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

5. 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

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 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

9. MOSFET Metal Oxide Semiconductor FET Has no PN junction Gate is separated from channel by SiO 2 (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 (VGS<0) – Enhancement (VGS>0)

11. 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)

12. Transfer Characteristic Curves D-MOSFET E-MOSFET Parabolic Curve VGS(TH) and ID(ON) can be found from the data sheet VSG ID

13. For more information click on each topic: TransistorTransistor, bipolar, NPNbipolarNPN TransistorTransistor, bipolar, PNPbipolarPNP TransistorTransistor, Junction FET, N-channelJunction FETN-channel TransistorTransistor, Junction FET, P-channelJunction FETP-channel TransistorTransistor, MOSFET, N-channel, depletion modeMOSFETN-channeldepletion mode TransistorTransistor, MOSFET, N-channel, enhancement modeMOSFETN-channelenhancement mode TransistorTransistor, MOSFET, P-channel, depletion modeMOSFETP-channeldepletion mode TransistorTransistor, MOSFET, P-channel, enhancement modeMOSFETP-channelenhancement mode Know the symbols!

14. 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 RS VG=VDD(R2/RT) VGS=VG-VS