AN7332 판매점 - Field-Effect Transistors

AN7332 반도체 회로 부품 판매점

Field-Effect Transistors

Intersil Corporation

The Application Of Conductivity-Modulated

Field-Effect Transistors

Application Note

May 1992

AN7332.1

Summary

The development of conductivity-modulated ﬁeld-effect

transistors, FETs, makes available to the system designer

another solid-state device that can be used to implement

power switching control. This paper reviews differences

between the standard and the newly developed FET. It

shows the signiﬁcant advantages that the conductivity-

modulated FET has over the standard FET. Several

applications are presented to show that this new type of

device works well in practical situations. The relative

immaturity of the conductivity-modulated FET may limit its

initial utilization. But as the family grows and product

innovation and reﬁnement takes place, this newest member

of the power semiconductor family will become a viable

alternative to the other members.

General Considerations

The development of the power ﬁeld-effect transistor has

made available to the power-stage designer an entire new

family of power semiconductors. Over the past 5 to 6 years,

the breadth of product has grown to encompass the require-

ments of a large number of applications. A limiting factor that

has slowed the utilization of power FETs in the high-current,

high-voltage applications is the fact that the on-state

resistance (RDS(ON)) in a standard FET is related to its

breakdown voltage (BVDSS) by a nearly cubic power, i.e.,

RDS(ON) ≈ BVDSS 2.8. What this implies, as Figure 1 shows,

is that as the breakdown voltage increases, the on-state

resistance climbs even faster.

1 P-CHANNEL MOSFETs

N-CHANNEL MOSFETs

0.1

0.01

N-CHANNEL

CONDUCTIVITY

MODULATED FET

P-CHANNEL CONDUCTIVITY

MODULATED FET

0.001

100 1000

DRAIN-SOURCE VOLTAGE (V)

FIGURE 1. SPECIFIC ON-RESISTANCE OF P AND N-CNANNEL

MOSFETS AND CONDUCTIVITY-MODULATED

FETS vs FORWARD BLOCKING VOLTAGE.

The MOSFET on-state resistance is contributed to primarily

by three components of the transistor: the MOS channel,

the neck region, and the extended drain region. The

extended drain region contributes the most to the on-state

resistance in high-voltage MOSFETs. To achieve a lower on-

state resistance at a given blocking voltage, the usual

technique is simply to make the die larger. However,

increasing the die size has its limitations from a

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manufacturing point of view, since MOSFETs, with their very

ﬁne horizontal geometries, are highly defect-yield sensitive.

As die size increases, the likelihood of a defect resulting in a

nonfunctional part increases exponentially. This tendency,

combined with a smaller number of parts per wafer, limits the

availability of low-on-state-resistance, high-voltage MOS-

FETs.

A change in the horizontal geometry of the MOSFET can

lower the speciﬁc on-state resistance per unit area. By using

more channel width with smaller source cells placed closer

together, a reduction in on-state resistance can be achieved.

A limitation on how close these cells can be placed arises

from a possible localization of ﬁeld concentrations that will

limit the voltage breakdown of the structure to less than the

theoretical rating due only to impurity concentrations.

Therefore, for a given breakdown voltage, there exists a

minimum spacing of the cell structure. Generally, the higher

the required breakdown voltage, the further apart the cells

must be placed.

As stated earlier, the extended drain region of the MOSFET

generally contributes the most to the on-state resistance in

high-voltage MOSFETs. As the required blocking voltage is

increased, this region must be made thicker and more lightly

doped to be able to support the desired voltage. It is this

region's contribution to on-state resistance that the conduc-

tivity-modulated ﬁeld-effect transistor drastically reduces.

This reduction occurs as the result of the injection of minority

carriers from the substrate and, in speciﬁc on-state resis-

tance per unit area, is about 10 times less than in a standard

MOSFET at the 400V BVDSS level, as shown in Figure 1.

Further analysis has shown that the speciﬁc on-state

resistance may be nearly independent of blocking-voltage

level. This ﬁnding implies that at a BVDSS of 1000V, the

reduction in conductivity-modulated FETs over the standard

MOSFETs could be perhaps 50 to 1. These reductions in

on-state resistance per unit area that the conductivity-

modulated FETs can achieve present the possibility that

high-voltage high-current FET-type devices can become

more readily available because of the smaller die sizes

associated with conductivity-modulated FETs.

Comparison of Standard and Conductivity-Modulated

FETs

Standard and conductivity-modulated FETs share some

characteristics, but are substantially different in others.

Shown in Table 1 is a listing of the major characteristics that

make the conductivity-modulated FETs unique among

power semiconductor families. Foremost, it is a voltage-

gated device; its input characteristics are similar to standard

power MOSFETs of comparable chip size. Very little drive

power is required at low to moderate switching frequencies.

The device remains under the control of the gate within its

normal operating conditions. It exhibits the normal linear

mode as well as the fully saturated on-state of conventional

power MOSFETs. When the gate voltage is removed, the

Application Note 7332

device turns off, unlike the thyristor family of power

semiconductors, which must be either externally or naturally

(internally) commutated.

TABLE 1. CONDUCTIVITY-MODULATED FET

CHARACTERISTICS

Voltage Gated

Turn Off

Nonlinear On-State

Voltage drop

Turn On Speed

Turn-Off Speed

Temperature Independent

On-State Voltage Drop

Small gate power required. Similar

to standard power MOSFET.

When gate drive is removed...

Unlike an SCR!

Like that of an SCR.

Fast! Comparable to a standard

power MOSFET.

Slow! Comparable to a bipolar

transistor.

Unlike the typical 2x variation of a

power MOSFET.

The on-state voltage drop or resistance characteristic of a

conductivity-modulated FET is markedly different from that

of a standard power MOSFET, and is similar to that of a

thyristor family member, the SCR. There is an offset voltage

component (typically 0.6V) due to the p-n junction on the

drain side, and a somewhat nonlinear resistive component,

both of which are in series between the drain and source

terminals. This series arrangement results in a highly

nonlinear equivalent resistance, unlike the linear resistive

characteristic of VDS(ON) of a standard FET.

The structure of the conductivity-modulated FET operates

during its turn on just as a standard FET does, hence its

turn-on speed is very similar to that of a standard FET. With

its high input impedance and its short propagation delay, the

turn-on transition of the conductivity-modulated FET, as well

as the standard power FET, is easily controlled by the gate

driving circuit. This characteristic allows the designer the

ability to control EMI and RPI generation easily. With other

power semiconductors, it may be necessary to employ elab-

orate circuit schemes to limit rapidly rising in-rush currents.

A signiﬁcant characteristic that must be considered in power

switching applications is that of turn-off speed. The internal

action that makes the conductivity-modulated FET such a

silicon-efﬁcient device also makes it an inherently slower

device during turn-off. The injection of the minority carriers

during the on-state conduction of current results in these

carriers being present at the moment of turn-off. Without any

way of removing these carriers by external means, they must

recombine within the structure itself before the device can

revert to its fully off-state condition. The quantity of these

carriers and how fast they can deplete themselves

determines the turn-off switching speed of the conductivity-

modulated FET. This process of recombination is

considerably slower than the simple discontinuance of

majority carrier ﬂow by which the standard power FET turns

off. Hence, again, the conductivity-modulated FET is an

inherently slower device. Its turn-off speed lies somewhere

between the performance of a thyristor and that of a bipolar

transistor.

The ﬁnal characteristic that makes the conductivity-

modulated FET different from a conventional FET is the

variance of on-state voltage with temperature. The

characteristic of the conductivity-modulated FET is similar to

that of an SCR, varying about -0.6mV/oC. The conventional

FET has a positive temperature coefﬁcient such that on

high-voltage devices the

value when the junction

RDS(ON) will

temperature

double from its +25oC

reaches +150oC. The

system designer must take this characteristic into consider-

ation when the heat sink is being designed for the system.

It is these similarities and differences that make the conduc-

tivity-modulated FET a unique member of the family of

power-semiconductor switching devices. Applications of this

alternative power switching device invariably make use of

one or more of its unique characteristics.

Applications

Automotive Ignition

An application that can take advantage of the low drive-

power capability of the conductivity-modulated FET is the

electronic automotive ignition system. In Figure 2, the control

IC takes the signal from the pickup coil located in the

distributor and regulates the current through the ignition coil.

At the proper time, the IC removes base drive from the

bipolar transistor, which all systems currently employ as their

coil driver. This removal of base drive allows the transistor to

shut off which, in turn, causes a rapid decrease in the

ignition-coil primary current. As the primary current

decreases to zero, the energy stored in the ﬁeld surrounding

the primary is transferred to the secondary coil. The

secondary coil, consisting of many more turns than the

primary, transforms this energy into a higher voltage,

resulting in a spark being generated in the cylinder. The

control IC determines when this spark occurs, so as to

derive usable power. With the use of a bipolar transistor, it is

estimated that approximately two-thirds of the power

dissipation that occurs in the control IC is the result of the

need to be able to drive the required base current of the

ignition output transistor. The high-impedance input of the

conductivity-modulated FET virtually eliminates the base-

current drive dissipation of the control IC.

With improved silicon usage, the conductivity-modulated

FET brings to power semiconductor switching devices the

die size necessary to attain the required voltage and current-

handling capabilities of the electronic ignition. This smaller-

sized die makes possible smaller modules, whether they be

hybrid or standard PC-based systems, than those currently

implemented with bipolar-transistor technology.

Brushless DC Motors

Another emerging application that can make use of

conductivity-modulated FETs is the emerging ﬁeld of

brushless DC motors. In this class of application, the solid-

state devices are used to electronically switch the voltage to

the multiplicity of windings that are employed. The motor

consists of an armature that has a number of N and S poles

consisting of high-strength permanent magnets. The stator

is made up of the multiplicity of windings that were

4-2

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AN7332 transistor

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Field-Effect Transistors

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