All of the measured AlGaN/GaN devices share some basic properties like identical substrate, layer sequence, and material quality. However, there are differences in the general geometry and AlGaN layer composition in the particular structures. As an illustration of the device surface geometry a scanning electron microscopy (SEM) image is shown in Fig. 5.1. The top metalization is the gate, the middle one is the source, and the bottom one is the drain. The active transistor area is the shallow channel between the source and drain metalization, with the thin gate electrode running along. In this figure two transistors are shown.
For good control of the sheet carrier concentration in the
two-dimensional electron gas (2DEG), the alloy composition and the
abruptness of the AlGaN/GaN interface has to be determined. Various
methods such as high resolution X-ray diffraction, transmission
electron microscopy, and elastic recoil detection have been
used [358,366,367]. A good estimate of the
effective channel thickness of the conducting region is required for
the simulator. A nominal value for the thickness of the 2DEG region
has been found in the literature to be in the order of 23 nm, see
e.g. [368], depending on the Al mole fraction in the AlGaN
layer. However, the effective thickness of the conducting region may
be wider than the 2DEG, albeit with a lower density. For the purpose
of calibrating the simulator to produce the same current density as in
the measured devices, various effective thicknesses of the defect-free
conducting GaN layer were analyzed. A value of 50 nm was used in all
simulations throughout this chapter. Self-heating effects are
accounted for by using a properly adapted ambient temperature. The
barrier height of the Schottky contact to GaN was experimentally
determined to be 1.0 eV at room temperature in agreement with
experiments by other groups [273].
Devices from three different HEMT generations are measured and simulated: first a device with field-plate structure (Device A), next a device with shield-plate structure (Device B), and last a state of the art device with T-gate only (Device C) [4]. The layer properties are summarized in Table 5.2 and the geometry is shown in Fig. 5.2.
Device A has gate length
, field-plate extension length
=0.6
m, and gate width 100
m. The Al composition in
the AlGaN supply layer is 30%. The latter is
-doped in order
to provide additional carriers and to improve access
resistance. Contact resistances of 4
mm are assumed.
Device B is a
device featuring a source shield-plate. The
gate is T-shaped. The Al composition in the barrier layer is 30% with
a
doping, too.
The last device has a T-shaped gate with
and a gate width
=2
50
m (taken as 1
100
m in the
simulations). The Al composition in the supply layer is 22%, contact
resistance is 0.2
mm.
Using the same setup the three generations of AlGaN/GaN based HEMTs are simulated and the results are compared to experimental data. In the following the results are discussed.
AC simulations are performed to compare the calculated and
experimental figures of merit e.g. cut-off and maximum
frequency. Fig. 5.9 shows the measured and simulated
cut-off frequency
(again at
=7 V). In order to
account for the parasitics introduced by the measurement equipment,
the intrinsic parameters obtained in the simulation are transformed
using a standard two-port pad parasitic equivalent circuit. Both
models provide a very good agreement with the experiment.
Fig. 5.10 compares the measured and simulated (using Model B) extrinsic
S-parameters at
V and
V. An excellent agreement
is achieved for all parameters in the frequency range 100 MHz
26 GHz.
The electron transport in the channel under the gate is studied at the
same bias point. As the electric field reaches its maximum under the
drain side of the gate [371], the peak of the electron
temperature is also found there (the gate edge is at
m in
Fig. 5.11). Consequently, in the same region a pronounced
velocity overshoot is observed. Interestingly, temperature and velocity
profiles obtained using both models do not differ significantly.