Characterization of Traps in AlGaN/GaN HEMTs with a Combined
Large Signal Network Analyzer/Deep Level Optical Spectrometer
System
Chieh-Kai Yang 1, Patrick Roblin 1, Andrew Malonis 1, Aaron Arehart 1, Steven Ringel 1,
Christiane Poblenz 2, Yi Pei 2, James Speck 2, and Umesh Mishra 2
1
Electrical and Computer Enginneering, The Ohio State University, Columbus, OH 43210, USA
2
Materials Science and Electrical Engineering departments, University of California,
Santa Barbara, CA 93106, USA
Abstract— A study of the effects of traps on the RF characteristics of AlGaN/GaN HEMTs is conducted using small and
large-signal microwave measurements with deep-level optical
spectroscopy. Different variations of the drain current swing
are observed for illuminations of different photon energies. The
time evolution of the current swing in transient measurements
enabled to determine the optical transient time-constant and the
equilibrium relaxation time-constant. The dependence of the Sparameters upon the illumination yielded results consistent with
the variation of the drain current swing under illumination.
A variation in the small signal transconductance and drain
conductance was extracted at 2 GHz which presumably arises
from the variation of the drain resistance under illumination. In
addition, it was verified that the SiN passivation greatly helps in
mitigating the RF performance degradation arising from deep
trapping centers.
Index Terms— Amplifier distortion, current, MODFETs, optical spectroscopy, charge carrier processes.
device with a monochromatic light beam of sufficiently large
enough photon energy will (1) release trapped electrons from
deep defects in the conduction band and (2) emit holes in the
valance band by capturing electron in traps on both AlGaN
and GaN. The de-trapping of the carriers leads in turn to a
corresponding increase in the drain current.
In section II, the experiment setup and the method of
analysis are presented. Section III includes the observation
results in RF loadline and S-parameters measurements subject
to illuminations and related discussions. Finally, in conclusion
we summarize our results in Section IV.
Large Signal Network Analyzer
Signal
Source
Port1
Tuner
DUT
Tuner
Current
Sensor
Shutter
Current
Sensor
Bias
Tee
Bias
Supply
Bias
Tee
Port2
I. I NTRODUCTION
GaN-based devices have induced great interests for high frequency, high power, and high-temperature applications owing
to the wide bandgap property of GaN and the characteristics
of AlGaN/GaN heterostructure favorable to high-power highefficiency operation [1]. Although the progress in the power
density and total power available from AlGaN/GaN HEMTs
has been remarkable, the phenomena of RF dispersion and
current collapse [2][3][4][5], have remained critical issues that
limit the performances of GaN-based devices and impacts its
degradation. Many efforts have been devoted to study the
causes of those negative effects and many possible explanations have been proposed[2][4]. Although the origins of those
effects are still under debate, the presence of trapping centers
related to surface, bulk, and/or interface states is considered
as the main reason responsible for those effects.
In this paper, we present a new system combining a large
signal network analyzer (LSNA) and a deep level optical spectrometer (DLOS) [6] to characterize possible deep trapping
centers responsible for RF dispersion or current collapse in
AlGaN/GaN HEMTs. The motivation for using photoionization spectroscopy is as follow: the illumination of a collapsed
978-1-4244-2804-5/09/$25.00 © 2009 IEEE
Monochromator
Deep Level
Optical Spectrometer
Fig. 1.
Experimental setup of LSNA/DLOS combined system.
II. E XPERIMENTAL S ETUP AND M ETHODS OF A NALYSIS
The experimental setup of a LSNA/DLOS combined system
is depicted in Fig. 1. Port 1 is used for the gate and port 2
for the drain of the device under test (DUT). The quiescent
operating point of the DUT is set by the two external bias
tees and DC power supplies. The voltage and current sensors
detect the low-frequency time variations of the device bias
voltage/current which are displayed on the oscilloscope. In
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IMS 2009
the DLOS system, a 1000 W Xenon Lamp is used as a light
source. The photon energy of the incident light in the 1.5-4.2
eV range is generated through a monochromator. A shutter is
placed between the focusing optics and the DUT to control
the duration of the illumination. For this study the DUTs used
are on-wafer unpassivated and SiN-passivated AlGaN/GaN
HEMTs on SiC substrate.
01
0.1
Pulsed-IV and RF loadline subject to 3.54 eV illumination
I DS
(A)
S
0.08
A. CW-RF loadlines measurement
0.04
SDARK
Pulsed-IV and RF
l dli in
loadline
i the
th dark
d k
A 2 GHz RF signal generated by a signal source is applied
to the gate terminal which is maintained at a fixed DC bias
point at VGS = -3.0 V and VDS = 5.0 V. The input RF
power and DC bias applied at the gate of the HEMT device
are selected to provide a gate voltage swing of VGS = from -6
to 0 V in order to ensure that the device reaches the pinchoff
and saturation conditions, respectively. A monochromatic light
beam generated by the DLOS system is then applied to the
HEMT device. Since the occupancy of the traps can be affected
by incident photons from the ambient light, the experiments
are carried out in the dark. To reduce memory effects and allow
for a reasonable relaxation toward equilibrium, the device
remains in the dark after each measurement for typically 3
minutes before the next measurement is performed.
The variation of the RF current swing is defined by:
0
0
B. S-parameters measurement
The LSNA can function also as a vector network analyzer to
measure the corresponding S-parameters of the HEMT device.
The device is maintained at fixed bias point at VGS = 3.0 V and VDS = 5.0 V. A monochromatic light generated
by the DLOS system is sent to the HEMT device. After the
measurement is completed, the illumination is turned off. The
5
10
VDS (V)
Fig. 2. Comparison of measured pulsed-IVs and RF loadlines at 2
GHz with and without illumination.
experiments were also carried out first in the dark. Again
the device were kept in the dark for three minutes between
successive measurements at different photon energies.
25
(1)
where SDARK = IDS,M AX (DARK) − IDS,M IN (DARK)
is the dark drain current swing and S = IDS,M AX −
IDS,M IN is the drain current swing under illumination. Both
drain current swings are defined in Fig. 2. Also plotted in
Fig. 2 are the pulsed-IV characteristics for the maximum gate
voltage measured using the same DC bias points. A good
correlation between the pulsed IV and RF loadlines is observed
for this particular device. Note, however, that this correlation
is not always observed and was found to be strongly foundry
dependent. This is presumably due to the fact that in high
performance devices the electron capture rate is believed to
be ultra fast and in such a case the DC bias point is no longer
setting the trap average occupancy.
In addition, time evolution measurements were also carried
out to study the trap response first when the illumination
turns on and next when it is turned off. The measurement
procedure is the same as described above except that the
LSNA measurements are performed for a fixed delay which
is controlled independently from the illumination duration. It
is to be noted that a LSNA measurement samples the RF
waveforms in 10 ms during which it is assumed the traps
state are not varying.
S
0.02
V
Variatio
on of c
currentt swing
g (mA)
IDS = S − SDARK
IDS
0.06
20
SiN-passivated
Unpassivated
15
10
5
0
-5
1 547
1.547
2 0474
2.0474
2 5476
2.5476
3 0484
3.0484
3 5449
3.5449
4 1357
4.1357
Photon Energy (eV)
Fig. 3. Photoionization spectrum of the variation of current swing
for SiN-passivated and unpassivated AlGaN/GaN HEMTs.
III. E XPERIMENTAL R ESULTS AND D ISCUSSIONS
A. CW-RF loadline photoionization spectrum measurements
Fig. 3 shows the variation in drain current swing of the RF
loadline relatively to the dark case as a function of the photon
energy for both unpassivated and SiN-passivated HEMTs. For
the unpassivated device, it can be observed that when the
energy is larger than about 3.3 eV, a significant increase of the
drain current swing is observed and two noticeable different
levels of increases are identified as well which correspond to
the onset energies of GaN bandgap and AlGaN bandgap at
3.4 eV and 4.1 ev, respectively. As the energy falls below
the GaN bandgap, unlike the result in [2] where only the light
with energy larger than the bandgap affected the drain current,
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TABLE I
T IME CONSTANTS OF THE VARIATION OF CURRENT SWING FOR DIFFERENT ENERGIES OF ILLUMINATIONS
3.10 eV
τ1
τ2
τ1
Optical Saturation
2.05 sec.
10.00 sec.
0.14 sec.
0.75 sec.
Thermal Relaxation
6.40 sec.
29.53 sec.
3.39 sec.
13.74 sec.
an increase, although small, can still be observed in the plot
in the range of energy between 2.1 eV and 3.3 eV which
may correlate with the corresponding energies of the deep
trapping centers inside the bandgap. However, no observable
increase can be seen when the energy falls further below about
2.1 eV. A possible explanation has been proposed in [2] to
interpret the large increase in drain current for energies above
bandgap as originating from the elimination of the virtual gate
on the surface between gate and drain. On the other hand,
for the smaller increase observed for energies below bandgap,
the work in [4] has pointed to a possible relationship with
the trapping centers in the GaN bulk region. For the SiNpassivated, the effect of the illuminations on the variation of
the current swing is largely reduced which shows that the
process of passivation with SiN has a great influence on the
performance of device.
V
Variati
ion of ccurrentt swing
g (mA))
12
10
3.10 eV illumination
3 54 eV illumination
3.54
6
4
C. S parameters measurement
In Fig. 5, the measured S21 parameters of both unpassivated and SiN-passivated HEMTs for different energies of
illumination are plotted together on the Smith Chart for a
frequency range sweeping from 1 GHz to 13 GHz. For the
unpassivated device, the results show that the value of S21
is strongly influenced by the illumination at low frequencies
when the transconductance dominates the DUT response.
The dependence of |S21 | versus photon energy dependence
obtained is similar to the results for the variation of the
drain current swing of the RF loadline. Again a significant
increase in magnitude of S21 appears when the energy of the
illumination is above the GaN and AlGaN bandgap. A small
increase is found in the 2.1 to 3.1 eV range of photon energy
below the GaN bandgap. Also at lower energies below 2.0 eV,
no noticeable change in the magnitude of S21 is observed.
2
0
0
10
20
30
40
50
60
70
80
Delay (s)
Fig. 4. Time evolution of the current swing variation subject to 3.54
eV and 3.10 eV illuminations for unpassivated AlGaN/GaN HEMT.
B. Time evolution CW-RF loadline measurements
The time evolution results of the variation in drain current
swing of the RF loadline for illumination with two different
photon energy of 3.54 eV and 3.10 eV respectively, are shown
in Fig. 4. The results show that when the illumination is turned
on, the amplitude of the current swing will initially increase
rapidly with time and then saturates after a certain illumination
time is reached. Similarly, when the illumination is turned
off, the amplitude of the drain current swing rapidly drops
but this decrease slows down for longer elapsed time. For
τ2
a photon energy of 3.54 eV (above the GaN bandgap), the
optical saturation and equilibrium relaxation time constants
are much shorter than that for the photon energy of 3.10 eV
(below the GaN bandgap). Presumably, these time constants
may be arise from the capture and the emission of carriers
from different deep-trapping centers. Table I lists the fitting
results obtained for those time constants under illuminations
with photon energies of 3.54 eV and 3.10 eV, respectively.
Imag
ginary part off reflec
ction co
oefficie
ent
Time constants
8
3.54 eV
3
S21
2
1GHz
1GHz
1
1.0 13GHz
2.0
0.5
SiN-passivated
p
13GHz
0.2
0
-0.7
-0.6
-0.4
0.0
Unpassivated
0.5
2.0
Inf
-0.2
02
-0.5
-1
-2.0
-1.0
Energy range 3.4-4.1 eV
-2
+ Energy range 2.1-3.3 eV
. Energy
E
range 1
1.5-2.0
5 2 0 eV
V
-3
-6
-4
-2
0
Real part of reflection coefficient
Fig. 5. Variation of S21 subject to different energy of illuminations
with frequency sweeping from 1 GHz to 13 GHz for SiN-passivated
and unpassivated AlGaN/GaN HEMTs.
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Again the process of SiN passivation helps to mitigate the
negative effects caused by the deep trapping centers in the
device resulting in an increase of the current swing and smallsignal gain.
are both very small due to the already large 2DEG charge
concentration in the drain region near the gate. Further detailed
device modeling will be reported elsewhere to backup these
assertions.
0.06
IV. C ONCLUSION
0.055
Real(Y21) ((S)
0.05
Unpassivated
p
SiN-passivated
0.045
0 04
0.04
0 035
0.035
0 03
0.03
0.025
15
1.5
2
25
2.5
3
35
3.5
4
45
4.5
Photon Energy (eV)
Fig. 6. Photoionization spectrum of the real part of Y21 at 2 GHz
for SiN-passivated and unpassivated AlGaN/GaN HEMTs.
6
x 10
-3
5.5
Real(Y
Y22) (S
S)
5
4.5
Unpassivated
U
i t d
SiN-passivated
4
3.5
A measurement system consisting of a combined large
signal network analyzer and a deep level optical spectrometer
was presented to characterize the effects of traps on the smallsignal and large-signal RF response of passivated and unpassivated AlGaN/GaN HEMTs. It was demonstrated that for
illuminations with photon energies above the GaN bandgap,
a significant increase of the variation of the drain current
swing was observed in the CW-RF loadline measurements.
For illumination with photon energy below the bandgap, a
residual variation was detected presumably originating from
deep energy levels. The time evolution of the drain current
swing for energy of illumination above and below the GaN
bandgap enabled to determine the optical transient timeconstant and the equilibrium relaxation time-constant.
The measured small-signal S21 and S22 parameters under
monochromatic illumination provided consistent results with
the large-signal drain-current swing measurements obtained
from the CW-RF loadlines. An improved performance for
illuminated devices is also observed in the variation of the real
part of Y21 and Y22 from low to high photon energies. This
presumably arises from the variation of the drain resistance
under illumination in unpassivated devices due to the traps
residing between the gate and drain region.
On the other hand it was seen that the process of SiNpassivation which deactivates the deep trapping centers at
the surface and desensitizes the device to both trapping and
illumination greatly helps in suppressing the degradation performance observed in unpassivated AlGaN/GaN HEMTs.
3
ACKNOWLEDGEMENT
2.5
2
5
1.5
2
2.5
3
3.5
Photon Energy (eV)
4
This work was supported by the Office of Naval Research
(ONR) under Grant N0014-08-1-0101 organized by Dr. Maki.
4.5
Fig. 7. Photoionization spectrum of the real part of Y22 at 2 GHz
for SiN-passivated and unpassivated AlGaN/GaN HEMTs.
To further evidence the influences of the illuminations on
the device performances, we plot in Fig. 6 and Fig. 7 the
real part of the small signal transconductance Y21 and drain
conductance Y22 respectively. The real part of Y21 increases
from 0.027S to 0.035S and the real part of Y22 decreases from
0.0056S to 0.0046S from low to high photon energies in the
unpassivated device whereas it remains approximately constant
in the passivated device. The increase in the transconductance
is presumably due to the decrease of the drain resistance under
illumination which negates the effect of the virtual surface
gate. The decrease of the drain conductance is itself certainly
due to the decrease of the 2DEG width under illumination [7].
In passivated devices theses effects although still present are
negligeable given the drain resistance and the 2DEG width
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