4.1.1.7 The Current Gain Cut-off Frequency

The bias dependence of , especially on , is closely related to the band gap and band gap discontinuity. As seen in Fig. 4.3 the overall maximum value for a given gate length increases as a function of decreasing channel band gap, which is related to a decreasing effective mass. The change of the bias dependence on is related to two principal mechanisms: the increase in  leads to increased parasitic charge modulation represented by increased  in a simple capacitor model, and to reduced . The tunneling probability of the carriers from the channel carrier confinement into the barrier drops with increasing band gap discontinuity, which explains this behavior.

Figure 4.3: as a function of for an InAlAs/InGaAs/InP HEMT, an AlGaAs/InGaAs/GaAs PHEMT and a AlGaN/GaN HEMT.

Figure 4.4: as a function of for several different AlGaAs/InGaAs/GaAs HEMT technologies.

Second, even within one materials system, e.g. AlGaAs/InGaAs shown in Fig. 4.4, there is still a significant spread of the decrease of depending on the different modifications of the layer structure and process influence. The quantitative physical factors related to this decrease are explained in Chapter 7.