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

${\it f}_\mathrm{T}$ at a given operation bias value can help to evaluate the performance of a device if gain is a restricting factor at the given frequency. However, several other factors than this small-signal quantity have to be considered to successfully design a high-power structure.

Figure 7.16: Experimental decrease of $\Delta f_T$/ $\Delta V_{DS}$ as a function of gate length $l_g$.

The physical origin for the bias dependence can by understood looking at the bias dependence of the three constituting elements ${\mit g}_{\mathrm{m}}$, ${\it C}_{\mathrm{gs}}$, and ${\it C}_{\mathrm{gd}}$. Aggressive scaling to obtain high ${\it f}_\mathrm{T}$ value at low bias ( ${\it V}_{\mathrm{DS}}$= 1V) does not necessarily increase the ${\it f}_\mathrm{T}$ values for higher ${\it V}_{\mathrm{DS}}$, e.g. ${\it V}_{\mathrm{DS}}$= 3 V or 5 V, but can, on the opposite, decrease them. The most important parameter for this decrease is the gate length itself. Fig. 7.16 shows the measured dependence of the decrease of $\Delta$ ${\it f}_\mathrm{T}$/ $\Delta {\it V}_{\mathrm{DS}}$ in a logarithmic scale versus the gate length ${\it l}_{\mathrm{g}}$. The data were consistently measured for one technology, which is similar to technology C, on one wafer. If the product ${\it f}_\mathrm{T}\times{\it l}_{\mathrm{g}}$ were a constant as a function of ${\it V}_{\mathrm{DS}}$, a straight line would be observed. However, the decrease enhances for shorter gate length ${\it l}_{\mathrm{g}}$$\leq$ 0.3 $\mu$m, which is the proof for short channel effects.

Fig. 7.17 shows the dependence of ${\it f}_\mathrm{T}$ on the contact situation given in Fig. 3.25. A difference can be observed for a device which is otherwise not changed. The direct contacting leads to increased ${\it f}_\mathrm{T}$ at low ${\it V}_{\mathrm{DS}}$ due to the lower contact resistance. However, for increasing bias the decrease in Case I is relatively stronger, so that for higher bias Case II appears more useful.

Figure 7.17: $f_T$ as a function of $V_{DS}$ bias for different contact situations.

Fig. 7.17 is a strong hint to the importance of RST and consequently parasitic charge modulation as a function of ${\it V}_{\mathrm{DS}}$ bias. Mostly, the term modulation efficiency ME, as given in (4.3) is used to compare different materials system, Fig. 7.17 shows the importance of ME as a function of bias within the same materials system. A comparison of Technology A and Technology C, which mainly differ in the contact situation, justifies the results shown in Fig. 4.4.