1. MOSFET Piecewise Models ECE 2204

2. Cut-off Region NMOSPMOS I G = I D = I S = 0 V GS ≤ V TN V GS ≥ V TP

3. Nonsaturation/Triode Region NMOSPMOS V GS ≥ V TN V DS ≤ V GS – V TN V GS ≤ V TP V DS ≥ V GS - V TP I D = Kn[(V GS – V TN )V DS – 0.5V DS 2 ]I D = Kp[(V GS - V TN )V DS – 0.5V DS 2 ] R DSon = V DS /I D I G = 0I D ≤ I Dsat

4. Saturation/Pinch-off Region NMOSPMOS V GS ≥ V TN V DS ≥ V DSsat V GS ≤ V TP V DS ≤ V DSsat V DSsat = V GS – V TN V DSsat = V GS - V TP I D = (Kn/2)(V GS – V TN ) 2 = (Kn/2)V DSsat 2 I D = (Kp/2)(V GS - V TP ) 2 = (Kp/2)V DSsat 2 I G = 0, I D = I Dsat (a constant)

5. Summary of I-V Relationships: Enhancement Mode FETs RegionNMOSPMOS Nonsaturation /Triode v DS ≤v DS (sat)v DS ≥v DS (sat) Saturation/ Pinch-off v DS ≥v DS (sat)v SD >v SD (sat) Enhancement Mode V TN > 0VV TP < 0V Note: Depletion Mode V TN < 0VV TP > 0V

6. Transition between models NMOS enhancement mode ▫ Cut-off and Saturation/Pinch-of ▫ Nonsaturation/Triode and Saturation/Pinch-off PMOS enhancement mode ▫ Cut-off and Saturation/Pinch-of ▫ Nonsaturation/Triode and Saturation/Pinch-off

7. Conduction Parameters NMOSFET PMOSFET where:

8. Questions How does V TN or V TP change with the doping concentration of the channel? To limit the power dissipated in a FET operating in the triode region, R DSon should be as small as possible. Should V GS be large or small? If the oxide is changed to a high K dielectric, will I D increase or decrease? To keep the drain current unchanged, should the thickness of the oxide increase or decrease? If you fabricated a NMOS transistor in Si and GaAs with the same doping concentrations, dimensions, and same oxide material), will I D (GasAs) be larger or smaller than I D (GasAs) when V DS = V GS – V TN ?