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AN211A

Fifeld Effect Transistors in Theory and Practice

Motorola => Freescale 

AN211A

Freescale Semiconductor, Inc.

SOURCE GATE

DRAIN

(-)

SiO2

-------------

ÍÍÍÍÍÍÍÍÍÍÍÍ + + + + + + + + + + + + +

ÍÍÍÍÍÍÍÍÍÍÍÍ ±±±±±±±±±±±±±

ÍÍÍÍÍÍÍÍÍÍÍÍ + ++ ++ ++ ++ ++ ++

Si3N4

N+ N

+++

N+

P (SUBSTRATE)

Figure 8. Channel Depletion Phenomenon.

Application of Negative Gate Voltage Causes

Redistribution of Minority Carriers in Diffused Channel

and Reduces Effective Channel Thickness. This Results

in Increased Channel Resistance.

ELECTRICAL CHARACTERISTICS

Because the basic mode of operation for field-effect

devices differs greatly from that of conventional junction

transistors, the terminology and specifications are

necessarily different. An understanding of FET terminology

and characteristics are necessary to evaluate their

comparative merits from data-sheet specifications.

Static Characteristics

Static characteristics define the operation of an active

device under the influence of applied dc operating conditions.

Of primary interest are those specifications that indicate the

effect of a control signal on the output current. The VGS –

ID transfer characteristics curves are illustrated in Figure 9

for the three types of FETs. Figure 10 lists the data-sheet

specifications normally employed to describe these curves,

as well as the test circuits that yield the indicated

specifications.

Of additional interest is the special case of

tetrode-connected devices in which the two gates are

separately accessible for the application of a control signal.

The pertinent specifications for a junction tetrode are those

which define drain-current cutoff when one of the gates is

connected to the source and the bias voltage is applied to

the second gate. These are usually specified as VG1S(off),

Gate 1 — source cutoff voltage (with Gate 2 connected to

source), and VG2S(off), Gate 2 — source cutoff voltage (with

Gate 1 connected to source). The gate voltage required for

drain current cutoff with one of the gates connected to the

DEPLETION

NĆCHANNEL

ID

ENHANCEMENT

NĆCHANNEL JUNCTION FET

VGS = 1 V/STEP

VGS = 0

DEPLETION ONLY

NOT DEFINED

IN THIS AREA

(-)

VGS

ID

DEPLETION

(+)

ENHANCEMENT

DEPLETION AND ENHANCEMENT MODE

(-)

VGS

(+)

DEPLETION

ID

ENHANCEMENT

ENHANCEMENT ONLY

-1V

-2V

0

VDS = 2 V/DIV

NĆCHANNEL MOS 2N3797

VGS = 1 V/STEP

VGS = 2 V

1V

0

-1V

-2V

0

VDS = 2 V/DIV

NĆCHANNEL MOS 2N4351

VGS = 1 V/STEP

VGS = 4 V

3V

2V

(-)

VGS

VGS(th)

(+)

0

VDS = 2 V/DIV

Figure 9. Transfer Characteristics and Associated Scope Traces for the Three FET Types

4

For More InformMaOtiToOnROOLnATShEiMsICPOrNoDdUuCcTtO, R APPLICATION INFORMATION

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