N. Zagni et al.: “Hole Redistribution” Model Explaining the Thermally Activated R
ON
Stress/Recovery Transients 1
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Abstract—R
ON
degradation due to stress in GaN-based
power devices is a critical issue that limits, among other
effects, long-term stable operation. Here, by means of two-
dimensional device simulations, we show that the R
ON
increase and decrease during stress and recovery
experiments in Carbon-doped AlGaN/GaN power MIS-
HEMTs can be explained with a model based on the
emission, redistribution, and re-trapping of holes within the
Carbon-doped buffer (‘hole redistribution’, in short). By
comparing simulation results with front- and back-gate off-
state stress experiments we provide an explanation for the
puzzling observation of both stress and recovery transients
being thermally activated with the same activation energy
of about 0.9 eV. This finds a straightforward justification in
a model in which both R
ON
degradation and recovery
processes are limited by hole emission by dominant
Carbon-related acceptors that are energetically located at
about 0.9 eV from the GaN valence band.
Index Terms—GaN MIS-HEMT, Current Collapse, Off-
State Stress, ON-Resistance Degradation, Hole
Redistribution
I. INTRODUCTION
OWER devices based on the AlGaN/GaN heterostructure
are becoming a popular technology solution as a
replacement for Silicon devices [1]. The High Electron
Mobility Transistors (HEMTs) based on this semiconductor
system exhibit enhanced switching speed and breakdown field
capability, enabling operation at higher operating
N. Zagni, A. Chini, F. M. Puglisi, and P. Pavan are with the
Department of Engineering “Enzo Ferrari”, Università di Modena e
Reggio Emilia, 41125, Modena, Italy. (e-mail: nicolo.zagni@unimore.it).
M. Meneghini, G. Meneghesso, and E. Zanoni are with the
Department of Information Engineering (DEI), University of Padova,
35131, Padova, Italy.
G. Verzellesi is with Department of Sciences and Methods for
Engineering (DISMI) and EN&TECH Center, University of Modena and
Reggio Emilia, 42122, Reggio Emilia, Italy.
voltage/frequency/temperature [1], [2]. However, the
introduction of AlGaN/GaN HEMTs into the mass market not
only depends on the demonstration of outstanding performance
but also on the stable and reliable operation of these devices.
Mitigation of the failure mechanisms to ensure long-term
reliable operation is therefore crucial to the success of GaN
power electronics. In this regard, several physical aspects
related to the so-called current-collapse effect (dynamic drain
current drop or on-resistance increase) and threshold voltage
drift (influencing current-collapse as well) occurring when
performing off-to-on (or on-to-off) switching still need to be
fully understood. Experimentally, “stress” tests are carried out
by applying bias conditions that are known to accelerate the so-
called dynamic R
ON
degradation, i.e. the increase in the device
on-state resistance (R
ON
) during the typical pulse-mode
operation of power transistors in power switching converters.
In this work, we propose an explanation for the on-resistance
(R
ON
) behavior during both front- and back-gating OFF-state
stress and recovery experiments reported for AlGaN/GaN
Metal-Insulator-Semiconductor HEMTs (MIS-HEMTs)
featuring a Carbon-doped buffer, which is based on a hole
emission/redistribution/re-trapping model (‘hole
redistribution’, in short). This paper is based on a recent
conference paper of ours [3] where only back-gating
stress/recovery were considered, and extends our analysis to
more customary front-gating stress/recovery experiments.
Carbon (C) doping is a widespread technology solution to
reduce buffer conductivity and increase breakdown voltage for
power applications [4], [5]. The introduction of acceptor traps
associated with C-doping, however, leads to an enhancement of
current collapse and dynamic R
ON
degradation during OFF-state
stress [6]. It is widely accepted that this is due to negative
charge build-up in the C-related traps within the buffer [7]–
[11]. This phenomenon is conventionally attributed to electron
capture into buffer traps [8]–[11]. The work by Meneghesso et
‘Hole Redistribution’ Model Explaining the
Thermally Activated R
ON
Stress/Recovery
Transients in Carbon-Doped AlGaN/GaN
Power MIS-HEMTs
Nicolò Zagni, Student Member, IEEE, Alessandro Chini, Francesco Maria Puglisi, Member,
IEEE, Matteo Meneghini, Senior Member, IEEE, Gaudenzio Meneghesso, Fellow, IEEE,
Enrico Zanoni, Fellow, IEEE, Paolo Pavan, Senior Member, IEEE, and Giovanni Verzellesi,
Senior Member, IEEE
P
N. Zagni et al.: “Hole Redistribution” Model Explaining the Thermally Activated R
ON
Stress/Recovery Transients 2
al. in [7] reports on the R
ON
stress and recovery behavior
measured in power MIS-HEMTs during front- and back-gating
experiments, finding that both transients under both test
conditions are thermally activated with the same activation
energy (E
A
) of about 0.9 eV. In [7], a possible explanation for
this was proposed, based on the observed correlation of the
thermally activated stress with the increase with temperature of
buffer leakage [8]. Moreover, the discharging of C-related
buffer traps during recovery transients was also attributed to
thermally activated vertical/lateral leakage paths in other works
[6], [12], [13].
Here, we propose that the activation energy of both stress and
recovery processes can be directly attributed to the dominant
acceptor trap energy level associated with Carbon in the buffer,
as a result of hole emission, redistribution, and re-trapping in
the C-doped buffer. Stress is performed either: i) by applying a
negative bias to the gate contact (V
G
) and a large positive bias
to the drain contact (V
D
) (with source and substrate contacts
grounded), or ii) by applying a negative bias to the substrate
contact (V
B
) (with all other contacts grounded). We will refer to
the above stress conditions as to front- and back-gating OFF-
state stress, respectively (FGOS/BGOS in short).
The proposed model: i) is seamlessly related to the
commonly accepted model for C doping in GaN, i.e., a
dominant acceptor trap level at about 0.9 eV from the valence
band edge (E
V
) [14], that turns the GaN buffer into a weakly p-
type region; ii) does not require, though does not exclude,
possible charging/discharging mechanism through leakage
paths; iii) self-consistently captures the dynamics of
stress/recovery processes up to drain bias for which C doping is
able to effectively suppress electron leakage current through the
GaN buffer, i.e., before significant electron injection into the
buffer from the source and/or from the substrate takes place.
The paper is organized as follows. In Section II, the modeling
framework is illustrated along with the analyzed device
structure and the relevant physical models employed. Results
are presented and discussed in Section III. Finally, conclusions
are drawn in Section IV.
II. MODELING FRAMEWORK
The structure employed in the simulations in this work is
sketched in Fig. 1, resembling a typical power AlGaN/GaN
MIS-HEMT. The two-dimensional (2D) numerical device
simulations were carried out with SDevice
TM
(Synopsys Inc.)
Technology CAD (TCAD) simulator [15]. We first calibrated
our TCAD simulation deck against experimental I
D
-V
GS
data
reported in [16]. The outcome is shown in Fig. 2. As reported
in [17], the calibrated device features a highly-conductive p-
type Si substrate, an AlN nucleation layer (200 nm), a C-doped
GaN buffer (2.3 µm), an unintentionally doped (UID) GaN
channel (150 nm), an Al
0.25
Ga
0.75
N barrier (10 nm) and a Si
3
N
4
passivation layer over the access regions (120 nm). The gate
insulator consists of an Al
2
O
3
layer (15 nm) that is added to the
structure after partially recessing the barrier and leaving 3.7 nm
of residual AlGaN under the gate [16]. The gate-to-drain access
region is 5 µm [16] (while it is 10 µm in [8], [17] from which
stress and recovery experimental data are taken).
Charge transport was modelled by means of the drift-
diffusion model. The piezoelectric polarization at the
heterointerfaces was included by using the default strain model
of the simulator [15]. A fully dynamic trap modelling approach
was adopted, with one Shockley-Read-Hall (SRH) trap-balance
equation for each distinct trap level included, describing the
dynamics of trap occupation without any quasi-static
approximation. A detailed description of the modeling
approach to describe device physics in AlGaN/GaN based
HEMTs can be found in [18]. The gate insulator (Al
2
O
3
) is
assumed ideal, i.e., gate leakage current is neglected.
C doping in the GaN buffer was modelled by considering a
dominant deep acceptor trap at 0.9 eV above E
V
partially
compensated by a shallow donor trap at 0.11 eV below E
C
[17].
To reproduce the experimental results, the adopted trap
concentrations were 2×10
18
cm
-3
and 1×10
18
cm
-3
, for C-related
acceptors and donors, respectively. This corresponds to an
effective acceptor density of 1×10
18
cm
-3
with a compensation
ratio of 50%. No additional traps were considered, while in all
nitride layers an n-type doping density of 1x10
15
cm
-3
was
assumed, as conventionally done when simulating GaN devices
to account for the unintentional n-type conductivity due to
shallow-donor impurities like Oxygen and Silicon during
growth [5], [19].
The C doping model employed in this work was developed
over the years by our group and allowed explaining measured
current-collapse, threshold voltage shifts, and breakdown
effects in different GaN power HEMTs [3], [20]–[25]. The
dependability of the acceptor-donor model for C doping is
further confirmed by its capability of reproducing source-drain
leakage currents and off-state breakdown as reported in [19].
The key assumption in the adopted C doping model is that the
dominant deep acceptor traps for holes are partially
compensated by shallow donor traps for electrons. The actual
energy position of donor traps however, if sufficiently shallow,
Fig. 1. Cross-section of the simulated MIS-HEMT with the recessed
AlGaN barrier.
Fig. 2. Calibration of simulated (blue solid lines) against experimental
data from [16] (black squares) I
D
-V
GS
characteristics.
N. Zagni et al.: “Hole Redistribution” Model Explaining the Thermally Activated R
ON
Stress/Recovery Transients 3
Fig. 3. R
ON
variations (normalized w.r.t. the pre-stress value) during FGOS (a, c) and consequent recovery (b, d) experiments (from [7]) and
simulations carried out at different temperatures (see legend). Stress/Recovery conditions are (V
G
, V
D
, V
B
) = (-8, 25, 0) V, and (V
G
, V
D
, V
B
) = (0,
0.5, 0) V, respectively.
Fig. 4. R
ON
variations (normalized w.r.t. the pre-stress value) during BGOS (a, c) and consequent recovery (b, d) experiments (from [7]) and
simulations carried out at different temperatures (see legend). Stress/Recovery conditions are (V
G
, V
D
, V
B
) = (0, 0, -25) V, and (V
G
, V
D
, V
B
) = (0,
0.5, 0) V, respectively.
N. Zagni et al.: “Hole Redistribution” Model Explaining the Thermally Activated R
ON
Stress/Recovery Transients 4
has little influence on simulation results. Indeed, C-related
donors could actually be closer to E
C
or even be modelled as
completely ionized doping, in agreement with recent hybrid-
functional DFT calculations [14], [26]. Further, an
experimental indication that C doping introduces donors
besides acceptors is found in [27]. According to previous
reports, for moderate C doping concentration (i.e., ≤10
18
cm
−3
)
it is appropriate to model dopants as discrete point defects,
whereas for concentrations of about (or higher than) 10
19
cm
−3
it is more appropriate to use a defect-band model [28].
III. RESULTS AND DISCUSSION
We will first compare simulation results with the
experimental data obtained from stress and recovery tests
carried out on the MIS-HEMT, as reported in [7]. The sketch of
the cross-section is shown in Fig. 1. Both stress conditions (i.e.,
FGOS and BGOS) bias the device in the subthreshold region.
However, the BGOS setup is useful to rule out surface trapping
effects – which can be present during FGOS instead – thus
allowing to attribute the observed phenomena to buffer traps
only [29], [30]. Under BGOS tests in fact, the formed 2DEG
channel screens the superficial layers from the field effect
induced by back-gating, so surface effects should be negligible
[29]. The fact that a similar kinetics was found for FGOS and
BGOS is an indication that buffer traps are mainly involved [8].
Since we only include buffer traps in our simulation setup, both
FGOS and BGOS conditions modify the state of C-related traps
only. The comparison between experimental data and
simulation results is shown in Figs. 3 and 4, with stress and
recovery conditions applied as follows. i) FGOS and recovery:
(V
G
, V
D
, V
B
) = (-8, 25,0) V and (V
G
, V
D
, V
B
) = (0, 0.5, 0) V,
respectively; ii) BGOS and recovery: (V
G
, V
D
, V
B
) = (0, 0, -25)
V and (V
G
, V
D
, V
B
) = (0, 0.5, 0) V, respectively. The chosen
experimental FGOS and BGOS conditions represent
“intermediate” OFF-state bias conditions, i.e., with drain
voltages that are large enough to have appreciable dynamic R
ON
effects but, at the same time, low enough not to promote
significant electron injection through the C-doped buffer due to
lateral source-drain punch-through or vertical leakage current.
During stress simulations, R
ON
values were obtained by fast
sweeping the device bias to measurement conditions (V
G
, V
D
) =
(0, 0.5) V in 10 ms to mimic on-the-fly (OTF) measurements
[7]. During recovery, instead, R
ON
was monitored throughout
the simulation as recovery and measurement conditions were
the same. R
ON
results in Figs. 3 and 4 are normalized with
respect to the fresh value at each temperature to purify results
from the R
ON
degradation induced by mobility reduction.
Recovery tests were performed immediately after the stress
phase was completed. Since R
ON
measurement takes about 50
ms [7] no measurement data points were acquired for recovery
time less than 100 ms. For both FGOS and BGOS, simulation
results can reproduce reasonably well the essential features
shown by the experimental results taken at different
temperatures. That is, simulations capture the thermally
activated processes at the basis of both R
ON
degradation and
recovery, as well as the time constant ranges.
As shown in [7], stress and recovery transients are found to
be thermally activated with similar activation energies in the
range 0.84-0.95 eV, irrespective of the stress condition. We
report the experimental Arrhenius plots shown in [7] (for stress
only) in Fig. 5 for both FGOS and BGOS conditions and
compare them with simulation results (in this case showing both
stress and recovery). The time constants at each temperature for
both experiments and simulations were extracted by fitting the
curves with the stretched exponential method [31]. The data at
60 °C were not suitable for the extraction of a time constant and
were therefore excluded from the Arrhenius plot, the latter still
having four reliable points for the extrapolation of E
A
. As it can
be noted, the Arrhenius signature of the stress process is well
reproduced by our simulations and, more importantly,
simulations predict the same activation energy of about 0.9 eV
for both stress and recovery in either FGOS/BGOS conditions.
Before providing the detailed explanation for this, it is
important to observe that the R
ON
increase shown in Figs. 3a)
and 3c) (as well as in Figs. 4a) and 4c)) can in principle be
induced by either electron trapping into or hole de-trapping
from the buffer traps. Consequently, the corresponding R
ON
recovery illustrated in Figs. 3b) and 3d) (as well as in Figs. 4b)
and 4d)) can be ascribed to either electron de-trapping or hole
trapping, respectively. However, regardless of the actual charge
carriers involved, the fact that both emission and capture
processes are thermally activated and, more importantly, exhibit
the same activation energy is not trivial. In fact, while carrier
emission is always a thermally activated process, carrier capture
can only be thermally activated in traps that feature a capture
barrier, although the associated activation energy is generally
different (and smaller) than the emission one [8], [11]. In the
case of the devices considered here, the extracted E
A
(for both
stress and recovery) correlates very well with the transition
energy of the dominant acceptor level (C
N
) related to Carbon in
GaN [13].
We will now show that this behavior can be explained with a
hole emission, redistribution, and re-trapping model. The model
applies to both FGOS and BGOS. Thus, we focus only on the
latter case for simplicity of data presentation, as under back-
gating conditions the buffer is uniformly exposed to the
backside bias and all internal quantities are characterized by an
almost one-dimensional distribution (along the device depth
direction). In the following, we will explain the processes
occurring during stress and recovery with the aid of the plots of
the net ionized acceptor trap density,
, and of the
Fig. 5. Arrhenius plot for the simulated a) FGOS and b) BGOS
transients as well as the relative recovery processes. Experimental
data from [7] of stress processes are reported for comparison. Lines
are the linear fitting of the data showing that both experiments and
simulations are characterized by the same E
A
0.9 eV.
N. Zagni et al.: “Hole Redistribution” Model Explaining the Thermally Activated R
ON
Stress/Recovery Transients 5
free hole density, p, shown in Figs. 6 and 7. These plots are
taken along a cutline parallel to the device depth drawn in the
middle of the gate-to-drain access region and zoomed in at the
top and bottom regions of the buffer, i.e., at the channel/buffer
and nucleation/buffer interface, respectively. Note that the
ionized donor trap density in the buffer remains constant (not
shown) because the energy level of these traps is shallow (i.e.,
0.11 eV from E
C
) [19]. The C-doped buffer behaves as a weakly
p-type region and the C-related donors simply have the role of
partially compensating the dominant acceptor levels, without
causing dynamic effects themselves. This picture can change at
higher voltages than those considered here, as approaching
lateral or vertical breakdown conditions significant electron
injection into the C-doped buffer can take place either from the
source or the substrate, respectively. Under such conditions,
transport processes may impact the stress/recovery kinetics, as
suggested in [32].
During stress,
(net negative charge) in the top
region of the buffer close to the channel increases because holes
are being emitted from the 0.9-eV C-related acceptor traps to
the valence band. This correlates with the observed R
ON
increase during stress. The variation in
close to the
channel is evident by comparing the cases before and after
stress in Figs. 6a) and 6b). The hole emission process is
Fig.6. Net ionized acceptor trap density,
, along the vertical direction in the Gate-to-Drain access region at different conditions,
namely: fresh (a, d), after 1000 s stress (b, e), and after 1000 s recovery (c, f) near the channel/buffer interface (a-c) and near the
buffer/nucleation interface (d-f). BGOS conditions were applied (T = 100 °C).
Fig. 7. Free hole density, p, along the vertical direction in the Gate-to-Drain access region at different conditions, namely: fresh (a, d), after
1000 s stress (b, e), and after 1000 s recovery (c, f) near the channel/buffer interface (a-c) and near the buffer/nucleation interface (d-f).
BGOS conditions were applied (T = 100 °C).
![Fig. 4. RON variations (normalized w.r.t. the pre-stress value) during BGOS (a, c) and consequent recovery (b, d) experiments (from [7]) and simulations carried out at different temperatures (see legend). Stress/Recovery conditions are (VG, VD, VB) = (0, 0, -25) V, and (VG, VD, VB) = (0, 0.5, 0) V, respectively.](/figures/fig-4-ron-variations-normalized-w-r-t-the-pre-stress-value-3s63tshc.webp)
![Fig. 3. RON variations (normalized w.r.t. the pre-stress value) during FGOS (a, c) and consequent recovery (b, d) experiments (from [7]) and simulations carried out at different temperatures (see legend). Stress/Recovery conditions are (VG, VD, VB) = (-8, 25, 0) V, and (VG, VD, VB) = (0, 0.5, 0) V, respectively.](/figures/fig-3-ron-variations-normalized-w-r-t-the-pre-stress-value-1u99ss4v.webp)
![Fig. 2. Calibration of simulated (blue solid lines) against experimental data from [16] (black squares) ID-VGS characteristics.](/figures/fig-2-calibration-of-simulated-blue-solid-lines-against-32pkb5d2.webp)
![Fig. 5. Arrhenius plot for the simulated a) FGOS and b) BGOS transients as well as the relative recovery processes. Experimental data from [7] of stress processes are reported for comparison. Lines are the linear fitting of the data showing that both experiments and simulations are characterized by the same EA 0.9 eV.](/figures/fig-5-arrhenius-plot-for-the-simulated-a-fgos-and-b-bgos-37thhsx3.webp)