Polysilicon Emitter Transistors (PETs) which are now being widely used in high speed bipolar circuits because of their extremely low emitter transit time τE, have broken-up interfacial oxide layer due to annealing process which they undergo. The effect of oxide layer break-up at the mono-poly interface of the emitter region of a PET on τE is studied here. The effect of doping profile, peak doping concentration, thickness of interfacial oxide layer on τE is studied also. Exponential doping distribution alongwith concentration dependent bandgap narrowing effect and concentration dependent diffusion constant in the monosilicon emitter region are considered.