©2002 Fairchild Semiconductor Corporation

Application Note 7500 Rev. A1

Because of the character of its silicon structure, a MOSFET

has a positive temperature coefficient of resistance, as

shown by the curves of Figure 4.

The positive temperature coefficient of resistance means

that a MOSFET is inherently more stable with temperature

fluctuation, and provides its own protection against thermal

runaway and second 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 electrically isolated from the body of the device,

theoretically no current can flow from the driving source into

the gate. In reality, however, a very small current, in the

range of tens of nanoamperes, does flow, and is identified on

current is so small, the input impedance of a MOSFET is

extremely high (in the megohm range) and, in fact, is largely

capacitive rather than resistive (because of the isolation of

the gate terminal).

Figure 5 illustrates the basic input circuit of a MOSFET. The ele-

ments are equivalent, rather than physical, resistance, R, and

data sheets, is a combination of the device's internal gate-to-

source and gate-to-drain capacitance. The resistance, R, repre-

sents the resistance of the material in the gate circuit. Together,

the equivalent R and C of the input circuit will determine the

upper frequency limit of MOSFET operation.

Operating Frequency

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 is increased, thereby reducing its upper oper-

ating frequency. Compared to a metal gate, a polysilicon

gate has a higher gate resistance. This property accounts for

the frequent use of metal-gate 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

the effective R and C of its gate terminal, a rough estimate

can be made of the upper operating frequency from

datasheet parameters. The resistive portion depends on the

sheet resistance of the polysilicon-gate overlay structure, a

value of approximately 20 ohms. But whereas the total R

recorded as both a maximum value and in graphical form as

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 will have slower

switching times than smaller chips, and are, therefore, 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.

0

100

200

300

400

500

600

10

6

4

2

1

0.6

0.4

0.2

0.1

0.06

0.04

0.02

0.01

CHIP

CHIP

LARGEST

SMALLEST

4

3

2

1

0

-50

0

50

100

150

200

FIGURE 4. MOSFETs HAVE A POSITIVE TEMPERATURE

COEFFICIENT OF RESISTANCE, WHICH

GREATLY REDUCES THE POSSIBILITY OF

THERMAL RUNAWAY AS TEMPERATURE

INCREASES

G

S

D

R

FIGURE 5. A MOSFETs SWITCHING SPEED IS DETERMINED

BY ITS INPUT RESISTANCE R AND ITS INPUT

Application Note 7500