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The gate drain capacitance $ {\it C}_{\mathrm{gd}}$ is written as:
    $\displaystyle {\it C}_{\mathrm{gd}} ({\it V}_{\mathrm{DS}}) = C_\mathrm {met} + C_\mathrm {pass}+ C_\mathrm {semi}({\it V}_{\mathrm{DS}})$ (4.5)

as was suggested in [65]. The first two contributions are independent of $ {\it V}_{\mathrm{DS}}$  representing the contributions of the gate and drain metal periphery (C $ _\mathrm {met}$) and the coupling of the gate metal with the semiconductor caps through the SiN passivation (C $ _\mathrm {pass}$). The $ {\it V}_{\mathrm{DS}}$  dependence of $ {\it C}_{\mathrm{gd}}$  arises mostly from the coupling of gate and drain through the barrier and channel layer, represented by C $ _\mathrm {semi}$.