An accurate analytical delay model for BiCMOS driver circuits

An analytical delay model for BiCMOS driver circuits is presented. The model is based on physical device parameters and can be used to estimate both the pull-up and the pull-down times for a variety of circuit configurations. The intrinsic delay associated with the bipolar transistors is taken into consideration by using a charge control model that incorporates the high-injection effects upon the current gain and the base transport factor. Separate sets of delay equations are derived for the pull-up and pull-down transient responses to account for significant differences between the two cases. The comparison with SPICE circuit simulation results shows that the new model predicts the respective delay times with less than 10% error in most cases. The influence of device dimensions upon the driver delay time is also investigated. The model has been applied to find an optimal area allocation between the CMOS and bipolar parts of the driver circuit when the total available area is limited as in the standard cell configuration