FET JFET Operating Characteristics: VGS = 0 V · The p-channel JFET behaves the same as the...

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CH 113 ��12

VGS = 0 and VDS increases from 0 to a more positive voltage:• Gate and Source terminals: at the same potential

• Drain: at positive potential => reverse biased

• Gate: at negative potential => reverse biased

• Source: negative potential => forward biased

The depletion region is wider near the top of both

p-type materials compared to lower parts

JFET Operating Characteristics: VGS = 0 V 14

difference of potential

FET

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The p-n junction is reversed biased for the length of the channel

results IG = 0A



FET

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• +VDS ↑ : ID ↑1. +VDS ↑ 0 to a few volts: ID ↑ linearly

(channel resistance is essentially constant)

2. +VDS ↑ further to a level VP: depletion region widens

=> noticeable reduction in channel width

=> reduced channel resistance

i.e., reduced path of conduction

finally, ID stops increasing

3. +VDS ↑ VP and above: ID constant (saturation level)



FET

CH 113 ��12

VGS = 0 and VDS increases from 0 to a more positive voltage:

JFET Operating Characteristics: VGS = 0 V 17FET

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• With VP ↑ the region of close encounter b/w two depletion regions increases in

length along the channel

• At VP in reality a very small channel still exists, with a very high density of

current

JFET Operating Characteristics: VGS = 0 V 18FET

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• JFET behaves like a current source

• IDSS : maximum drain current for a JFET and is defined by the conditions

VGS = 0V and VDS > |VP|


DS PV V>

FET

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VGS = -ve and VDS increases from 0 to a more positive voltage:

• As VGS becomes more negative, the depletion region increases

• Saturation level of ID reduced

• For more –ve VGS, saturation

levels of ID reduced

JFET Operating Characteristics 20

0 ; 0 GS DS

V V V to ve value< = +

FET

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As VGS becomes more negative:

• The JFET experiences pinch-off at a lower voltage (VP).

• ID decreases (ID < IDSS) even though VDS is increased.

• Eventually ID reaches 0 A. VGS at this point is called Vp or VGS(off)

Also note that at high levels of VDS the JFET reaches a breakdown

situation. ID increases uncontrollably if VDS > VDSmax

JFET Operating Characteristics 21FET

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.

JFET Operating Characteristics 22FET

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Voltage Controlled Resistor:

JFET Operating Characteristics 23

2

1

o

d

GS

P

rr

V

V

=

−

The resistance at a

particular level of

VGS

The resistance with

VGS = 0V

FET

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p-channel JFETs 24

The p-channel JFET behaves the same as the n-channel JFET,

except the voltage polarities and current directions are reversed.

FET

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p-channel JFETs 25

Also note that at high levels of VDS the JFET reaches a breakdown situation: ID

increases uncontrollably if VDS > VDSmax

As VGS increases more positively

• The depletion zone

increases

• ID decreases (ID < IDSS)

• Eventually ID = 0 A

FET

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Symbol 26FET

n-channel JFET symbol p-channel JFET symbol

CH 113 ��12

Summary 27FET

o The maximum current is

defined as IDSS and occurs

when VGS = 0V and VDS ≥ |VP|:

o For gate-to-source voltage VGS

less than (more negative than)

the pinch-off level, the drain

current is 0A (ID = 0A):

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Summary 28FET

o For all levels of VGS between 0V and the pinch-off level, the current ID

will range between IDSS and 0A, respectively:

o A similar list can be developed for p-channel JFETs.

CH 113 ��12

FET

Transfer

Characteristics

29

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Transfer Characteristics of JFETs 30FET

o TC of a device: input-to-output characteristics

o For BJT β relates the IB (input) and IC(output)

o A linear relationship b/w both parameters

FET relationship:

o Defined by ‘Shockley’s equation’

o Non-linear relationship b/w ID and VGS (due to square)

o The curve grows exponentially with decreasing magnitude of VGS

BCI Iβ=

constant

Controlvariable

GS

PDSS V

2

1V

D II −

=

constant

Controlvariable

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Plotting Transfer Characteristics of JFETs 31FET

o Transfer characteristic curve of a JFET:

→ a curve of ID vs VDS for VGS (0V to VP)

o Three methods:

1. Drain characteristic curve

2. Shockley’s equation

3. Shortcut method

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1. Uisng Drain characteristic curve:

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1. Using Drain characteristic curve :

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1. Using Drain characteristic curve:

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A parabolic curve results

as the changes in VGS is

uniform while the resulting

current (ID) changes non-linearly

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Transfer Characteristic Curve of a JFET

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2. Using Shockley’s Equation:

o Required: IDSS and VP (from data sheets)

=> (VGS , IDSS)

=> (VP ,0)

Solving for VGS = 0V ID = IDSS

2

P

GSDSSD

V

V1II

−−−−====

Step 1

Solving for VGS = Vp (VGS(off)) ID = 0A

2

P

GSDSSD

V

V1II

−−−−====

Step 2

Solving for VGS = 0V to Vp

2

P

GSDSSD

V

V1II

−−−−====

Step 3

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3. Using Shockley’s Equation (Shortcut method):

o Required: IDSS and VP (from data sheets)

VGS ID

0 IDSS

0.3VP IDSS/2

0.5VP IDSS/4

VP 0mA

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Example 6.1:

Sketch the transfer function curve define by IDSS = 12 mA and VP = − 6V.

Example 6.2:

Sketch the transfer function curve for a p-channel device with IDSS = 4 mA

And VP = 3V.

FET JFET Operating Characteristics: VGS = 0 V · The p-channel JFET behaves the same as the...

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Transcript of FET JFET Operating Characteristics: VGS = 0 V · The p-channel JFET behaves the same as the...