Reduced temperature S-parameter measurements of 400+ GHz sub-micron InP DHBTs

The high operating power density and aggressively scaled geometries associated with 400+ GHz InP-Based DHBTs present a new challenge in device design and thermal management. In order to assess the effects of self-heating on the RF performance, S-parameters of six InP DHBTs with varying emitter dimensions were measured over a 75 °C ambient temperature range. An 8–10% increase in peak fT is observed as the temperature is reduced. Data analysis indicates that reductions in the base and collector transit times and the base–emitter charging times are responsible for the peak fT improvement. The calculated electron velocities exceed 6 × 107 cm/s, indicating velocity overshoot plays a critical role in the reduction of the transit times. When emitter scaling are considered, the total transit time variation is directly correlated to the rise in junction temperature. Using previously measured thermal resistance values, a 77–116 °C minimum junction temperature rise is estimated from self-heating. Therefore, the 8–10% increase in peak fT is a reasonable estimate of the performance to be recovered by minimizing self-heating. Improved intra-device thermal management through device design is an important supplement to geometry scaling as a means to enhance device performance.