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
and
shown in Fig. 4.2 account for
different gate leakage mechanisms from either the drain or the source side into the gate. Similar
to
and
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
at high
bias, the onset of impact ionization
generating carriers can be interpreted as an additional contribution to the transconductance
, when looking at transfer characteristics [230]. Therefore, the additional element
is defined which can be extracted to determine the onset of impact ionization in the device
from the bias dependent S-parameters.
The quantities
and
introduce an additional time constant to explain the
inductive behavior of the S-parameters for low frequencies. The capacitance
is used to
model the additional carriers during impact ionization in analogy to the relation of
and
.