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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.


\includegraphics[width=10 cm]{D:/Userquay/Promotion/HtmlDiss/fig30.eps}
Figure 4.4: $ f_T$ as a function of $ V_{DS}$ for several different AlGaAs/InGaAs/GaAs HEMT technologies.

\includegraphics[width=10 cm]{D:/Userquay/Promotion/HtmlDiss/fig30a.eps}

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.


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Next: 4.1.2 Extended Small-Signal Equivalent Up: 4.1.1 The Basic Model Previous: 4.1.1.6 Contributions to
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2001-12-21