GaN and GaN-based III-V alloy semiconductors are likely the most promising new materials
for the next generation of high-power electronic, microwave and optoelectronic devices.
Apart from the continuing search for the optimum substrate and p-type doping method, other
for the GaN transistor technology (see Figure 1) are the self-heating effects, traps and the high-levels of
Professor Balandin's Nano-Device Laboratory (NDL) research group
is involved in investigation of the thermal transport in AlGaN materials
and study of the traps and trap-relarted flicker noise in GaN-based transistors.
The self-heating effects in GaN transistors are related to the extremely
high power densities involved and a large thermal resistance of the
Self-heating in GaN-based field-effect transistors (FETs) results in abrupt increase of local temperature
and can lead to the drain-source current reduction and
thermal breakdowns at bias voltages much lower than the theoretically predicted
(see V.O. Tourin and A.A. Balandin, Electron Letters, 2004).
To solve the problem of thermal management
of the high-power GaN transistors, NDL researchers carry out
experimental and theoretical studies of
thermal conduction in GaN thin films and
AlGaN alloys used in fabrication of AlGaN/GaN heterostructure field-effect transistors (HFETs).
Figure 1. GaN/AlGaN heterostructure field-effect transistors (HFETs)
used for experimental study of self-heating effects.
NDL researchers have carried out a systematic study of
the thermal conductivity in the AlGaN alloy system as a function
of the Al mole fraction in the temperature range from 4K to 600K (see Figure 2).
The experimental data have been
explained on the basis of the the virtual crystal model.
The thermal conductivity variation with Al
mole fraction, as demonstrated in both the theoretical and experimental data,
shows a characteristic abrupt reduction when the Al content
increases from 0.0 to about 0.1,
followed by a graduate approaches to minimum. Details of the
study have been reported in
W.L. Liu and A.A. Balandin, J. Applied Physics, 2005.
This study clarified the role of the alloy and boundary
phonon scattering in AlGaN thin films, and allowed
the NDL researchers to extract the parameters required for
accurate modeling of GaN FET performance.
Figure 2. Measured thermal conductivity in AlGaN thin films with different
Al mole fraction.
The results are after
W.L. Liu and and A.A. Balandin, Appl. Phys. Lett., 2004.
In collaboration with the group of Professor Kang L. Wang (UCLA), the NDL
researchers investigated the effects of the ambient temperature on the
performance of GaN/AlGaN HFETs in the temperature
range from 25 C to 250 C (see Figure 3). Understanding the ambient effects on the
transistor performance is important for designing a reliable
The measured data indicates about 30%
degradation in the saturation current and transconductance with
temperature increase to 250 C. Experimental results have
been used as input parameters for the physics-based modeling using
ISE DESSIS software (see also NDL Device Modeling).
The obtained experimental dependencies
and simulation data can be used for predicting GaN-based
device performance and reliability in changing, high
Details of the work have been published in
W.L. Liu, V.O. Turin, A.A. Balandin, Y.L. Chen and K.L. Wang, J. Nitride Research, 04.
Figure 3. Ambient temperature effects on the GaN transistor performance.
Professor A.A. Balandin in collaboration with Professor K.L. Wang (UCLA) were first to demonstrate
experimentally that GaN HFETs can operate with the low flicker noise levels (Hooge parameter ~0.0001),
required for the high-tech microwave applications (see A.A. Balandin et al,
IEEE Electron Device Letters, 1998).
The flicker noise, which manifests itself at low-frequencies (0.01 kHz – 100 kHz) with
the ~1/f spectral density, is an important figure-of-merit for the
semiconductor device technology
since this type of noise is the limiting phase-noise factor for all kinds of transistors.
In addition, the low-frequency noise spectral density is a characteristic of the material quality.
Professors Balandin and Wang have also proposed an original method for the
noise suppresion in GaN/AlGaN HFETs using the high-Al content ("piezo-dopped") barriers
without the channel doping, which led to
the two-orders-of-magnitude noise spectral density suppression (see Figure 4).
Figure 4. Noise spectra density in the conventional and "piezo-doped"
AlGaN/GaN HFET. After A.A. Balandin and K.L. Wang, Appl. Phys. Lett., 2000.
In order to minimize the device noise one should understand where the noise
sources are located: in the device channel, contacts, etc. Analysis of the gate and
drain-source bias dependences may clarify the noise type and sources.
Figure 5 shows that in the subsaturation region, the noise spectral density
measured for GaN/AlGaN HFET has two different
regions characterized by the dependence ~1/V^m, where m is close to 1
(for small Vg<3 V), or is equal or larger than 3.
This corresponds to the case when
the dominant noise source is in the device channel.
Details of the noise-source analysis have been
reported in A.A. Balandin,
Electron Letters, 2000.
Figure 5. Effective gate-bias dependence of flicker noise in AlGaN/GaN HFET.
Capacitance-voltage (CV) profiling is a well-developed characterization
technique for investigation of the carrier and trap distribution in semiconductor heterostructures.
In CV measurements, the frequency dispersion provides useful information on the trap states .
Dr. W.L. Liu and other NDL group members
apply this technique to study
surface and channel layer trap characteristics in
in GaN/AlGaN HFETs (see Figure 6). The results of the CV measurements
performed for surface passivated GaN-based transistors have been
recently reported at the SPIE Noise Conference.
Figure 6. Capacitance-voltage spectroscopy of interface traps in GaN/AlGaN HFETs.
Results are after W.L. Liu, A.A. in collaboration with K.L. Wang, 2005.
More information on the projects
currently under way in the Nano-Device Laboratory (NDL) can be
To join NDL as a graduate student or postdoctoral research visit
the web-page HERE.
To learn more about course offering in the field of Materials,
Devices and Circuits visit the web-page HERE.