AN-7500 Datasheet, PDF(1/5 Page) Fairchild Semiconductor

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AN-7500 Datasheet, PDF (1/5 Pages) Fairchild Semiconductor – Understanding Power MOSFETs

Application Note

Understanding Power MOSFETs

October 1999

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Introduction

Power MOSFETs (Metal Oxide Semiconductor, Field Effect

Transistors) differ from bipolar transistors in operating

principles, specifications, and performance. In fact, the

performance characteristics of MOSFETs are generally supe-

rior to those of bipolar transistors: significantly faster switching

time, simpler drive circuitry, the absence of or reduction of the

second-breakdown failure mechanism, the ability to be

paralleled, and stable gain and response time over a wide

temperature range. This note provides a basic explanation of

general MOSFET characteristics, and a more thorough

discussion of structure, thermal characteristics, gate

parameters, operating frequency, output characteristics, and

drive requirements.

General Characteristics

A conventional n-p-n bipolar power transistor is a current-

driven device whose three terminals (base, emitter, and

collector) are connected to the silicon by alloyed metal con-

tacts. 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. And because of its current-

driven base-emitter input, 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, Figure 1(a), is electrically isolated from

its silicon body by a thin layer of silicon dioxide (SiO2). 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 field 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 1(b). This conversion,

called the surface-inversion phenomenon, allows current to

flow 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, which always

retain its n-p-n characteristic.

By virtue of its electrically-isolated gate, a MOSFET is

described as a high-input impedance, voltage-controlled

device, whereas a bipolar transistor is a low-input-impedance,

current-controlled device. 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 (where mobility

is a term that defines the average velocity of a carrier in terms

of the electrical field imposed on it) makes a MOSFET more

resistive at elevated temperatures, and much more immune to

the thermal-runaway problem experienced by bipolar devices.

A useful by-product of the MOSFET process is the internal par-

asitic diode formed between source and drain, Figure 1(c).

(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.

ALUM GATE

SOURCE

p CONVERTED

TO n CHANNEL

n+ n+

DRAIN

(a)

n+ n+

(b)

(c)

FIGURE 1. THE MOSFET, A VOLTAGE-CONTROLLED DEVICE

WITH AN ELECTRICALLY ISOLATED GATE, USES

MAJORITY CARRIERS TO MOVE CURRENT FROM

SOURCE TO DRAIN (A). THE KEY TO MOSFET

OPERATION IS THE CREATION OF THE INVER-

SION CHANNEL BENEATH THE GATE WHEN AN

ELECTRIC CHARGE IS APPLIED TO THE GATE (B).

BECAUSE OF THE MOSFETs CONSTRUCTION, AN

INTEGRAL DIODE IS FORMED ON THE DEVICE

(C), AND THE DESIGNER CAN USE THIS DIODE

FOR A NUMBER OF CIRCUIT FUNCTIONS

Structure

Fairchild Power MOSFETs are manufactured using a vertical

double-diffused process, called VDMOS or simply DMOS. A

DMOS MOSFET is a single silicon chip structured with a

large number of closely packed, hexagonal cells. The

number of cell varies according to the dimensions of the

chip. For example, a 120-mil2 chip contains about 5,000

cells; a 240-mil2 chip has more than 25,000 cells.

One of the aims of multiple-cells construction is to minimize

the MOSFET parameter rDS(ON), or resistance from drain to

source, when the device is in the on-state. When rDS(ON) is

minimized, the device provides superior power-switching

Application Note 7500 Rev. A1

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