FET-Electronics-Lecture Slides, Slides of Electronics

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Department of Electrical Engineering

FET (Field Effect Transistors) 2

4

FET

JFET

MOSFET

BJT FET

BJT transistor is a current-controlled device. Example : The current IC in Fig. is a direct function of the level of IB JFET transistor is a voltage-controlled device Example: The current ID will be a function of the voltage VGS applied to the input circuit There are npn and pnp bipolar transistors There are n-channel and p-channel field-effect transistors BJT transistor is a bipolar device. Bi- revealing that the conduction level is a function of two charge carriers, electrons and holes The FET is a unipolar device depending solely on either electron ( n-channel) or hole (p- channel) conduction. Input resistance levels of the BJT transistor is less compared to FET FET has high input impedance. At a level of 1 to several 100 meg ohms. BJT transistor has a much higher sensitivity to changes in the applied signal. Variation in output current is typically a great deal more for BJTs Variation in output current is typically a not great deal in FETs for the change in applied voltage BJTs are less temperature stable FETs are more temperature stable BJTS usually have more spacius construction. FETs are usually smaller in construction 5 docsity.com

Conditions for JFET Operation

V

GS

= 0V , V

DS

> 0 V.

G 7

V

GS

= 0V , V

DS

= V

p

(Pintch – off Voltage )

Why IDSS exist if junction is block ?????

I DSS  Drain-to-Source current with a Short-circuit connection from gate to source

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Voltage-Controlled Resistor

 The region to the left of the pinch-off locus is referred to as

the ohmic or voltage-controlled resistance region.

 In this region the JFET can actually be employed as a variable

resistor whose resistance is controlled by the applied gate-to-

source voltage.

 The resistance of the device between drain and source for

V

DS

< V

P

is a function of the applied voltageV

GS

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 As V

GS

becomes more and more negative, the slope of each curve

becomes more and more horizontal, corresponding with an

increasing resistance level.

where r

o

is the resistance withV

GS

= 0V ,

and

r

d

the resistance at a particular level of V

GS 11

Characteristic curve of p- channel JFET 13

Symbol Denotation

 N-Type

 P-Type

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 For gate-to-source voltages V

GS

less than (more negative than)

the pinch-off level, the drain current is 0 A (I

D

= 0 A).

 For all levels of V

GS

between 0 V and the pinch-off level, the

current I

D

will range between IDSS and 0 A,

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Transfer Characteristics

 For the BJT transistor the output current I

C

and input

controlling current I

B

were related by beta β.

 But for JFET we have a Shockley’s Equation.

The squared term of the equation will result in a nonlinear relationship between ID and VGS, producing a curve that grows exponentially with decreasing magnitudes of VGS. 17

Verifying Shokleys Equation. ( *) 19

Similarly V

GS

can be obtained from Eq. (*)

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