Tutorial Solutions on MOS Threshold Voltage and CV Curve

PwnHero VT N = 0.25 V. The parameter Kn is related to the slope of the line ( slope )2 = Kn = µnWκoxϵ0 2Ltox ( slope )2 · (2tox) (W/L)κoxϵ0 = µn Taking our best estimate of the slope: µn = 0.033 √ A 2.75 V 2 · 2 · 4.25 × 10−6 cm 10 · 3.9 · 552000 e V·cm = 5.686 × 10−17 C · cm2 e · V · s × e 1.602 × 10−19C ≈355 cm2 V · s (53) For an enhancement type NMOS, the flat-band and threshold voltages are related as 0 < VFB < VT , assuming that the device is in accumulation mode at VG = 0. Write similar relations for the situations below: (a) Enhancement type PMOS which in accumulation mode at VG = 0. Solution: VT < VFB < 0. Remember for an enhancement type PMOS, the threshold voltage is negative. If the device is in accumulation mode at VG = 0, then one first needs to enter depletion mode by crossing the flat band voltage, and then afterward to enter inversion mode by crossing the threshold. (b) Depletion type NMOS which is in inversion mode at VG = 0. Solution: VFB < VT < 0. The depletion type NMOS is already active at VG = 0. We need to apply a negative gate bias to turn offthe device. First it must go back into depletion mode (by crossing the threshold voltage), and later it must cross the flat band voltage to go into accumulation mode. (c) Enhancement type PMOS which is in depletion mode at VG = 0 Solution: VT P < 0 < VF B. Recall that for an enhacement type PMOS, the threshold voltage is negative. If the device is in the depletion mode at VG = 0, it means that the gate voltage (of zero) is between the flatband and threshold voltages. 48 PwnHero.com