4.1.1.4 Contributions to the Output Conductance ${\it g}_{\mathrm{ds}}$

The output conductance ${\it g}_{\mathrm{ds}}$, respectively the channel resistance ${\it R}_{\mathrm{ds}}$ are subject to a number of influences that must be carefully separated. Special care has to be taken when comparing DC- ${\it g}_{\mathrm{ds}}$  and RF- ${\it g}_{\mathrm{ds}}$. DC- ${\it g}_{\mathrm{ds}}$ values obtained from the output characteristics can appear to be negative. A first effect to be separated is the effect of self-heating and the temperature dependence of the parasitic resistances, especially the gate resistance ${\it R}_{\mathrm{G}}$. Self-heating leads to a rise of ${\it R}_{\mathrm{G}}$ and thus to a reduction of the internally applied ${\it V}_{\mathrm{GS}}$ bias, so self-heating causes an internal bias shift [169]. A second effect is the influence of carrier generation/recombination. As reported especially for GaN HEMTs [81], but also for pseudomorphic HEMTs [211], the occurrence of traps leads to a hysteresis in the DC-output characteristics, an effect, which is not seen in RF-parameters. The RF-value of ${\it g}_{\mathrm{ds}}$ extracted of measured data is therefore always positive.

In a compact current model extending one-dimensional charge models, a saturated velocity model leads to a constant DC-output conductance as a function of ${\it V}_{\mathrm{DS}}$ in the so-called saturation region [95]. Introducing first order compact approaches [20] for impact ionization leads to an exponential increase of the drain current ${\it I}_{\mathrm{D}}$ as a function of ${\it V}_{\mathrm{DS}}$ bias.