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AN211A

Fifeld Effect Transistors in Theory and Practice

Motorola => Freescale 

Freescale Semiconductor, Inc.

AN211A

SOURCE GATE (+)

DRAIN

ALUMINUM

Si3N4

SiO2

+++++++++

ÍÍÍÍÍÍÍÍÍÍÍÍ %%%%%%%%%

ÍÍÍÍÍÍÍÍÍÍÍÍ + + + + + + + + +

ÍÍÍÍÍÍÍÍÍÍÍÍ - - - - - - - - -

N+

N+

INDUCED

CHANNEL

P (SUBSTRATE)

Figure 5. Channel Enhancement. Application of

Positive Gate Voltage Causes Redistribution of Minority

Carriers in the Substrate and Results in the Formation

of a Conductive Channel Between Source and Drain

An equivalent circuit for the MOSFET is shown in Figure

6. Here, Cg(ch) is the distributed gate-to-channel capacitance

representing the nitride-oxide capacitance. Cgs is the

gate-source capacitance of the metal gate area overlapping

the source, while Cgd is the gate-drain capacitance of the

metal gate area overlapping the drain. Cd(sub) and Cs(sub)

are junction capacitances from drain to substrate and source

to substrate. Yfs is the transadmittance between drain current

and gate-source voltage. The modulated channel resistance

is rds. RD and RS are the bulk resistances of the drain and

source.

The input resistance of the MOSFET is exceptionally high

because the gate behaves as a capacitor with very low

leakage (rin [ 1014 Ω). The output impedance is a function

of rds (which is related to the gate voltage) and the drain

and source bulk resistances (RD and RS).

To turn the MOSFET “on”, the gate-channel capacitance,

Cg(ch), and the Miller capacitance, Cgd, must be charged.

In turning “on”, the drain-substrate capacitance, Cd(sub), must

be discharged. The resistance of the substrate determines

the peak discharge current for this capacitance.

The FET just described is called an enhancement-type

MOSFET. A depletion-type MOSFET can be made in the

following manner: Starting with the basic structure of Figure

4, a moderate resistivity n-channel is diffused between the

source and drain so that drain current can flow when the

gate potential is at zero volts (Figure 7). In this manner, the

MOSFET can be made to exhibit depletion characteristics.

For positive gate voltages, the structure enhances in the

same manner as the device of Figure 4. With negative gate

voltage, the enhancement process is reversed and the

channel begins to deplete of carriers as seen in Figure 8.

As with the JFET, drain-current flow depletes the channel

area nearest the drain first.

The structure of Figure 7, therefore, is both a

depletion-mode and an enhancement-mode device.

MODES OF OPERATION

to the increase of carriers in the channel due to application

of gate voltage. A third type of FET that can operate in both

the depletion and the enhancement modes has also been

described.

The basic differences between these modes can most

easily be understood by examining the transfer

characteristics of Figure 9. The depletion-mode device has

considerable drain-current flow for zero gate voltage. Drain

current is reduced by applying a reverse voltage to the gate

terminal. The depletion-type FET is not characterized with

forward gate voltage.

The depletion/enhancement mode type device also has

considerable drain current with zero gate voltage. This type

device is defined in the forward region and may have usable

forward characteristics for quite large gate voltages. Notice

that for the junction FET, drain current may be enhanced

by forward gate voltage only until the gate-source p-n

junction becomes forward biased.

The third type of FET operates only in the enhancement

mode. This FET has extremely low drain current flow for zero

gate-source voltage. Drain current conduction occurs for a

VGS greater than some threshold value, VGS(th). For gate

voltages greater than the threshold, the transfer

characteristics are similar to the depletion/enhancement

mode FET.

SOURCE GATE

DRAIN

RS

CS(sub)

Cgs Cgd

Cg(ch)

rds

Yfs VGS

P SUBSTRATE

RD

CD(sub)

Figure 6. Equivalent Circuit of Enhancement-

Mode MOSFET

SOURCE

DIFFUSED

CHANNEL

GATE

DRAIN

ALUMINUM

SiOSÍÍÍi23N4ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍN ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ

N+

N+

P (SUBSTRATE)

There are two basic modes of operation of FET’s —

depletion and enhancement. Depletion mode, as previously

mentioned, refers to the decrease of carriers in the channel

due to variation in gate voltage. Enhancement mode refers

Figure 7. Depletion Mode MOSFET Structure.

This Type of Device May Be Designed to Operate in

Both the Enhancement and Depletion Modes

MOTOROLA SEMICONDUCTOR AFPPoLrICMAoTrIOeNInINfFoOrmRMaAtTioIOnNOn This Product,

3

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