5.3.1 Identification of the Most
Critical Parameters
A figure of merit for HEMTs is the maximum transconductance gm
max. One of the most important parameters governing gm
max is the gate-to-channel separation dGC.
The thickness of the epitaxial layers can be controlled very precisely
by MBE growth. The critical technological step which determines the magnitude
of dGC is the gate recess. In the recess region it is
intended to remove the GaAs cap layer completely but to leave the AlGaAs
Schottky barrier intact. In practice, this etching can only be performed
with a finite selectivity, and the effective recess depth can vary a few
nanometers across the wafer or from on processing run to another. Therefore,
small deviations in the order of 2 nm of the actual dGC
from the nominal values must be considered to be realistic. Deviations
of the gate length LG from their nominal values are also
technological inevitable. These deviations highly depend on the used technology
to define the gate structure.
First the impact of the most important parameters on the intrinsic device
should be investigated using an analytical formula. The intrinsic gm
max int (i. e. gm max for vanishing source
resistance RS) of a HEMT is given by [58]:
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The parameters m and vsat
depend on the channel material whereas LG and dGC
are geometry quantities defined by process technology. The equilibrium
electron concentration ns0 depends on various parameters
such as doping concentration in the barriers, the distance of the doping
layer to the channel, and the height of the energy barriers. For practical
devices with typical values ns0 this will have almost
no influence on gm max int as shown in Figure
5.15 but only determine the threshold voltage VT.
In the following gm max int will be plotted versus the
parameters m, vsat, LG,
and dGC to qualitatively analyze what has to be expected
for the measured and simulated devices investigated in Chapter
6. Typical values of an Al0.2Ga0.8As/In0.2Ga0.8As
HEMT used for the plots are given in Table
5.1. The parameters which are not varied actually are kept at these
constant values.
The magnitude of the material parameter m
can be determined by Hall measurements. The absolute measured values have
fairly large uncertainties because there is no standardized epitaxial Hall
structure. Figure
5.16 reveals that gm max int is nearly independent
of m for LG < 250
nm and m higher than about 4000 cm2/Vs.
With increasing LG the influence of m
increases.
The experimental determination of vsat is even more
difficult than that of µ and can be done directly only for bulk material.
When the same material is used in a quantum well additional effects have
to be taken into account. These are the change in alloy scattering depending
on the growth technique, interface roughness, Coulomb scattering, electron
scattering and change in the v(E) characteristics due to quantization
effects. Today there are no models suitable for device simulation which
can describe these effects in AlGaAs/InGaAs quantum wells. It has to be
expected that vsat is reduced significantly compared
to the bulk material values. Therefore especially vsat
is considered to be a fitting parameter. In Figure
5.17 the dependency on vsat is shown. Practical devices
operate in the saturation region when they reach their maximum gm.
Therefore gm max int is almost linearly dependent on
vsat as expected.
The dependence of gm max int on LG
is plotted in Figure
5.18. For LG < 250 nm only a small influence can
be observed. The decrease is almost linear for LG > 500
nm. The expected reduction proportional to
occurs for gate lengths of several microns (not shown in Figure
5.18).
As shown in Figure
5.19 the dependence of gm max int on (dGC
+ DdGC) is only moderate for
values larger than about 30 nm. It appears that gm max int
gets extremely sensitive to (dGC + DdGC)
for much smaller values which nowadays can be manufactured with an accuracy
below 2 nm. Considering (dGC + DdGC)
= 20 nm a reduction of 1 nm (which corresponds to only 4 atomic layers)
increases gm max int by about 50 mS/mm. This establishes
dGC as the most important technological parameter for
the transconductance of HEMTs with LG below 1 µm
which are nowadays used for practical applications.
Next: 5.3.2 Fitting Procedure Up:
5.3 Determination of the Parameter Set for the Simulation
Previous: 5.3 Determination of the Parameter Set
for the Simulation
Helmut Brech 1998-03-11