Proceedings of the 12th European Microwave Integrated Circuits Conference
Anomaly and Threshold Voltage Shifts in GaN and
GaAs HEMTs Over Temperature
Mohammad A Alim1, Ali A Rezazadeh2, and Christophe Gaquiere3
1
Applied Physics, Electronic and Communication Engineering, University of Chittagong, Bangladesh.
Microwave and Communication System Group, School of EEE, The University of Manchester, UK.
3
Institute of Electronic, Microelectronic and Nanotechnology (IEMN), The University of Lille, France.
Tel: +8801775822146, Email: mohammadabdulalim@cu.ac.bd
2
Abstract— The anomalies and the threshold voltage shifts in
GaN and GaAs based high electron mobility transistors over
temperature were reported and analyzed using on wafer
measurements. Discrepancies are noticed; most conspicuously
that the thermal trends of the threshold voltage of the two device
technologies are utterly contrasting. This anomaly extends for the
other parameters of the devices such as sheet carrier densities of
the two-dimension electron gas. In addition barrier
inhomogeneities and the band offset of the semiconductor
heterojunction with temperature provides some valuable insights
between the two competitive device technologies.
study with temperature for GaN and GaAs based HEMTsin
order to provide detailed analysis and experimental results.
This is rather essential to effectively identify the variations of
device behaviour and underlying device physical parameters
changes with temperature. Usually, the VT of HEMTs depends
on the design of the epitaxial structure, charge carrier density
of the two dimension electron gas (2-DEG), thickness of the
barrier layer [8]−[9]. However the barrier inhomogeneities
e.g., variation in Schottky barrier height (SBH) with T, the
semiconductor heterojunction e.g., band offset ΔE with T, and
polarization fields depending on the dielectric are the main
contributors to the VT shift with temperature [10].
Keywords—GaN and GaAs based high electron mobility
transistor (HEMT), anomalies, threshold voltage, temperature.
I. INTRODUCTION
Over the years there has been a growing concern in the
study of thermal performance of high power and wide
bandgap (GaN compared to GaAs) semiconductor materials
which present a challenging demand in understanding the
temperature issues. The GaN based HEMT grown on silicon
carbide substrate has attained a very great extent of attention
in present-day being a auspicious candidate for high-power
applications. It exhibits remarkable power performance in
comparison with the more matured GaAs material and up to
now became the technology of choice for ongoing and
forthcoming applications [1]. The SiC has better thermal
conductivity, and the fact that GaN is a wide bandgap
semiconductor so that GaN coupled with SiC can sustain
higher temperature as well as higher electric field [2]. For the
time being, GaAs is more established device technology with
better device terminal contacts. These GaAs based devices are
an exquisite choice for high power, low noise, good linearity,
and high efficiency applications [3]. The Schottky parameters
controls the electron current at the hetero-interface and
depletion width in the semiconductor are the key parameter for
the junction based devices. For the conduction of a HEMT
device; the threshold voltage VT is an important indicator and
as well as the deriving element for the other parameters.
Various reports have highlighted the VT shifts with
temperature (T) for MESFET [4], MOSFET [5], FET [6] and
HEMTS [7]−[10]. However there is a lack of threshold voltage
978-2-87487-048-4 © 2017 EuMA
Fig. 1. A HEMT in a simplified structure showing the relationships of
threshold voltage and the physical device properties (not to scale).
A complete set of data and analysis of the threshold
voltage VT and related parameters with T for GaN and GaAs
based HEMTs are presented in this paper. The main findings
are the deterioration or altitude trend of VT with T as a
function of drain-source voltage Vds for the GaN and GaAs
devices are thoroughly antithetical. Furthermore the sheet
carrier densities of the 2-DEG, the barrier inhomogeneities
and the band offset behaviour with T are also analysed. To
the best of authors’ knowledge this is the first time that such a
study is reported based on two diferent semiconductor device
tecnologies to. These effective comparisons are useful to
identify the underlying physics of the device behaviour with
temperature for future designs and optimizations.
33
9–10 Oct 2017, Nuremberg, Germany
II. DEVICE FABFRICATION AND EXPERIMENTAL
n s (T ) =
The top views of fabricated two devices are shown in Fig.
2. The AlGaAs/GaAs heterojunction consists of 3 nm undoped
AlGaAs spacer layer and 0.5 ȝm GaAs buffer layer on a 500
ȝm semi-insulating GaAs substrates. A 20 nm AlGaAs supply
layer (n-type doped) along with an undoped 10 nm AlGaAs
was grown beneath of a highly doped GaAs cap layer (15 nm)
and at the top of the spacer layer. The Schottky mushroom
gate (Ti/Pt/Au) geometry of two fingers (0.5 ȝm ×100 ȝm)
and the separations between contacts were LSG of 1 μm and
LGD of 2.5 μm. On the other hand, the AlGaN/GaN
heterojunction consists of 25 nm undoped (25.3% Al) AlGaN
and 1.5 ȝm undoped GaN layer on a 400 ȝm silicon carbide
substrate . A thick AlN nucleation layer of 0.3 ȝm was grown
between buffer and substrate layer. The ohmic contacts were
formed by Ti/Al/Ni/Au (12 nm/200 nm/40 nm/1000 nm) using
photo-lithography and metal evaporation alloyed at the
temperature of 900°C. The mushroom shaped Schottky gate
(Pt/Ti/Pt/Au) has two gate fingers with (0.25ȝm ×100 ȝm)
dimensions. Si3N4 passivation with thickness of 240 nm was
deposited and the separations between contacts are LSG of 1μm
and LGD of 2.75 μm.
GaAs HEMT
GaN HEMT
S
S
G
D
G
D
S
S
S
S
S
(a)
0.5 μm × (2×100) μm
εs
ªVgs − VT (T ) º¼
qd barrier ¬
(3)
where μ0 is the electron mobility of the channel and εs is the
dielectric constant with a thermal coefficients of ~10-4/°C, the
thickness of the barrier layer, dbarrier is 25 and 35 nm for the
GaN and GaAs device respectively, Vgs is the applied voltage,
and VT is threshold voltage.
III. RESULTS AND ANALYSIS
Threshold voltage VT is an important electrical parameters
in modeling of HEMTs, and its extraction with good accuracy
and reliability is necessary. The commonly used extraction
method for VT based on Ids–Vgs transfer characteristics at a
reference Vgs from linear region is employed. The variation of
threshold voltage with temperature in terms of drain-source
(VT – Vds– T) is presented in Fig. 3 for the two devices. The VT
shifts to more negative values of Vgs when the values of Vds
increase at all the measured temperature. The GaN device
shows an increasing trend of VT and the GaAs device shows a
decreasing trend when the temperature rises from - 40°C to
150°C. Here, it is aid noting that the shift of VT with
temperature in GaN is absolutely the converse direction of the
GaAs devices.
S
(b) 0.25 μm × (2×100) μm
Fig. 2. The top view of: (a). 0.5 ȝm × 200 ȝm GaAs based HEMT and (b).
0.25 ȝm × 200 ȝm GaN based HEMT.
The presented investigation is based on DC on-wafer
measurements at eight ambient temperatures of – 40°, – 25, 0,
25, 50, 75, 125 and 150°C. The temperature of the wafer
chuck is controlled by a temperature control unit integrated
with the vector network analyzer (VNA) with probable error
of 2%. The normal transistor action is the control of the 2-D
electron gas through a Schottky gate contact to the barrier
layer where the charge under the gate Qg can be estimated
from [8]:
LG
Q g = − ³ Wqn s (V gs , V x ) dx
(1)
0
where W, LG, q, Vgs, and Vx are the gate-width, gate-length,
electron charge, gate voltage and potential at any point x in the
channel respectively. The dx and the 2-DEG sheet carrier
density ns can be derived from the drain-current Ids as [8]:
dx =
μ 0Wqns (Vgs , Vx ) dV x
I ds
(2)
Fig. 3. Temperature-dependency of the threshold voltage VT for: (a) the GaN
and (b) the GaAs based HEMTs as a function of drain-source voltage, Vds.
34
The change of VT in per unit °C temperature, ∂VT shows
an almost linear function of T and of Vds and it can be
presented using a phenomenological expression as in [10]:
VT (T ) V = VT (T0 ) V [1 + β (T − T0 )]
ds
higher than unity. This specifies that the dominant conduction
mechanism for the devices is not the thermionic emission.
Traps assisted tunnelling phenomena at the metal and
semiconductor hetero-interface are identified for this
conduction [11]. The 2-DEG sheet carrier concentration, ns
increases with input bias, Vgs for the two devices. However,
the ns degrade with T in GaN device and increases with T in
GaAs device adopting (3) as depicted in Fig.5.
(4)
ds
where, the parameter at reference temperature (T0 in °C) is
denoted as VT (T0)⏐Vds, final temperature as VT (T)⏐Vds and ȕ is
denoted as the temperature coefficient (units/°C) which can be
estimated from the measured data using (4). Before any
conclusion drawn for the opposite directions of the VT of the
two devices; one needs to analyze the barrier inhomogeneities
e.g., variation in SBH, φb with T, band offset ΔE with T,
charge carrier density of the 2-DEG, ns with T as in (5) and as
well as the drain-bias dependent transfer characteristics.
VT = φb − Δ E −
2
qns d barrie
r
2ε s
(5)
The values of the ideality factor (n) and the SBH (φb) have
been determined for the Schottky junction of the two devices
with the variation of temperatures. The measured value of
SBH increases from 0.55 to 1.12 eV and 0.45 to 0.74 eV while
the ideality factor, n decreases from 2.4 to 1.02 and 1.55 to
0.99 for the GaN and GaAs devices respectively. The
correlation between effective φb and n with temperature may
be approximated by using a linear relationship [11] as shown
in Fig. 4. The value of the Schottky barrier extrapolated at n=1
is different for the two devices which is only related to the
uniformity of the process and materials.
Fig. 5. Bias and temperature-dependency of the sheet carrier density of the 2DEG, ns for: (a) GaN HEMT at Vds = 12 V and (b) GaAs HEMT at Vds = 3 V.
Fig. 4. The linear correlation between the ideality factor, n and the Schottky
barrier height, φb with temperature for the GaN and GaAs based HEMTs.
It is very difficult to have exactly the same quality of
device terminal contacts; this quality will directly impact on
this parameter in the two devices. If the etching just before the
metal deposition is not exactly the same, the ideality factor
will be impacted. When the gate length is very small, it
becomes very difficult to well clean before the metal
deposition and in this case a huge work on the process is
required to obtain a good Schottky contact. The experimental
values of the ideality factor with T for GaN device are all
Fig. 6. Conduction band discontinuity with temperature for the AlGaN/GaN
and AlGaAs/GaAs heterointerface.
35
TABLE I: COMPARISON BETWEEN THE DEVICE PARAMETERS AT ROOM
The semiconductor heterojunction: band-offset ΔE with T
were estimated from the empirical expressions available in the
open literature: for the GaN device [10] and for the GaAs
device [15] as presented in Fig. 6. The behaviour of the
conduction band discontinuity ΔE with T for AlGaAs/GaAs
heterojunction shows higher degradation in comparison with
the AlGaN/GaN heterojunction. Upto now all the results show
an impact on the threshold voltage shifts with temperature for
the two technologies studied in this work. The drain-bias
dependent transfer characteristics for the two devices exihibits
zero temperature coefficient (ZTC) point as presented in Fig 7.
The achieved ZTC point is seen approximately at – 5.9 V of
Vgs trace for the GaN device at the drain bias of 12 V before
threshold voltage VT (see inset Fig. 7a). On the other hand, the
GaAs device exihibits the ZTC point at – 0.06 V of Vgs trace
and the drain bias of 3 V after VT. At all the drain bias the
anomalies of ZTC point can be noticed before VT for GaN
and after VT for the GaAs devices. These ZTC points are
arised due to threshold voltage trapping and de-trapping
phenomena where the VT reduction is compensated by the
reduction of mobility [10]. A comparison between the device
parameters of the two devices are presented in Table I. To sum
up all results, it can be concluded that, the traping/de-traping
phenomena before and after the VT point is one of the
principle reasons for the shifts of the threshold voltage with
temperature in GaN and GaAs devices.
TEMPERATURE
Parameters
Threshold voltage, VT (V)
Zero temperature coefficient (ZTC), (V)
Schottky contact barrier height, φb (eV)
Ideality factor, n
Conduction band discontinuity, ΔE (eV)
GaN
- 5.45
- 5.9
0.73
1.81
0.27
GaAs
- 0.63
- 0.06
0.38
1.95
0.19
IV. CONCLUSIONS
The anomalies and the shift of the threshold voltage in the
GaN and GaAs based HEMTs against temperature were
reported and analyzed based on measured results. An
understanding of the proficiency of these technologies with
temperature was established in terms of barrier
inhomogeneities,
2-DEG
sheet
charge
densities,
semiconductor band offset and drain-source voltage variations.
Barrier inhomogeneities of the two devices confirm that, the
trap assisted tunneling mechanism at the metal/semi-conductor
interface is the dominant factor of the conduction mechanism
for the GaN HEMT. On the other hand, the trapping/detrapping phenomenon before zero temperature coefficient
point is identified as a reason of the threshold voltage shifts of
the GaAs HEMT. The 2-DEG sheet charge densities with
temperature show negative trend for the GaN and positive
trend for the GaAs devices are also a reason of the threshold
voltage shifts for the two devices. This work provides a new
analysis for the device parameters of GaN and GaAs HEMTs
and their anomalies with temperature.
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[1]
Fig. 7. Temperature-dependence of the transfer charateristics exihibits zero
temperature coefficient, ZTC point: (a) before threshold voltage for the GaN
HEMT (inset), and (b) after threshold voltage for the GaAs HEMT.
36