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Next: 7.2.6 and Related Quantities Up: 7.2 The Bias Dependence Previous: 7.2.4 Further Small-Signal Elements

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$.

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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.

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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.

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Next: 7.2.6 and Related Quantities Up: 7.2 The Bias Dependence Previous: 7.2.4 Further Small-Signal Elements