-type material. In effect, the MOSFET ceases
to be an n-p-n device when in this state. The region between the drain and source can be
represented as a resistor, although it does not behave linearly, as a conventional resistor
would. Because of this surface-inversion phenomenon, then, the operation of a MOSFET
is entirely different from that of a bipolar transistor.
227Power Semiconductors
By virtue of its electrically isolated gate, a MOSFET is described as a high-input
impedance, v
oltage-controlled device, compared to a bipolar transistor. As a majority-
carrier semiconductor, a MOSFET stores no charge, and so can switch faster than a
bipolar device. Majority-carrier semiconductors also tend to slow down as temperature
increases. This effect brought about by another phenomenon called carrier mobility
makes a MOSFET more resistive at elevated temperatures, and much more immune to the
thermal runaway problem experienced by bipolar devices. Mobility is a term that defi nes
the average velocity of a carrier in terms of the electrical fi eld imposed on it.
A useful byproduct of the MOSFET process is the internal parasitic diode formed
between source and drain, Figure 7.25(b) . (There is no equivalent for this diode in a
bipolar transistor other than in a bipolar Darlington transistor.) Its characteristics make it
useful as a clamp diode in inductive-load switching.
Different manufacturers use different techniques for constructing a power FET, and
names like HEXFET, VMOS, TMOS, etc., have become trademarks of specifi c
7.5.3 MOSFET Structures and On Resistance
Most power MOSFETs are manufactured using various proprietary processes by various
manufacturers on a single silicon chip structured with a large number of closely packed
identical cells. For example, Harris Power MOSFETs are manufactured using a vertical
double-diffused process, called VDMOS or simply DMOS. In these cases, a 120-mil
chip contains about 5,000 cells and a 240-mil
chip has more than 25,000 cells.
One of the aims of multiple-cells construction is to minimize the MOSFET parameter
when the device is in the on-state. When R
is minimized, the device provides
superior power-switching performance because the voltage drop from drain to source is also
minimized for a given value of drain-source current. Reference 6 provides more details.
(a) (b)
Figure 7.25 : Structure of N-channel MOSFET and symbol (a) Structure (b) Symbol

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