AN211A Motorola, AN211A Datasheet

AN211A

Manufacturer Part Number

AN211A

Description

AN211AFIFELD EFFECT TRANSISTORS IN THEORY AND PRACTICE

Manufacturer

Motorola

Datasheet

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SEMICONDUCTOR APPLICATION NOTE

INTRODUCTION

Field-Effect Transistor (JFET) and the “Metal-Oxide

Semiconductor” Field-Effect Transistor (MOSFET), or

Insulated-Gate Field-Effect Transistor (IGFET). The

principles on which these devices operate (current controlled

by an electric field) are very similar — the primary difference

being in the methods by which the control element is made.

This difference, however, results in a considerable difference

in device characteristics and necessitates variances in circuit

design, which are discussed in this note.

JUNCTION FIELD-EFFECT TRANSISTOR (JFET)

with nothing more than a bar of doped silicon that behaves

as a resistor (Figure 1a). By convention, the terminal into

which current is injected is called the source terminal, since,

as far as the FET is concerned, current originates from this

terminal. The other terminal is called the drain terminal.

Current flow between source and drain is related to the

drain-source voltage by the resistance of the intervening

material. In Figure 1b, p-type regions have been diffused into

the n-type substrate of Figure 1a leaving an n-type channel

between the source and drain. (A complementary p-type

device is made by reversing all of the material types.) These

p-type regions will be used to control the current flow

between the source and the drain and are thus called gate

regions.

the p-n junctions when the junctions are reverse biased

(Figure 1c). As the reverse voltage is increased, the

depletion regions spread into the channel until they meet,

creating an almost infinite resistance between the source and

the drain.

(Figure 1d) with zero gate voltage, drain current flow in the

channel sets up a reverse bias along the surface of the gate,

parallel to the channel. As the drain-source voltage

increases, the depletion regions again spread into the

channel because of the voltage drop in the channel which

reverse biases the junctions. As V

depletion regions grow until they meet, whereby any further

increase in voltage is counterbalanced by an increase in the

depletion region toward the drain. There is an effective

REV 0

MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION

Motorola, Inc. 1993

There are two types of field-effect transistors, the Junction

In its simplest form the junction field-effect transistor starts

As with any p-n junction, a depletion region surrounds

If an external voltage is applied between source and drain

Freescale Semiconductor, Inc.

For More Information On This Product,

is increased, the

Go to: www.freescale.com

increase in channel resistance that prevents any further

increase in drain current. The drain-source voltage that

causes this current limiting condition is called the “pinchoff”

voltage (V

produces only a slight increase in drain current.

voltage (V

in Figure 2a. In the low-current region, the drain current is

linearly related to V

to deplete and the slope of the I

the V

constant until drain-to-gate avalanche, V

If a reverse voltage is applied to the gates, channel pinch-off

occurs at a lower I

region spread caused by the reverse-biased gates adds to

that produced by V

for any value of V

The variation in drain current (I

Figure 2. Drain Current Characteristics

is equal to V

Figure 1. Development of Junction

Ç Ç Ç

Ç Ç

Ç Ç Ç

Ç Ç

). A further increase in drain-source voltage

) at zero gate-source voltage (V

Field-Effect Transistors

È È

level (Figure 2b) because the depletion

. As I

. Thus reducing the maximum current

, I

È È È È

È È

“saturates” and stays relatively

increases, the “channel” begins

Ç Ç Ç

Ç Ç Ç Ç

Ç Ç

curve decreases. When

È È

) with drain-source

Order this document

BR(DSS)

Ç Ç Ç Ç

Ç Ç

È È

by AN211A/D

is reached.

) is shown

È È

Related parts for AN211A

AN211A Summary of contents

Page 1

... Figure 1. Development of Junction is increased, the Figure 2. Drain Current Characteristics Go to: www.freescale.com Order this document by AN211A/D ) with drain-source shown increases, the “channel” begins DS D curve decreases. When “ ...

Page 2

... Figure 4. Development of Enhancement-Mode N-Channel MOSFET MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION Go to: www.freescale.com Ë Ë Ë Ë Ë Ë Ë Ë Ë Ë Ë Ë ...

Page 3

... Depletion mode, as previously mentioned, refers to the decrease of carriers in the channel due to variation in gate voltage. Enhancement mode refers MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION For More Information On This Product, 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 ...

Page 4

... These are usually specified as V Gate 1 — source cutoff voltage (with Gate 2 connected to source), and V G2S(off) Gate 1 connected to source). The gate voltage required for drain current cutoff with one of the gates connected to the MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION Go to: www.freescale.com – G1S(off) ...

Page 5

... Junction breakdown is indicated by an increase in gate current (beyond I ) which signals the beginning of GSS avalanche. MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION For More Information On This Product, Some reflection will reveal that for junction FETs, the V specification really provides the same information (BR)DGO as V ...

Page 6

... Freescale Semiconductor, Inc. Figure 10. Static Characteristics for the Three FET Types Are Defined by the Above Curves, Tables, and Test Circuits Figure 11. Test Circuit for Leakage Current 6 For More Information On This Product, MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION Go to: www.freescale.com † † ...

Page 7

... D(on) DSS fs imaginary components, but is dominated by the real component at low frequency, the 1 kHz characteristic is given as an absolute magnitude and indicated as y MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION For More Information On This Product interesting to note that y I due to nonlinearity in the I D variation, for a typical n-channel, JFET is illustrated in Figure 14 ...

Page 8

... Typical graphs of such variations are illustrated in . Like Figure 19 for the 2N5458. From graphs of this kind the designer can anticipate the noise level inherent in his design. MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION Go to: www.freescale.com and y is the amplification fac ...

Page 9

... The magnitude of the ac voltage should be kept low so that there will be no pinchoff in the channel. Insulated-gate FETs may be measured with dc gate bias in the enhancement mode. MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION For More Information On This Product, Figure 20. Circuit for Measuring JFET APPLICATIONS Device Selection ...

Page 10

... With its high input impedance, the field-effect transistor iss will play an important role in input circuitry for instrumentation and audio applications where low-impedance junction transistors have generally been least successful iss rrs Figure 21. RF Stage of Broadcast Auto Radio Figure 22. Line Operated Phono Amplifiers MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION Go to: www.freescale.com ...

Page 11

... Freescale Semiconductor, Inc. MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION For More Information On This Product, Figure 23. FET Chopper Circuits Go to: www.freescale.com 11 ...

Page 12

... JAPAN: Nippon Motorola Ltd.; 4-32-1, Nishi-Gotanda, Shinagawa-ku, Tokyo 141, Japan. ASIA PACIFIC: Motorola Semiconductors H.K. Ltd.; Silicon Harbour Center, No. 2 Dai King Street, Tai Po Industrial Estate, Tai Po, N.T., Hong Kong. 12 For More Information On This Product, MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION Go to: www.freescale.com AN211A/D ...

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