Chapter 4 characteristics of field effect transistors

Val de Loire Program p.57 CHAPTER 4: CHARACTERISTICS OF FIELD-EFFECT TRANSISTOR Table of Contents 4.1.. CHAPTER 4: Characteristics Field-Effect Transistor CHAPTER 4: CHARACTERISTIC

Val de Loire Program p.57

CHAPTER 4:

CHARACTERISTICS OF FIELD-EFFECT

TRANSISTOR

Table of Contents

4.1 INTRODUCTION 58

4.2 JFET CONSTRUCTION AND SYMBOLS 58

4.3 JFET TERMINAL CHARACTERISTICS 59

4.4 JFET BIAS LINE AND LOAD LINE 62

4.5 MOSFET CONSTRUCTION AND SYMBOLS 63

4.6 MOSFET TERMINAL CHARACTERISTICS 64

Table of Figures Fig 4-1 JFET Constructions and Symbols 59

Fig 4-2 JFET terminal characteristics 61

Fig 4-3 JFET amplifier bias 62

Fig 4-4 MOSFET construction and symbol 64

Fig 4.5 MOSFET terminal characteristics 65

Fig 4-6 MOSFET amplifier bias 66

CHAPTER 4: Characteristics Field-Effect Transistor

CHAPTER 4:

CHARACTERISTICS OF FIELD-EFFECT

TRANSISTOR

4.1 INTRODUCTION

The operation of the field-effect transistor (FET) can be explained in

terms of only majority-carrier (one-polarity) charge flow; the transistor is

therefore called unipolar

Two kinds of effect devices are widely used: the junction field-effect transistor (JFET) and the metal-oxide semiconductor field-field-effect transistor (MOSFET)

4.2 JFET CONSTRUCTION AND SYMBOLS

Conduction is by the passage of charge carriers from source (S) to

drain (D) through the channel between the gate (G) elements

The transistor can be an n-channel device (conduction by electrons)

or a p-channel device (conduction by holes); a discussion of n-chanel devices applies equally to p-channel devices if complementary (opposite

in sign) voltages and currents are used

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Fig 4-1 JFET Constructions and Symbols 4.3 JFET TERMINAL CHARACTERISTICS

Output or drain charactersistics for an n-channel JFET in

common-source (CS) connection with v GS  0

CHAPTER 4: Characteristics Field-Effect Transistor

For a constant value of v , JFET acts as a linear resistive device (in GS

the ohmic region) until the depletion region of the reverse-biased gate-source junction extends the width of the channel (a condition called

pinchoff)

Above pinchoff but below avalanche breakdown, drain current i D

remains nearly constant as v is increased The shorted-gate parameters DS

DSS

I and V are defined as indicated in Fig 4-2(a); typically 0 V is 0

between 4 and 5 V

As gate potential decreases, the pinchoff voltage, that is, the source-to-drain voltage V at which pinchoff occurs, also decreases, p

approximately obeying the equation:

0

V V v

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Fig 4-2 JFET terminal characteristics

The drain current shows an approximate square-law dependence on source-to-gate voltage for constant values of v in the pinchoff region: DS

2

0

1 GS

p

v

i I

V

CHAPTER 4: Characteristics Field-Effect Transistor

4.4 JFET BIAS LINE AND LOAD LINE

Fig 4-3 JFET amplifier bias

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The commonly used voltage-divider bias arrangement of Fig 4-3(a)

can be reduced to its equivalent in Fig 4-3 (b), where the Thévenin parameters are given by:

1 2

G

R R R



 and

1

R



With i  , application of KVL around the gate-source loop of Fig G 0

4-3(b) yields the equation of the transfer bias line,

D

i

Which can be solved simultaneously with transfer characteristics or

plotted as indicated on Fig 4-2(b) to yield I DQ and V GSQ, two of the necessary three quiescent variables

Application of KVL around the drain-source loop of Fig.4-3(b) leads

to the equation of the dc load line,

D

i

So: V DSQV DD R S R I D DQ

4.5 MOSFET CONSTRUCTION AND SYMBOLS

The n-channel MOSFET has only a single p region (called the

substrate), one side of which acts as a conducting channel A metallic

gate is separated from the conducting channel by an insulating metal oxide (usually SiO2), where the name insulated-gate FET (IGFET) for the

CHAPTER 4: Characteristics Field-Effect Transistor

device The p-channel MOSFET, formed by interchanging p and n

semiconductor materials, is described by complementary voltages and currents

Fig 4-4 MOSFET construction and symbol 4.6 MOSFET TERMINAL CHARACTERISTICS

In an n-channel MOSFET, the gate (positive plate), metal oxide film

(dielectric), and substrate (negative plate) form a capacitor, the electric field of which controls channel resistance

When the positive of the gate reaches a threshold voltage

T

V (typically 2 to 4 V), sufficient free electrons attracted to the region

immediately beside the metal oxide film (this is called

enhancement-mode operation) to induce a conducting channel of low resistivity If the

source-to-drain voltage is increased, in the JFET

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Fig 4.5 MOSFET terminal characteristics

CHAPTER 4: Characteristics Field-Effect Transistor

Fig 4-6 MOSFET amplifier bias

The enhancement-mode MOSFET, operating in the pinchoff region, and if the substrate is shorted to the source Then:

2

1 GS

T

v

i I

V

where v GS V T