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 ${\it C}_{\mathrm{gd}}$.