Filed Effect Transistors (FETs)

1
Filed Effect Transistors (FETs)

• Chapter 7

2
JFET
3
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

4
N and P-type JFETs

• N-type
• VDDgt 0
• VGSlt0
• VGS(cutoff) lt 0 ID0
• P-type
• VDDlt 0
• VGSgt0
• VGS(cutoff) gt 0 ID0

5
Drain Characteristic VDS-ID Curve

• VGS is shorted ? VGS0
• IDSS is the maximum drain current allowed at the
  Vp (Pinch-off voltage)

VGD reverse biased voltage causes large
depletion area ? Very narrow channel ? Fast
change of ID due to VDS - breakdown
ID is proportional to VDD ? Constant chan.
Resistance (ohmic area)
ID is almost constant (constant current-area)
6
Drain Characteristic Curves

• VGS 0 to -5
• VGS -5 cutoff voltage ? ID is almost zero

7
JFET Transfer Characteristic
Q point (ID, VGS)
8
JFET Transfer Characteristic
VGS is zero Remember For self-biased dc load
line VGSID x RS
Self-Biased --------------- VGS-ID x RS
Self-biased DC Load Line
9
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

10
D-MOSFET

• P-type and n-type have different voltage
  polarities
• The gate voltage can be positive or negative
• Two modes of operations
• Depletion (VGSlt0)
• Enhancement (VGSgt0)

11
E-MOSFET
NOTE The channel is closed until voltage is
applied to the gate VGSgtVGS(TH)

• P-type and n-type have different voltage
  polarities
• Operates only in Enhancement Mode
• If n-type ? VGS gt0
• If p-type ? VGS lt0

12
Transfer Characteristic Curves
VGS(TH) and ID(ON) can be found from the data
sheet
Parabolic Curve
E-MOSFET
D-MOSFET
ID
VSG
13
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!
14
JFET Transfer Characteristic
VGS is15 Remember Voltage divider-biased slot
is VG/Rs
ID IDSS (1 VGS/VGF(off) ) 2
Voltage-Divider- Biased --------------- VSID x
RS VGVDD(R2/RT) VGSVG-VS
Load Line