A "Step recovery" switching transistor

Fast rise time pulses can be generated with a special high-frequency silicon transistor structure having a collector diffusion profile designed to control the charge storage in the collector. When this transistor is switched out of saturation, the field in the collector forces the stored minority carriers towards the center of the base-collector junction. At the end of the storage phase, the transistor switches abruptly to the open condition in the same manner as a step recovery diode. The switching current flows through the base, and thus a very low base resistance is required. This is accomplished using a high-frequency interdigitated transistor geometry with metal over oxide connections. The transistor also requires a uniform impurity profile through the whole collector base junction which is obtained in a mesa-type collector junction. An unusual structure is used to combine these two requirements.