The effect of punch through on the small signal properties of single drift n+ np+ silicon IMPATT diode near 94 GHz atmospheric propagation window has been studied. The computed results show that for a fixed bias current density, when the space charge effect is not prominent, the optimum negative conductance decreases, and the corresponding frequency for highest negative conductance increases with the increase of punch through determined by doping density and the thickness of the active layer. The increase of punch through produces an upward shift in the operating frequency of the device. It is shown that an increase of punch through leads to a decrease in the conversion efficiency, and the negative conductance of the device.