Experimental and Simulation Study on the Electrical Characteristics of Proton-irradiated
AlGaN/GaN HEMT
RaJeonSo1
LeeSang Ho1
ParkJin1
KimMin Seok1
BaeSeung Ji1
HongJeong Woo1
KohWon Suk1
YunGang San1
KangIn Man1,*
YoonYoung Jun2,**
-
(School of Electronic and Electrical Engineering, Kyungpook National University, Daegu
41566, Korea)
-
(Department of Electronic Engineering, Andong National University, Andong 36729, Korea)
Copyright © The Institute of Electronics and Information Engineers(IEIE)
Index Terms
AlGaN/GaN HEMT, proton irradiation effects, reliability analysis, simulation modeling
I. INTRODUCTION
Gallium nitride (GaN), with its wide bandgap (WBG) characteristics, is emerging as
a promising candidate for next-generation high-efficiency power devices and radio
frequency (RF) electronic applications. This is due to its superior material properties,
such as a high electric break-down field, high-electron saturation velocity, and high
mobility in a readily available heterojunction 2-D electron gas (2DEG) channel [1]. GaN's high breakdown field of 3.3 MV/cm [2] is 11 times higher than Si at 0.3 MV/cm. Consequently, for a given voltage, a GaN
layer can be 11 times thinner than its Si counterpart, resulting in reduced resistivity.
This significantly lowers resistive losses in switching mode power supplies (SMPSs)
[3]. High-electron mobility transistors (HEMTs) based on the AlGaN/GaN heterojunction
are exemplary devices for space applications due to GaN's superior properties. Satellites
and their electronic components are exposed to harsh radiation environments [4], particularly form Earth's geomagnetic fields. Space radiation includes heavy ions
and protons, which generate neutrons and other byproducts upon collision with devices.
For materials and devices to be useful in high orbit and space environments, they
must withstand particle irradiation damage [5]. GaN HEMTs, used in low Earth orbit satellites, are typically subjected to fluxes
of high-energy protons and electrons, as well as neutrons or gam-ma rays if used in
radiation-hard electronics for nuclear or military systems [6]. GaN is recongnized for its radiation resistance [7,8,9], making it a highly researche topic. Previous research has focused on evaluating
HEMTs (MIS-HEMTs) with various gate dielectric layers, including SiN [10,11], MgO/Sc${}_{2}$O${}_{3}$ [12], Gd${}_{2}$O${}_{3}$ [13], SiN/Al${}_{2}$O${}_{3}$ [14,15], and poly-AIN/SiN [16]. Studies have also explored InAlAs/InGaAs hetero junction structures of InP-based
HEMTs at different incident angles [17]. However, these studies primarily focused on the impact of dielectrics on radiation
resistance and varied proton incident angles [17,18]. We identified and analyzed trends to determine how proton irradiation afftects the
device and why its reliability deteriorates. The degradation in reliability after
proton irradiation leads to performance issues, which must be addressed. In this study,
we used the AlGaN/GaN HEMT process to measure DC characteristics and current collapse.
Following this, the devices were irraddiated with protons at 5 MeV energy and a fluence
of $5 \times 10^{13}$ cm${}^{-2}$. We then remeasured the DC characteristics and current
collapse to analyze the changes in device performance. We measured the current characteristics
of the devices before and after proton irradiation to evaluated the changes and assess
the influence of protons Additionally, we performed simulation fitting for further
analysis. This study provides valuable data on the impact of proton radiation on power
semiconductor devices, which can be utilized in developing high-reliability power
semiconductor technology.
II. EXPERIMENTAL PROCEDURE
Fig. 1 shows the schematic cross-sectional view of the AlGaN/GaN high-electron-mobility
transistor (HEMT) and Table 1 summarizes the parameters used in this study. The gate length (L${}_{\rm G}$) is
3 $\mu$m, the gate-to-source distance (D${}_{\rm GS}$) is 5 $\mu$m, and the gate-to-drain
distance (D${}_{\rm GD}$) is 5 $\mu$m. The epitaxial growth details are as follows:
the AlGaN barriere thickness (T${}_{\rm AlGaN}$) is 20 nm, the GaN channel thickness
(T${}_{\rm Channel}$) is 180 nm, the GaN buffer thickness (T${}_{\rm buffer}$) is
2.3 $\mu$m, and the substrate thickness (T${}_{\rm sub}$) is 430 $\mu$m. Fig. 2 illustrates the growth of AlGaN/GaN HEMT structures on sapphire using metal-organic
chemical vapor deposition (MOCVD) equipment. Fabrication began with the dry etching
process for electrical isolation between devices, targeting a depth of $\sim$300 nm
using ICP equipment, with Cl2 gas (100 sccm), Ar gas (10 sccm), RFT (power 800) and
RFB (power 200). We deposited a 30 nm-thick SiN as passivation layer using plasma-enhanced
chemical vapor deposition (PECVD). After the SiN deposition layer, a spin coater was
used to apply photoresist (PR) to 5000 RPM, for 35 s. Following a soft bake ($90^\circ{\rm
C}$, for 90 s), exposure (810 W/cm${}^{2}$, for 30 s) was performed using MA6_1KW
equipment, PR and SiN wet etching were conducted using PS-800, and BOE, respectively.
Source and drain metallization (Si/Ti/Al/Ni/Au) was performed using an electronic-beam
evaporator (ERC). Lift-off was executed with PS-800, and RTP ($850^\circ{\rm C}$,
for 80 s) converted the Schottky contact into ohmic contact. Subsequent wet etching
removed the SiN layer removal, forming the source and drain. The ohmic contact was
confirmed by measuring TLM. Finally, after gate photolithography, gate metallization
(Ni/Al/Ni) was carried out using an ERC completing the HEMT device fabrication. The
HEMTs were irradiated with 5 MeV protons at a fluence of $5 \times 10^{13}$ cm${}^{-2}$
using the proton linear accelerator at the Korea Multi-Purpose Accelerator Complex
(KOMAC). The current characteristics of the fully fabricated devices were measured
using a B1500 semiconductor device analyzer (K Keysight, Santa Roda, California, USA).
After proton irradiation, the devices were re-measured to assess the changes.
Fig. 1. Schematic cross-sectional view of the AlGaN/GaN high-electron-mobility transistor
(HEMT).
Fig. 2. Process flow of the HEMT and proton irradiation.
Table 1. Parametes of the AlGaN/GaN HEMT.
|
Parameter
|
Value
|
|
Gate Length (LG)
|
3 $\mu$mm
|
|
Gate to Source Distance (DGS)
|
5 $\mu$mm
|
|
Gate to Drain Distance (DGD)
|
5 $\mu$mm
|
|
AlGaN Barrier Thickness (TAlGaN)
|
20 nm
|
|
GaN Channel Thickness (TChannel)
|
180 nm
|
|
GaN Buffer Thickness (Tbuffer)
|
2.3 $\mu$mm
|
|
Substrate Thickness (Tsub)
|
430 $\mu$mm
|
III. RESULTS AND DISCUSSION
1. Direct Current (DC) Characteristics
Figs. 3(a) and 3(b) show the output curves (I${}_{\rm D}$-V${}_{\rm D}$) before and after proton irradiation
at a drain voltage (V${}_{\rm D}$) of 10 V. As seen in Fig. 3(b), the current characteristics deteriorated significantly after irradiation with 5
MeV protons at a fluence of $5 \times 10^{13}$ cm${}^{-2}$. The static resistance
(R${}_{\rm on}$) value was measured at a gate voltage (V${}_{\rm G}$) of 1 V and drain
voltage (V${}_{\rm D}$) of 0.1V. The R${}_{\rm on}$value increased from 62.5 $\Omega\cdot$mm
before irradiation to 98.135 $\Omega\cdot$mm after irradiation, at V${}_{\rm G} =
1$ V, respresenting a 57% increase. This indicates substantial damage to the devices
from proton irradiation. Additionally, we evaluated the buffer leakage current characteristics
by measuring the current. Fig.~4 shows the buffer current before and after proton
irradiation. Post-irradiation data, represented by the red open symbols (A and A$'$),
show higher current values at oltages below 10 V. As the voltage value increases,
the current initially rises (marked as A$'$) and then decreases again indicating changes
in the buffer current characteristics due to proton irradiation. Because of thisinflection,
it cannot be definitively stated the buffer current decreased after proton irradiation.
The observed current characteristics are due to measurements taken through a buffer
pattern, which captures both buffer and surface leakage currents. As a result, at
voltages below 10 V (marked as A), the post-irradiation current is larger than the
pre-irradiation current As the voltage increases, electrons become trapped in th buffer
layer's trap, causing the current to decrease.
Figs. 5(a) and 5(b) show the transfer curves (I${}_{\rm D}$-V${}_{\rm GS}$) before and after proton irradiation
at a drain voltages (V${}_{\rm D}$) of 0, 0.1, and 7 V. The on-current (I${}_{\rm
on}$) values of the before and after proton irradiation are $3.51 \times 10^{-1}$
and $2.57 \times 10^{-1}$ A/mm at V${}_{\rm D} = 7$ V, respectively. The corresponding
off-current (I${}_{\rm off}$) values for the AlGaN/GaN HEMT are $1.35 \times 10 ^{-6}$
and $2.56 \times 10^{-6}$ A/mm, respectively, at V${}_{\rm D} = 7$ V. Additionally,
the I${}_{\rm on} /$I${}_{\rm off}$ ratios are $2.6 \times 10^{5}$ and $1.0 \times
10^{5}$, respectively. The threshold voltage (V${}_{\rm t}$) was calculated using
the g${}_{\rm m,max}$ method, with the V${}_{\rm t}$ values $-3.24$ V before irradiation
and $-3.27$ V after irradiation. This indicates that the I${}_{\rm on}$ decreased
by 26.78% and the surface leakage current increased, leading to an 89.63% increase
in I${}_{\rm off}$.
However, there was no significant change in V${}_{t}$ suggesting minimal damage in
the 2DEG channel region that determines V${}_{\rm t}$. For more accurate analysis,
the R${}_{\rm on}$ value as a function of D${}_{\rm GD}$ was obtained. Fig. 6 shows the R${}_{\rm on}$ values as a function of D${}_{\rm GD}$ before and after
proton irradiation. Closed black symbols represent R${}_{\rm on}$ values before proton
irradiation, while open red symbols indicate R${}_{\rm on}$ values after irradiation.
For a D${}_{\rm GD}$ of 5 $\mu$m, the R${}_{\rm on}$ value increases from 6.58 $\Omega\cdot$mm
before irradiation to 7.66 $\Omega\cdot$mm after irradiation. Similarly, for D${}_{\rm
GD}$ values of 10, 20, and 30 $\mu$m, the R${}_{\rm on}$ values change from 7.58,
11.83, and 15.46 $\Omega\cdot$mm before irradiation to 9.14, 13.04, and 16.73 $\Omega\cdot$mm
after proton irradiation. The key point in this figure is that the slope of the line
representing the R${}_{\rm on}$ values does not significantly change after proton
irradiation. This slope corresponds to the sheet resistance (R${}_{sh}$), which is
the resistance of the 2DEG channel region formed between the AlGaN and the GaN layers.
The slope values before and after irradiation. This slope corresponds to the sheet
resistance (R${}_{sh}$), which is the resistance of the 2DEG channel region formed
between the AlGaN and the GaN layers. The slope values before and after rradiation
are $\sim$0.3552 and 0.3628, respectively, indicating that the degradation of device
characteristics is not due to the 2DEG channel layer. Therefore, the changes in device
characteristics must be attributed to other factors, specifically device resistance
Fig. 7 illustrates the AlGaN/GaN HEMT, where the R${}_{\rm on}$value is the sum of R${}_{\rm
S}$, R${}_{\rm Ch}$, R${}_{\rm GD}$, and R${}_{\rm D}$, as shown in Equation (1).
Here, R${}_{\rm S}$ and R${}_{\rm D}$ are the source and drain resistances, which
correspond to metal contacts, and R${}_{\rm Sh}$ represents the sheet resistance or
2DEG resistance. Since the slope value representing R${}_{\rm Sh}$ remains nearly
unchanged, the increase in R${}_{\rm on}$ value after proton irradiation must be due
to the chanages in the source and drain resistances, i.e., the metal resistance. Additionally,
it was observed that the metal regions suffered more damage compared to the 2DEG regions
when subjected to the same proton irradiation across the device. Figs. 8(a) and 8(b) schematically illustrate proton irradiation throughout the AlGaN/GaN HEMT and the
interaction when a proton collides with metal. The higher density ad specific properties
of the metal contacts result in greater damage compared to the 2DEG regions when exposed
to proton irradiation. When a hydrogen proton irradiates with the device, it collides
with the very small metal atoms. This collision produces secondary particles, such
as protons or neutrons leading to significant damage around the metal areas.
Fig. 3. Output curves (I${}_{\rm D}$-V${}_{\rm D}$) (a) before and (b) after proton
irradiation at a drain voltage (V${}_{\rm D}$) of 10 V.
(a)
(b)
Fig. 4. Buffer current characteristics of AlGaN/GaN HEMT before and after proton irradiation.
Fig. 5. Transfer curves (I${}_{\rm D}$-V${}_{\rm GS}$) (a) before and (b) after proton
irradiation at drain voltages (V${}_{\rm D}$) of 0, 0.1, and 7 V.
(a)
(b)
Fig. 6. The R${}_{\rm on}$ value according to D${}_{\rm GD}$ before and after proton
irradiation.
Fig. 7. Resistance characteristics of the AlGaN/GaN HEMT.
Fig. 8. (a) Schematic of proton irradiation throughout the device, and (b) interation
when a proton collides with metal.
(a)
(b)
2. TCAD Simulation-based Analysis of Proton Irradiation Impact
We used the Atlas TCAD semiconductor device simulator from SILVACO for simulation
modeling. The modeled HEMT structure consists of a gate, source, and drain are formed
on an epitaxy layer with a GaN buffer layer, a channel layer, and an AlGaN layer are
grown on a sapphire substrate. The AlGaN layer is 20-nm thick, while the GaN layer
is 180-nm thick, with the Al composition ratio in AlGaN being $\sim$25%. The polarization
between he AlGaN/GaN heterostructure is controlled based on the AlGaN layer's thickness
and Al composition ratio to determine electron configuration and mobility. The model
incorporates a polarization model for implementing 2DEG and a thermal model to account
for heat dissipation during device operation. Based on this model, we performed transfer
curve fitting before proton irradiation.We also accounted for contact resistance,
which significantly affects device characteristics, particularly after irradiation.
The SRIM simulation results show vacancies occurring when protons irradiate the AlGaN/GaN
HEMT [19].
At a standard energy of 5 MeV protons are injected to a depth of 120 $\mu$m. Since
the protons passe through the AlGaN/GaN heterojunction structure to the sapphire substrate,
they affect the entire device in terms of depth, producing many Ga and N vacancies.
Figs. 9(a) and 9(b) show the AlGaN/GaN HEMT simulation fitting before and after proton irradiation. The
measurement values are represented by lines, while symbols indicate the simulation
fitting values. The simulation incorporates polarization for 2DEG implementation and
the self-heating phenomenon during device operation, The initial R${}_{\rm C}$ value
was based on $1.3 \times 10^{-5}$ as referenced in [21]. As shown in Fig. 9(b), when R${}_{\rm C}$ increased to $1.9 \times 10^{-5}$, the simulated current most
closely matched the measured value, indicating that R${}_{\rm C}$ increased by $\sim$1.46
times after irradiation compared to that before irradiation.
Fig. 9. (a) and (b) AlGaN/GaN HEMT simulation fitting before and after proton irradiation.
(a)
(b)
3. Proton-induced degradation of current collapse characteristics
Fig. 10 shows the current collapse characteristics of the AlGaN HEMT before and after the
proton irradiation at V${}_{\rm G} = 1$ V. The gate and drain biases (V${}_{\rm G_B}$,
and V${}_{\rm D_B,\ }$, respectively) were $(0$ V, $0$ V$)$, $(-5$ V, $0$ V$)$, $(0$
V, $10$ V$)$, and $(-5$ V, $10$ V$)$. The pulse period and width (P${}_{\rm period}$
and P${}_{\rm width}$) were 5 ms and 100 $\mu$s, respectively. The pink symbols indicate
the base voltage section without applied stress voltage. The blue and red symbols
represent stress voltage applied to the gate and drain, respectively. This separation
of base voltage helps in analyzing the degradation of device characteristics after
proton irradiation. The R${}_{\rm on}$ value for gate lag increased from 7.37 $\Omega\cdot$mm
before irradiation to 8.59 $\Omega\cdot$mm after irradiation, an increase of 16.58%.
For drain lag, R${}_{\rm on}$ increased from 8.16 $\Omega\cdot$mm before irradiation
to 10.88 $\Omega\cdot$mm after irradiation, an increase of 33.33%. Table 2 presents the corresponding I${}_{\rm D}$ current values at V${}_{\rm D} = 10$ V.
Under the $($V${}_{\rm G_B}$, V${}_{\rm D_B}) = (0$ V, $0$ V$)$ condition, the I${}_{\rm
D}$ value decreased by 5.57% after proton radiation, for the $(-5$ V, $0$ V$)$ condition,
the decrease was 3.81%. Under $(0$ V, $10$ V$)$ and $(-5$ V, $10$ V$)$ conditions,
the decreases were 10.29% and 10.41%, respectively. he overall current characteristic
deteriorated after proton irradiation, with the drain lag showing a greater difference
than the gate lag. This suggests that more traps formed in the GaN buffer layer than
at the AlGaN layer interface. Consequently, as voltage is applied, electron trapping
increases, leading to a decrease in the I${}_{\rm D}$ value.
Fig. 10. Pulse output curves before and after proton irradiation.
Table 2. The corresponding I${}_{\rm D}$ current values at V${}_{\rm D}$ 10 V.
|
(VG_B, VD_B)
|
Before
|
After
|
|
(0, 0)
|
3.59 ×10-1 [A/mm]
|
3.39 ×10-1 [A/mm]
|
|
(-5, 0)
|
3.15 ×10-1 [A/mm]
|
3.03 ×10-1 [A/mm]
|
|
(0, 10)
|
3.40 ×10-1 [A/mm]
|
3.05 ×10-1 [A/mm]
|
|
(-5, 10)
|
2.88 ×10-1 [A/mm]
|
2.58 ×10-1 [A/mm]
|
IV. CONCLUSIONS
In this study, AlGaN/GaN HEMT was fabricated, with protons at 5 MeV energy and a dose
of $5 \times 10^{13}$ cm${}^{-2}$ to analyze their behavior. The DC characteristics
analysis showed that I${}_{\rm on}$ decreased by 26.78% and I${}_{\rm off}$ increased
by 89.63% after irradiation. The V${}_{\rm t}$ remained relatively unchanged, at $-3.24$
V before irradiation and $-3.27$ V after irradiation, indicating that the 2DEG density,
which determines V${}_{\rm t}$ did not suffer significant damage. However, the R${}_{\rm
on}$ value increased by 57% from 62.5 to 98.14 $\Omega\cdot$mm. Simulation fitting
results suggest that this increase is due to the contact resistance increasing by
$\sim$1.46 from $1.3 \times 10^{-5}$ to $1.9\times 10^{-5}$. Additionally, the gate
lag increased the R${}_{\rm on}$ value by 16.58%, from 7.37 to 8.59 $\Omega\cdot$mm,
while the drain lag increased by 33.33% from 8.16 to 10.88 $\Omega\cdot$mm. This indicates
that more traps were formed in the GaN buffer layer than at the AlGaN layer interface
after proton irradiation. In conclusion, we believe that these findings will contribute
to pre- verification research for high reliability applications, such as space and
aviation semiconductors.
ACKNOWLEDGMENTS
This work was supported by the National Research Foundation of Korea (NRF) grant
funded by the Korea government (MSIT) (No. NRF-2020R1A2C1005087). This research was
supported by National R&D Program through the National Research Foundation of Korea
(NRF) funded by Ministry of Science and ICT (2022M3I7A1078936). This study was supported
by the BK21 FOUR project funded by the Ministry of Education. Korea (4199990113966).
This investigation was financially supported by Semiconductor Industry Collaborative
Project between Kyungpook National University and Samsung Electronics Co. Ltd. The
EDA tool was supported by the IC Design Education Center (IDEC), Korea. This work
was also supported by a Research Grant of Andong National University.
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So Ra Jeon received her B.Sc. degree in electronic engineering from the School
of Electronics and Information Engineering, Korea University (KU) Sejong Campus, Korea,
in 2020, where she is currently pursuing an M.S. degree in the School of Electronic
and Electrical Engineering. Her research interests include the design, fabrication,
and characterization of GaN devices.
Sang Ho Lee received his B.Sc. degree in electronics engineering from the School
of Electronics Engineering (SEE), Kyungpook National University (KNU), Daegu, Korea,
in 2019, where he is currently pursuing a Ph.D. in the School of Electronic and Electrical
Engineering. His research interests include the design, fabrication, and characterization
of gate-all-around logic devices and capacitor-less 1T-DRAM transistors.
Jin Park received her B.Sc. degree in electronic engineering from the School of
Electronics Engineering (SEE), Kyungpook National University (KNU), Daegu, Korea,
in 2020, where she is pursuing a Ph.D. in the School of Electronic and Electrical
Engineering. Her research interests include the design, fabrication, and characterization
of gate-all-around logic devices and capacitor-less 1T-DRAM transistors.
Min Seok Kim received his B.Sc. degree in electronics engineering from the School
of Electronics Engineering (SEE), Kyungpook National University (KNU), Daegu, Korea,
in 2023, where he is currently pursuing an M.S. in school of Electronic and Electrical
Engineering. His research interests include the design, fabrication, and characterization
of logic devices and capacitor-less 1T-DRAM transistors.
Seung Ji Bae received her B.Sc. degree in electronic engineering from the School
of Electronics Engineering (SEE), Kyungpook National University (KNU), Daegu, Korea,
in 2023, where she is currently pursuing an M.S. degree in the School of Electronic
and Electrical Engineering. Her research interests include the design, fabrication,
and characterization of capacitor-less 1T-DRAM transistors.
Jeong Woo Hong received his B.Sc. degree in electronic engineering from the School
of Electronics Engineering (SEE), Kyungpook National University (KNU), Daegu, Korea,
in 2023, where he is currently pursuing an M.S. degree in school of Electronic and
Electrical Engineering. His research interests include the design, fabrication, and
characterization of GaN power devices.
Won Suk Koh received his B.Sc. degree in electronic engineering from the School
of Electronics Engineering (SEE), Kyungpook National University (KNU), Daegu, Korea,
in 2024, where he is currently pursuing an M.S. degree in school of Electronic and
Electrical Engineering. His research interests include the design, fabrication, and
characterization of capacitor-less 1T-DRAM transistors.
Gang San Yun received his B.Sc. degree in electronic engineering from the School
of Electronics Engineering (SEE), Kyungpook National University (KNU), Daegu, Korea,
in 2024, where he is currently pursuing an M.S. degree in school of Electronic and
Electrical Engineering. His research interests include the design, fabrication, and
characterization of capacitor-less 1T-DRAM transistors.
In Man Kang received his B.S. degree in electronic and electrical engineering
from the School of Electronics and Electrical Engineering, Kyungpook National University
(KNU), Daegu, Korea, in 2001, and a Ph.D. degree in electrical engineering from the
School of Electrical Engineering and Computer Science (EECS), Seoul National University
(SNU), Seoul, Korea, in 2007. He worked as a teaching assistant for semiconductor
process education from 2001 to 2006 at Inter-university Semiconductor Research Center
(ISRC) in SNU. From 2007 to 2010, he worked as a senior engineer at Design Technology
Team of Samsung Electronics Company. In 2010, he joined KNU as a full-time lecturer
of the School of Electronics Engineering (SEE). Now, he is currently working as a
professor. His current research interests include CMOS RF modeling, silicon nanowire
devices, tunneling transistor, low-power nano CMOS, and III-V compound semiconductors.
He is a member of IEEE EDS.
Young Jun Yoon received his B.S. and Ph.D. degrees in electronics engineering
from the School of Electronics Engineering (SEE), Kyungpook National University (KNU),
Daegu, Korea, in 2013 and 2019, respectively. He is currently assistant professor
with the Deparment of Electronic Engineering, Andong National University (ANU). His
research interests include simulation, fabrication, and characterization of semiconductor
device.