AlGaAs/GaAs HBT with Enhanced Forward Diffusion

One of the key limits of high-frequency operation of bipolar transistors is the base transient time, which is proportional to the square of the base width when the base transport is dominated by diffusion. Consequently, high-frequency bipolar transistors tend to use thin bases (<100 nm) that results in a short base transient time and a high cut-off frequency fT. However, for high frequency operations, it is not the current gain that matters most. Rather, it is the unilateral power gain that determines the operating frequency of any three-terminal devices. The frequency fmax, at which the power gain is unity, is determined by both fT and RC time constant. Because of the peculiar geometry of bipolar transistors, the electrical contact to the base is always made from the side. Thus, a thin base, which is important to yield a high fT, will inevitably result in a high sheet resistance and a lowering of fmax. It is this difficult trade-off between fT and fmax that lead Prof. S. Luryi and his coworkers to propose a novel heterostructure bipolar transistor, whose band diagram is shown in Figure 26.