Experimental probe of minority carrier velocity profile

We experimentally investigate the details of the minority electron transport across the base of a GaAs heterojunction bipolar transistor (HBT). A stepwise doping change in the base layer is employed to locally accelerate the electrons and enhance the dc current gain. The position of the step change is varied to probe the electron velocity across the base layer in multiple sample sets with different base thicknesses. Positioning the doping change within 200 Å of the collector provides the largest velocity increase (∼15%), independent of base thickness. This indicates that the electrons are moving slower as they approach the collector, an observation which sharply contrasts with the conventional quasi-ballistic drift-diffusion models of base transport that predict the highest electron velocities are reached near the collector.