4.1.2 Extended Small-Signal Equivalent Circuit Model

In [230] several additional elements are introduced into the standard circuit for the analysis of noise, impact ionization, and gate leakage phenomena. The method allows to extract information on the impact ionization behavior for GaAs devices close to breakdown and to distinguish different InAlAs/InGaAs HEMTs on metamorphic buffer or InP substrate.

The two additional resistances ${\it R}_{\mathrm{pgs}}$ and ${\it R}_{\mathrm{pgd}}$ shown in Fig. 4.2 account for different gate leakage mechanisms from either the drain or the source side into the gate. Similar to ${\it R}_{\mathrm{gs}}$ and ${\it R}_{\mathrm{gd}}$ they represent a compact effective conductivity into the gate. They represent the dominating leakage mechanism, so for pseudomorphic HEMTs an electron tunneling current, while for InAlAs/InGaAs a hole current dominates [75].

For the output conductance ${\it g}_{\mathrm{ds}}$ at high ${\it V}_{\mathrm{DS}}$ bias, the onset of impact ionization generating carriers can be interpreted as an additional contribution to the transconductance ${\mit g}_{\mathrm{m}}$, when looking at transfer characteristics [230]. Therefore, the additional element ${\it g}_{\mathrm{m im}}$ is defined which can be extracted to determine the onset of impact ionization in the device from the bias dependent S-parameters.

The quantities ${\it C}_{\mathrm {im}}$ and ${\it R}_{\mathrm{im}}$ introduce an additional time constant to explain the inductive behavior of the S-parameters for low frequencies. The capacitance ${\it C}_{\mathrm {im}}$ is used to model the additional carriers during impact ionization in analogy to the relation of ${\mit g}_{\mathrm{m}}$ and ${\it g}_{\mathrm{m im}}$.