4.1.1.7 The Current Gain Cut-off Frequency ${\it f}_\mathrm{T}$

The bias dependence of ${\it f}_\mathrm{T}$, especially on ${\it V}_{\mathrm{DS}}$, is closely related to the band gap and band gap discontinuity. As seen in Fig. 4.3 the overall maximum ${\it f}_\mathrm{T}$ value for a given gate length ${\it l}_{\mathrm{g}}$ increases as a function of decreasing channel band gap, which is related to a decreasing effective mass. The change of the bias dependence on ${\it V}_{\mathrm{DS}}$ is related to two principal mechanisms: the increase in ${\it V}_{\mathrm{DS}}$ leads to increased parasitic charge modulation represented by increased ${\it C}_{\mathrm{gs}}$ in a simple capacitor model, and to reduced ${\mit g}_{\mathrm{m}}$. 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: $f_T$ as a function of $V_{DS}$ for an InAlAs/InGaAs/InP HEMT, an AlGaAs/InGaAs/GaAs PHEMT and a AlGaN/GaN HEMT.

Figure 4.4: $f_T$ as a function of $V_{DS}$ 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 ${\it f}_\mathrm{T}$ 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.