Govt. Exams
Entrance Exams
For n-channel depletion MOSFET, when VGS = 0 V (which is greater than Vto = -3 V), the channel is open and device conducts. With VGS > Vto and positive VDS, the device operates in saturation. Depletion-mode devices conduct at VGS = 0.
Germanium has bandgap Eg ≈ 0.67 eV versus silicon's 1.12 eV. Lower bandgap requires less voltage to forward bias. However, silicon is preferred due to better temperature stability and lower leakage current.
Thermal voltage VT = kT/q, where k = 1.38×10⁻²³ J/K, T = 300K, q = 1.6×10⁻¹⁹ C. VT ≈ 26 mV at room temperature, a fundamental parameter in semiconductor device physics.
IS doubles approximately every 5°C for Si. Over 50°C increase: IS(75°C) = IS(25°C)·2^(50/5) = 2^10 ≈ 1000 times
GaAs has electron mobility ~8500 cm²/Vs, significantly higher than Si (~1350 cm²/Vs), enabling higher frequency operation.
Wide bandgap emitter (e.g., AlGaAs) blocks minority carrier injection from base, improving γE and allowing lower base doping for higher β.
Pconduction = ID²·RDS(on) = (20)²×0.5 = 400×0.5 = 200 W
For p-channel: |VGS - VT| = |-8 - (-2)| = |-6| = 6 V overdrive, or algebraically VGS - VT = -8 - (-2) = -6 V
MESFET uses Schottky (metal-semiconductor) junction for gate control instead of p-n junction, enabling high-frequency GaAs implementation.
Bypass capacitor shorts RE to ground at signal frequencies, eliminating degeneration feedback and increasing Av = gm·RC approximately.
