Power dissipation calculation of the base spreading and contact resistance of transistors at low currents and low frequencies

The base spreading resistance of transistors is examined by the power dissipation method at low currents and low frequencies using analytical approximations and computer simulations. Results indicate that because of geometrical considerations the resistance values are reduced by a factor FR, which decreases with increasing aspect ratio. The results also show that for a rectangular-shaped intrinsic base region, when surrounded by an extrinsic one having negligible resistivity, FRvaries between 12 and 29 for an aspect ratio of infinity and 1:1, respectively. It is further shown that when the extrinsic region resistivity is accounted for, FRdecreases further as the extrinsic to intrinsic base sheet resistivity ratio increases. For completeness, the contact resistance expression is also derived with the power dissipation method, and it is shown to agree with that obtained using the transmission line approach. This method, therefore, is suited for the calculation of the total equivalent base resistance of a transistor.