APPLICATION NOTE
AN7244
Rev.3.00
October 1, 1999

Understanding Power MOSFETs

Introduction
Power MOSFETs (Metal Oxide Semiconductor, Field EffectTransistors) differ from bipolar transistors in operating
principles, specifications, and performance. The performance
characteristics of MOSFETs are generally superior to those ofbipolar transistors: significantly faster switching time, simpler
drive circuitry, the absence of or reduction of the secondbreakdown failure mechanism, the ability to be paralleled, andstable 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.

A useful by-product of the MOSFET process is the internalparasitic diode formed between source and drain, Figure 1,
section C. (There is no equivalent for this diode in a bipolartransistor other than in a bipolar Darlington transistor.) Its
characteristics make it useful as a clamp diode in inductive-load
switching.
ALUM
SOURCEn+

By contrast, a power MOSFET is a voltage-driven device whosegate terminal, Figure 1(a), is electrically isolated from its
silicon body by a thin layer of silicon dioxide (SiO2). As amajority-carrier semiconductor, the MOSFET operates at muchhigher speed than its bipolar counterpart because there is nocharge-storage mechanism. A positive voltage applied to thegate of an n-type MOSFET creates an electric field in thechannel region beneath the gate; that is, the electric charge on
the gate causes the p-region beneath the gate to convert to ann-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 thisstate. 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 surfaceinversion phenomenon, then, the operation of a MOSFET isentirely 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 asa high-input impedance, voltage-controlled device, whereas abipolar transistor is a low-input-impedance, current-controlled
device. As a majority-carrier semiconductor, a MOSFET stores nocharge, and so can switch faster than a bipolar device. Majoritycarrier 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 theaverage velocity of a carrier in terms of the electrical field
imposed on it) makes a MOSFET more resistive at elevatedtemperatures, and much more immune to the thermal-runaway
problem experienced by bipolar devices.

AN7244 Rev.3.00
October 1, 1999

p CONVERTED
TO n CHANNEL

n+

n+

n+
p

p
n-

n+

General Characteristics
A conventional n-p-n bipolar power transistor is a current-driven
device whose three terminals (base, emitter, and collector) areconnected to the silicon by alloyed metal contacts. Bipolartransistors 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.

GATE

DRAIN
(a)
D
G

N
N

+
D

+

P
–

N
+

+

G

S
S

–

+
(b)

(c)

FIGURE 1. THE MOSFET, A VOLTAGE-CONTROLLED DEVICE WITH AN
ELECTRICALLY ISOLATED GATE, USES MAJORITYCARRIERS TO MOVE CURRENT FROM SOURCE TO
DRAIN (A). THE KEY TO MOSFET OPERATION IS THE
CREATION OF THE INVERSION 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 FUNCTIONSSTRUCTURE
Intersil 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 tosource, when the device is in the on-state. When rDS(ON) is
minimized, the device provides superior power-switching
performance because the voltage drop from drain to source isalso minimized for a given value of drain-to-source current.
Since the path between drain and source is essentiallyresistive, because of the surface-inversion phenomenon, eachcell in the device can be assumed to contribute an amount,
RN, to the total resistance. An individual cell has a fairly low

Page 1 of 5

Understanding Power MOSFETs

resistance, but to minimize rDS(ON), it is necessary to put a large
number of cells in parallel on a chip. In general, therefore, thegreater the number of paralleled cells on a chip, the lower its
rDS(ON) value:rDS(ON) = RN/N, where N is the number of cells.
3000
REXT

rDS(ON) (m)1000rDS(ON)

RCHAN

RBULK

rDS(ON)
100

RBULK

30

100

1000

BVDSS (V)FIGURE 2. THE DRAIN-TO-SOURCE RESISTANCE, RDS(ON) , OF A
MOSFET IS NOT ONE BUT THREE SEPARATERESISTANCE COMPONENTSTABLE 1. PERCENTAGE RESISTANCE COMPONENTS FOR A TYPICAL
CHIP
BVDSS

The significance of RBULK in devices with a high voltage
capability is due to the fact that thick, lightly doped epi layers are
required for the drain region in order to avoid producing highelectric fields (and premature breakdown) within the device. And
as the epi layers are made thicker and less resistive to support
high voltages, the bulk component of resistance rapidly
increases (see Figure 2) and begins to dominate the channel andexternal resistance. The rDS(ON) therefore, increases withincreasing breakdown voltage capability, and low rDS(ON) mustbe sacrificed if the MOSFET is to withstand even higher
breakdown voltages.
There is a way around these obstacles. The rDS(ON) in Figure 2
holds only for a given cell and chip size. Using a larger chip
results in a lower value for rDS(ON) because a large chip has morecells (see Figure 3), shifting the vertical axis for each of theconstituent parts.

RCHAN
REXT

10

Two conclusions, inherent consequences of the laws ofsemiconductor physics, and valid for any DMOS device, can be
drawn from the preceding discussion: First, rDS(ON) obviously
increases with increasing breakdown-voltage capability of a
MOSFET. Second, minimum rDS(ON) performance must be
sacrificed if the MOSFET must withstand ever-higher breakdown
voltages.

The penalty for using a larger chip, however, is an increase incost, since chip size is a major cost factor. And because chip area
increases exponentially, not linearly, with voltage, the additional
cost can be substantial. For example, to obtain a given rDS(ON) ata breakdown voltage twice as great as the original, the new chiprequires an area four or five times larger than the original.
Although the cost does not rise exponentially, it is substantiallymore than the original cost.

40V

150V

500V

EFFECTS OF TEMPERATURE

RCHANNEL

50%

23%

2.4%

RBULK

35%

70%

97%

REXTERNAL

15%

7%

<1%

The high operating temperatures of bipolar transistors are
frequent cause of failure. The high temperatures are caused by
hot-spotting, the tendency of current in a bipolar device toconcentrate in areas around the emitter. Unchecked, this hotspotting results in the mechanism of thermal runaway, andeventual destruction of the device. MOSFETs do not suffer thisdisadvantage because their current flow is in the form of majority
carriers. The mobility of majority carriers (where, again, mobility
is a term that defines the average velocity of a carrier in terms of
the electrical field imposed on it) is temperature dependent insilicon: mobility decreases with increasing temperature. This
inverse relationship dictates that the carriers slowdown as thechip gets hotter. In effect, the resistance of the silicon path isincreased, which prevents the concentrations of current that leadto hot spots. In fact, if hot spots do attempt to form in a MOSFET,
the local resistance increases and defocuses or spreads out thecurrent, rerouting it to cooler portions of the chip.

In reality, rDS(ON) is composed of three separate resistances.
Figure 2 shows a curve of the three resistive components for asingle cell and their contributions to the overall value of rDS(ON).
The value of rDS(ON) at any point of the curve is found by adding
the values of the three components at that point:rDS(ON) = RBULK + RCHAN + REXT
where RCHAN represents the resistance of the channel beneaththe gate, and REXT includes all resistances resulting from thesubstrate, solder connections, leads, and the package. RBULK
represents the resistance resulting from the narrow neck of nmaterial between the two layers, as shown in Figure 1(a), plus theresistance of the current path below the neck and through the
body to the substrate region of the device.
Note in Figure 2 that RCHAN and REXT are completely
independent of voltage, while RBULK is highly dependent on
applied voltage. Note also that below about 150 volts, rDS(ON) is
dominated by the sum of RCHAN and REXT. Above 150 volts,rDS(ON) is increasingly dominated by RBULK. Table 1 gives apercentage breakdown of the contribution of each resistance forthree values of voltage.

AN7244 Rev.3.00
October 1, 1999

Page 2 of 5

Understanding Power MOSFETs

10
6
4
2

Because the gate current is so small, the input impedance of a
MOSFET is extremely high (in the megohm range) and, in fact, islargely capacitive rather than resistive (because of the isolationof the gate terminal).

SMALLESTCHIP

Figure 5 illustrates the basic input circuit of a MOSFET. The elements
are equivalent, rather than physical, resistance, R, and capacitance,
C. The capacitance, called CISS on MOSFET data sheets, is acombination of the device's internal gate-to-source and gate-to-drain
capacitance. The resistance, R, represents the resistance of thematerial in the gate circuit. Together, the equivalent R and C of the
input circuit will determine the upper frequency limit of MOSFET
operation.

rDS(ON) ()

1
0.6
0.4
0.2

LARGEST
CHIP

0.1
0.06
0.04
0.02
0.01
0

100

200

300

400

500

600

BVDSS (V)D

FIGURE 3. AS CHIP SIZE INCREASES, RDS(ON) DECREASES

R

GS

Because of the character of its silicon structure, a MOSFET has apositive temperature coefficient of resistance, as shown by the
curves of Figure 4.

CISS

4
ID = 4A
VGS = 10VrDS(ON) ()3

FIGURE 5. A MOSFETS SWITCHING SPEED IS DETERMINED BY
ITS INPUT RESISTANCE R AND ITS INPUTCAPACITANCE CISS

2

OPERATING FREQUENCY

1

0
-50

0

50

100

150

200

JUNCTION TEMPERATURE - TJ (oC)FIGURE 4. MOSFETS HAVE A POSITIVE TEMPERATURE
COEFFICIENT OF RESISTANCE, WHICH GREATLY
REDUCES THE POSSIBILITY OF THERMAL RUNAWAYAS TEMPERATURE INCREASESThe positive temperature coefficient of resistance means that a
MOSFET is inherently more stable with temperature fluctuation,and provides its own protection against thermal runaway andsecond breakdown. Another benefit of this characteristic is that
MOSFETs can be operated in parallel without fear that one device
will rob current from the others. If any device begins to overheat,
its resistance will increase, and its current will be directed away
to cooler chips.

GATE PARAMETERS
To permit the flow of drain-to-source current in an n-type MOSFET,
a positive voltage must be applied between the gate and source
terminals. Since, as described above, the gate is electricallyisolated from the body of the device, theoretically no current canflow from the driving source into the gate. In reality, however, avery small current, in the range of tens of nanoamperes, does
flow, and is identified on data sheets as a leakage current, IGSS.

AN7244 Rev.3.00
October 1, 1999

Most DMOS processes use a polysilicon gate structure rather
than the metal-gate type. If the resistance of the gate structure(R in Figure 5) is high, the switching time of the DMOS device isincreased, thereby reducing its upper operating frequency.
Compared to a metal gate, a polysilicon gate has a higher gate
resistance. This property accounts for the frequent use of metalgate MOSFETs in high-frequency (greater than 20MHz)
applications, and polysilicon-gate MOSFETs in higher-power but
lower-frequency systems.
Since the frequency response of a MOSFET is controlled by theeffective R and C of its gate terminal, a rough estimate can bemade of the upper operating frequency from datasheet
parameters. The resistive portion depends on the sheet
resistance of the polysilicon-gate overlay structure, a value ofapproximately 20 ohms. But whereas the total R value is notfound on datasheets, the C value (CISS) is; it is recorded as both amaximum value and in graphical form as a function of drain-tosource voltage. The value of CISS is closely related to chip size;
the larger the chip, the greater the value. Since the RC
combination of the input circuit must be charged and discharged
by the driving circuit, and since the capacitance dominates,
larger chips have slower switching times than smaller chips, andare more useful in lower-frequency circuits. In general, the upper
frequency limit of most power MOSFETs spans a fairly broad
range, from 1MHz to 10MHz.

Page 3 of 5

Understanding Power MOSFETs

Probably the most used MOSFET graphical data is the output
characteristics or a plot of drain-to-source current (IDS) as afunction of drain-to-source voltage (VDS). A typical characteristic,
shown in Figure 6, gives the drain current that flows at various
VDS values as a function of the gate-to-source voltage (Vgs). The
curve is divided into two regions: a linear region in which VDS issmall and drain current increases linearly with drain voltage, anda saturated region in which increasing drain voltage has no effecton drain current (the device acts as a constant-current source).
The current level at which the linear portion of the curve joinswith the saturated portion is called the pinch-off region.

DRIVE REQUIREMENTS
When considering the Vgs level required to operate a MOSFET,
note from Figure 6, that the device is not turned on (no draincurrent flows) unless Vgs is greater than a certain level (called thethreshold voltage). In other words, the threshold voltage must be
exceeded before an appreciable increase in drain current can be
expected. Generally Vgs for many types of DMOS devices is atleast 2V. This is an important consideration when selecting
devices or designing circuits to drive a MOSFET gate: the gatedrive circuit must provide at least the threshold-voltage level, butpreferably, a much higher one.

As Figure 6 shows, a MOSFET must be driven by a fairly highvoltage, on the order of 10V, to ensure maximum saturated
drain-current flow. However, integrated circuits, such as TTL
types, cannot deliver the necessary voltage levels unless they aremodified with external pull-up resistors. Even with a pull-up to 5V,
a TTL driver cannot fully saturate most MOSFETs. Thus, TTL
drivers are most suitable when the current to be switched is far
less than the rated current of the MOSFET. CMOS ICs can run
from supplies of 10V, and these devices are capable of driving a
MOSFET into full saturation. On the other hand, a CMOS driverwill not switch the MOSFET gate circuit as fast as a TTL driver. The
best results, whether TTL or CMOS ICs provide the drive, areachieved when special buffering chips are inserted between the
IC output and gate input to match the needs of the MOSFET gate.
VG = 20V

VG = 8V

30
VG = 10V

25
DRAIN CURRENT - ID (A)OUTPUT CHARACTERISTICS

VG = 7V
20

PULSE TESTPULSE DURATION = 80S
DUTY CYCLE  2%
TC = 25oC
VG = 6V

15

10
VG = 5V

5

VG = 4V
0
0

2

4

6

8

10

12

14

DRAIN-TO-SOURCE VOLTAGE - VDS (V)FIGURE 6. MOSFETS REQUIRE A HIGH INPUT VOLTAGE (ATLEAST 10V) IN ORDER TO DELIVER THEIR FULLRATED DRAIN CURRENT

AN7244 Rev.3.00
October 1, 1999

Page 4 of 5

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