The effect of emitter doping gradient on fTin microwave bipolar transistors

The effect of the emitter doping gradient on the transit time in microwave bipolar transistor structures has been investigated. The range of gradients was chosen to cover those typically found in shallow phosphorus- and arsenic-emitter diffusions. The results show that there is appreciable free carrier storage in the emitter space charge layer. The Storage decreases when the doping gradient in the junction is increased. This effect can account for the improved fTvalues of arsenic emitter transistors. The free carrier storage in the emitter space charge layer is compared with predictions deduced from the theoretical analysis of Morgan and Smits [1].