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Next: 7.2.4 Further Small-Signal Elements Up: 7.2 The Bias Dependence Previous: 7.2.2 The Gate Source

7.2.3 The Gate Drain Capacitance $ {\it C}_{\mathrm{gd}}$

The bias dependence of $ {\it C}_{\mathrm{gd}}$ is addressed in (4.5). The three contributions to $ {\it C}_{\mathrm{gd}}$ can be evaluated by device simulation. The contribution $ {\it C}_{\mathrm {met}}$ and $ {\it C}_{\mathrm{pass}}$ are independent of the bias. They can be reduced by the gate geometry, see [50], to reduce the fringe parts or by low-k dielectrics. $ {\it C}_{\mathrm{gd}}$ is a decreasing function of $ {\it V}_{\mathrm{DS}}$. The value of $ {\it C}_{\mathrm{gd}}$ saturates for high $ {\it V}_{\mathrm{DS}}$, as can best be seen for a doping concentration of $ {\it N}_{\mathrm{D}}$ = 1$ \times $10$ ^{18}$ cm$ ^{-3}$ in Fig. 7.12. Next to the $ {\it V}_{\mathrm{DS}}$ bias, the value of the semiconductor contribution strongly depends on the cap concentration. Thus, for high-power HEMTs with depleted caps this contribution reduces to very small values of the contribution $ {\it C}_{\mathrm{semi}}$ of $ {\it C}_{\mathrm{gd}}$.