Evaluation of Electron Carrier Trap Effects
in GaN HEMTs using I-V Voltage Measurements
D. Fernandez, K. Foster
Department of Electrical and Computer Engineering, Naval Postgraduate
School, Monterey, CA
M. Porter, Research Associate
Introduction
Recent advances in semiconductor materials science has begun the
instillation of novel materials other than Silicon (Si) in semiconductor
transistors. Our research group is currently investigating Gallium Nitride
(GaN), a material which promises to increase the frequency range and
power output of radar and communications system of transistor devices
[1]. However, a number of problems remain in the fabrication of GaN
power transistors which leaves them vulnerable to defects [2] . It is
necessary to fully understand how these defects may cause early failure
in devices made from GaN in order to maximize reliability and efficiency.
Results
Materials and
Methodology
In order to successfully carry out carry out current-voltage (IV)
measurements, it is necessary to use specialized equipment beyond
standard multimeters and electrical measurement tools. The
laboratory setup at NPS which was used is shown below. The setup
includes high-accuracy semiconductor characterization equipment
which can be used to take measurements directly on semiconductor
wafers, before they are packaged.
All Nitronex devices were tested (Cal Poly’s & NPS) with and without
filters at the gate, drain and both. The tests with filters attached to the
gate or drain or both significantly suppressed the noise going through
the device. Although, it showed the devices were still noisy. This
resulted in more testing to only prove the devices themselves were
faulty.
Device 2 – IV Curves
Figure 6 (a). The
device without the
filter is showing
signs of
amplified noise due
to large gain.
This project investigates the defects present in a commercial GaN-on-Si
high electron mobility transistor (HEMT), by measuring the electrical
characteristics of the device under a variety of conditions. Based on the
measurements of continuous current-voltage (IV), pulsed IV, and
capacitance-voltage (CV) curves under varying channel temperature
conditions, this investigation seeks to characterize the density, energy
level and location of charge carrier traps in the commercial device.
Once the data are obtained, a simulation of the effects of the traps upon
the operation of the device will then be performed using the Silvaco
ATLAS TCAD Suite to confirm and explain our results. Obtaining this
data will contribute to ongoing investigations into the nature of GaN
device reliability.
Figure 1. A Nitronex GaN HighElectron Mobility Transistor to be used
for testing.
Figure 6 (b). The
RC filter at the gate
suppressed the
noise.
Background: Gallium
Nitride HEMTs - Physics
and Applications
• Gallium Nitride (GaN) has been recently demonstrated as a promising material
to replace Silicon in high-power, high-frequency Department of Defense (DOD)
applications.
• Using GaN promises to decrease losses in applications such as radar and
power converter modules, enabling higher-power applications to become more
mobile. Some examples of the applications of GaN devices include in
aeronautical and naval applications.
GaN high electron mobility transistors
(HEMTs) are a type of field-effect
transistor (FET) design that is currently
being investigated by NPS. Field effect
transistors act as voltage controlled
amplifiers by controlling the flow of
current through a conducting channel
by an applied voltage. The structure
of the GaN HEMTs derive their advantages
Figure 2. The use of GaN devices, for
from the formation of a two-dimensional
example, will greatly benefit the
electron gas (2DEG) at the interface
performance of ballistic-missile defense
of the GaN substrate with a layer of
systems.
Aluminum-GaN allow. Strain in the
material induces electrons to fill the
2DEG and creates high values of conductivity. However, this strain is directly
responsible for defects in the devices leading to early degradation.
Device 4 – IV Curves
Gate
Filter
Figure 7 (a). No RC
filter at the gate.
Figure 4. The three RC low
pass filters used on the gate
terminal of the Nitronex
devices measured in order
to effectively reduce noise
and feedback to provide
accurate measurements.
Figure 9. The cartoon on the
left describes the process for
the formation of the drain
current suppression "humps" in
the Id-Vd characteristic.
Electrons normally flow through
the channel, from the source to
the drain terminals. However, there are a significant number of traps in the
material below the channel, which remove electrons from the channel until a
large enough value of drain voltage is applied to the device."
Further work to understand the imperfections and physics of device
breakdown will be performed at NPS in the near future. Tests such as
capacitance-voltage measurements will directly examine the energy
levels, density and location of traps. Once the non-ideal
characteristics of these devices are understood, it is planned that they
will be used in a amateur radio amplifier design. This will be a direct
demonstration of the superiority of GaN devices for high frequency,
high power applications.
Figure 7 (b). With
the RC filter in place
It helped get rid of
unwanted noise
in the tests.
Works cited
Device 5 – IV Curves
•The RC low pass filters built were designed to have a maximum cutoff
frequency at 50 hertz, suppressing feedback from common power
sources, such as wall sockets and power strips in the measurements
taken. These three gate terminal filters varied in capacitance and
resistance ratios, however all contributed to reduced feedback and
overall less noise in the IV measurements taken on the Nitronex
devices.
Successful measurements and characterization of GaN HEMT currentvoltage characteristics were accomplished utilizing a semiconductor
device analyzer,. However, initial problems due to the feedback of
amplified noise through the transistor reduced the ability to take clean
measurements that could be reliably reproduced. This problem was
found to be unavoidable due to the large gain of the HEMT. A
necessary solution in the form of filtering had to be used. As
described, a filter with a cutoff frequency of 50 Hz was designed using
a resistor and a capacitor. Using this circuit in the path of the gate
terminal of the device enabled it to eliminate any effects of amplified
noise in the measurements.
Resulting IV curves showed a unique shape, which was not expected
for a field-effect type device. Specifically, drain current levels
consistently were depressed below their saturation values for any
given gate voltage, until a critical value of drain voltage was reached.
Beyond this voltage, drain current quickly recovered to a much higher
value of saturation current. Though the reasons for this behavior are
unclear, it can be assumed that electron traps, which act to remove
electrons from the conduction channel of the device, are responsible
[3]. When a high enough electric field is reached at a given critical
drain voltage, the traps release the electrons and the drain current is
restored. This hypothesis remains to be verified through further
testing.
Figure 3. Layout of lab setup, showing various instruments used to
make measurements. Primary among these is the Agilent B1500A
Semiconductor Parameter Analyzer, which is used to make the IV
measurements as explained above.
Initial attempts to carry out
measurements
simply involved
utilizing the B1500 to take IV data.
The circuit involved in this setup is
shown to the right. However, due to
the high gain of the Nitronex device,
a feedback loop formed due to the
gate-drain capacitance. This caused
noise to couple to the gate voltage
and interfere with the gate voltage
level, leading the curves shown at
right. To remedy this problem, low
cutoff frequency resistor-capacitor
(RC) low pass filters were
implemented.
Conclusions and Further
Work
Figure 8 (a). Notice
The oscillations. This
Device without a gate
Filter showed a lot of
Noise due to high
Gain.
[1] Khan, M.A.; Simin, G.; Pytel, S.G.; Monti, A.; Santi, E.; Hudgins,
J.L.; , "New Developments in Gallium Nitride and the Impact on
Power Electronics," Power Electronics Specialists Conference,
2005. PESC '05. IEEE 36th , vol., no., pp.15-26.
[2] Rosker, Mark J. “Recent Advances in GaN-on-SiC HEMT Reliability
and Microwave Performance within the DARPA WBGS-RF
program.” CSICS 2007 Proceedings, 2007. 1-4.
[3] Wang, M. and Chen, K.J. “Kink Effect in AlGaN/GaN HEMTs
Induced by Drain and Gate Pumping.” IEEE Electron Device
Letters. Vol. 32, No. 4, Apr. 2011, pp. 482.
Objective of Study
Currently, NPS is carrying out research into the physics of the long-term
degradation of GaN HEMTs and other GaN devices. In order to fully
characterize these devices, the dependency of current upon the voltage
applied to terminals must be fully understood.
Using Nitronex
commercial GaN HEMTs, the objective was to carry out a successful
current-versus-voltage measurement in order to understand the defects
present in these devices. By accomplishing this objective, statements
could be made about the effects that imperfections in the Nitronex
device materials would have upon electronic systems which used them
as amplifiers.
Figure 8 (b). With
The RC filter at the
gate the interfering
noise frequencies
were suppressed as
Expected.
Figure 5. An oscilloscope measurement used to verify
the 50 hz cutoff frequency of each RC lowpass filter built.
Acknowledgments
We would like to thank Dr. Weatherford, our mentor Matthew Porter,
Jason Brunton, Kevin Pham, Paul Richardson, Jeff Knight.
We would also like thank the Hartnell and Cebrowski STEM
coordinators Joe Welch, Alison Kerr, Casi Martin, and Pat McNeil, Ana
Hernandez, Andy Newton, for all their dedication.