Filed Effect Transistors (FETs)

Chapter 7

JFET

VGS Effect

G-S junction is reverse-biased with negative voltage (VG) depletion region

VG less than zero VG controls the drain current, ID

Higher VG narrows the channel Larger depleted region

– Larger barrier voltage larger resistance less current through ID

Less VG wider channel larger ID

Note: In this case VGG = VGS

N and P-type JFETs

N-type – VDD>= 0– VGS<=0– VGS(cutoff) < 0; ID=0

P-type– VDD<= 0– VGS>=0– VGS(cutoff) > 0; ID=0

Drain Characteristic: VDS-ID Curve

VGS is shorted VGS=0 IDSS is the maximum drain

current allowed at the Vp (Pinch-off voltage)

ID is proportional to VDD Constant chan. Resistance

(ohmic area)ID is almost

constant(constant current-

area)

VGD – reverse biased voltage causes large

depletion area Very narrow

channel Fast change of ID due

to VDS - breakdown

Drain Characteristic Curves

VGS = [0 to -5] VGS = -5 cutoff voltage ID is almost zero

JFET Transfer Characteristic

Q point : (ID, VGS)

2

)(

)1(offGS

GSDSSD V

VII

JFET Transfer Characteristic

Self-biased DC Load Line

VGS is zero; Remember: For self-biased dc

load line VGS=ID x RS

Self-Biased---------------VGS=-ID x RS

2

)(

)1(offGS

GSDSSD V

VII

MOSFET

Metal Oxide Semiconductor FET

Has no PN junction Gate is separated from

channel by SiO2 (silicon dioxide)

– Also called Insolated Gate (IGFET)

Can be n or p channel MOSFET

D-MOSFET

P-type and n-type have different voltage polarities

The gate voltage can be positive or negative Two modes of operations:

– Depletion (VGS<0)– Enhancement (VGS>0)

E-MOSFET

P-type and n-type have different voltage polarities

Operates only in Enhancement Mode– If n-type VGS >0 – If p-type VGS <0

NOTE: The channel is closed until voltage is applied to the gate:

VGS>VGS(TH)

Transfer Characteristic Curves

D-MOSFETE-MOSFET

Parabolic Curve

2)( )( THGSGSD VVKI

2

)(

)1(offGS

GSDSSD V

VII

2)(

)(

)( THGSGS

OND

Vv

IK

VGS(TH) and ID(ON) can be found from the data sheet

VSG

ID

For more information click on each topic:

          

Transistor, bipolar, NPN

          

Transistor, bipolar, PNP

          

Transistor, Junction FET, N-channel

          

Transistor, Junction FET, P-channel

           

Transistor, MOSFET, N-channel, depletion mode

           

Transistor, MOSFET, N-channel, enhancement mode

           

Transistor, MOSFET, P-channel, depletion mode

           

Transistor, MOSFET, P-channel, enhancement mode

Know the symbols!

JFET Transfer Characteristic

ID = IDSS (1 – VGS/VGF(off) ) 2

Load Line

VGS is15; Remember: Voltage

divider-biased; slot is VG/Rs

Voltage-Divider- Biased---------------VS=ID x RSVG=VDD(R2/RT)VGS=VG-VS