An MOS transistor is fabricated on a p-type silicon substrate with N(a)=2x10^15 cm^-3. The oxide thickness is t(ox)=450A and the equivalent fixed oxide charge is Q'(ss)=5x10^10 cm^-2. Calculate the threshold voltage for (a) an aluminum gate, (b) an n+ polysilicon gate, and (c) a p+ polysilicon gate.