Solid-State Electronics 54 (2010) 1367–1371

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Solid-State Electronics

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AlGaN/GaN MOSHFETs with HfO2 gate oxide: A simulation study

Y. Hayashi, S. Sugiura, S. Kishimoto, T. Mizutani *

Department of Quantum Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

a r t i c l e i n f o

Article history:Received 27 January 2010Received in revised form 30 March 2010Accepted 30 March 2010Available online 3 June 2010The review of this paper was arranged byE. Calleja

Keywords:MOSFETGaNHfO2

Device simulationInterface trap

0038-1101/$ - see front matter � 2010 Published bydoi:10.1016/j.sse.2010.03.022

* Corresponding author. Tel./fax: +81 052 789 5455E-mail address: tmizu@nuee.nagoya-u.ac.jp (T. Mi

a b s t r a c t

Two-dimensional device simulations of a HfO2/AlGaN/GaN metal–oxide–semiconductor–heterostructureFET (MOSHFET) have been carried out based on the drift–diffusion model focusing on the effects of HfO2/AlGaN interface properties. In the case of MOSHFETs with no trap at the HfO2/AlGaN interface, the trans-conductance was found to decrease at large VGS due to channel formation at the HfO2/AlGaN interface,resulting in a plateau structure of gm. When the interface states were incorporated at the HfO2/AlGaNinterface, gm decreased due to electron capture by the trap at a smaller gate voltage than the onset ofgm decrease for the case with no trap at the HfO2/AlGaN interface. This is because the trap level reachedEF earlier than the channel formation at the HfO2/AlGaN interface. This resulted in a peak structure of thegm when the interface states were deep, which is consistent with experimental results. It was pointed outthat if the trap concentration was less than 4 � 1011 cm�2, the threshold voltage shift was less than 0.3 Vand the gm decrease was less than 10%.

� 2010 Published by Elsevier Ltd.

1. Introduction

GaN is attracting considerable attention for high-frequency andhigh-power devices due to its remarkable material properties suchas a high breakdown voltage and a high electron velocity [1]. In or-der to apply these advantages to high-power switching systems, itis important to design for normally-off operation to ensure a fail-safe system, that is, one that avoids the problem of circuit burnout when the gate signal drops to ground voltage [2]. The conven-tional normally-off HEMTs require a precise etching control for thegate recess [3,4] or heavy p+-doping for the junction gate [5]. Inaddition to these process requirements, the normally-off HEMTssometimes show a problem of small transconductance (gm) dueto a large parasitic resistance [4]. Recently, the normally-off HEMTsfabricated by fluoride-based plasma treatment, which introducedfluorine atoms into the AlGaN barrier layer, have been reported[6,7]. This process seems promising because it does not requirethe etching precision or p+-doping of previous designs for nor-mally-off operation. However, our present understanding of therole played by the fluorine atoms is not sufficient. In addition, itis not clear at present whether the fluorine atoms incorporatedin the AlGaN barrier layer are stable, even though the thresholdvoltage of the device has been found to be stable for 80 days at200 �C [8].

In most of the normally-off devices published in the literature,there is an issue that the maximum drain current is small because

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.zutani).

of the small available forward gate voltage. It is expected that GaNMOSFETs will be suitable for solving these problems because it ispossible to apply a large forward voltage to the MOS gate. Eventhough there are many reports of normally-on GaN MOSFETs [9–12], there are not so many reports of normally-off GaN MOSFETs[13–15].

In previous work, GaN MOSFETs using SiO2 as a gate oxideshowed a small drain current and a small transconductance (gm)probably due to their small gate input capacitance and insufficientinterface property. To improve the performance we have fabricatedGaN MOSFETs using HfO2 with a large dielectric constant andachieved a maximum transconductance (gmmax) of 45 mS/mmand maximum drain current of 400 mA/mm [16]. Even thoughthese values are several times larger than those of conventionalGaN MOSFETs, they are still smaller than those of AlGaN/GaNHEMTs. This is probably because the quality of the HfO2/GaN inter-face where the channel is formed is lower than that of the AlGaN/GaN hetero interface, and also because polarization-inducedcharge is not available for the GaN MOSFETs.

To improve the device performance we have proposed aHfO2/AlGaN/GaN metal–oxide–semiconductor-heterostructure FET(MOSHFET) in which a high-quality interface was used as a chan-nel [17]. The maximum drain current (ID) of 780 mA/mm and themaximum gm of 185 mS/mm were realized at VGS = 10 V with agate leakage current of 1 mA/mm. These results were fairly good.However, the gm decreased at large gate voltage. It has beenpointed out that a 2-dimensional electron gas (2DEG) is formedat the oxide/semiconductor interface at high positive gate bias[18]. If the interface quality is not good, the gm degradation is

Table 1Polarization charge density at each interface.

nSP(GaN)(cm�2)

nSP(AlGaN)(cm�2)

nPE(AlGaN)(cm�2)

Total (cm�2)

HfO2/AlGaN �2.62 � 1013 �5.74 � 1012 �3.20 � 1013

AlGaN/GaN �1.81 � 1013 2.62 � 1013 5.74 � 1012 1.39 � 1013

Bottom of GaN 1.81 � 1013 1.81 � 1013

1368 Y. Hayashi et al. / Solid-State Electronics 54 (2010) 1367–1371

expected. Another possibility is electron trapping at the interfacetraps. Until now, however, there has been little research publishedon these phenomena.

In this paper, two-dimensional device simulations of HfO2/Al-GaN/GaN MOSHFETs have been carried out to study the electricalcharacteristics of the AlGaN/GaN MOSHFETs and the effects ofinterface traps on the device characteristics.

Table 2Values of m–E curve parameters in GaN and AlGaN used in the simulations.

GaN AlGaN

llow (cm2/Vs) 800 180msat (cm/s) 3.3 � 107 1.8 � 107

b 1 2

2. Simulation setup

Two-dimensional numerical device simulations based on adrift–diffusion model were carried out using the commercial sim-ulation program DESSIS. Fig. 1a and b shows a schematic cross sec-tion of the device and the corresponding energy band diagramperpendicular to the device surface used in the simulation. Thechannel length was 1.5 lm. Aluminum mole fraction in the AlGaNbarrier was 0.25 and the thickness was 12 nm. The thickness waschosen to realize normally-off operation for the HEMTs. The GaNand AlGaN were assumed to be lightly p-doped, with a concentra-tion of 1 � 1014 cm�3. The AlGaN/GaN heterostructure has sponta-neous and piezoelectric polarizations (PSP and PPE). To take theseeffects into account, fixed charges (nSP and nPE) shown in Table 1were introduced at each interface [19]. Spontaneous polarizationcharges at both sides of the GaN were also included in the model

(b)

(a)

Fig. 1. Schematic cross section of the device and the corresponding energy banddiagram used in the simulations. Donor-type traps at an energy level ET below theconduction band minimum EC of AlGaN were assumed.

to align the modeled system closer to the real device. If no polari-zation charges are assumed as employed in the conventional sim-ulation, threshold voltage shifts to the negative direction. Inaddition to the polarization charge, fixed positive charge with aconcentration of 1.5 � 1013 cm�2 was introduced at the HfO2/Al-GaN interface in order to match the simulated threshold voltageswith those observed in experiments with the HEMTs. The introduc-tion of the positive charge and its concentration are consistentwith Ref. [20]. Even though we do not know the origin of the posi-tive charge at the surface, if the Fermi level of the HfO2/AlGaNinterface is below the charge neutrality level, positive charge willbe formed at the HfO2/AlGaN interface. Another possibility is thenitrogen vacancy which is believed to be donor-type defect [21].Further study is necessary to clarify the origin of the positivecharge.

Mobility and velocity of electrons used in the simulation aresummarized in Table 2 assuming the electron velocity–electricfield (m–E) curve of mðEÞ ¼ llowE

1þðllowEVsat

Þbh i1=b. The parameters for GaN

and AlGaN listed in Table 2 were chosen to fit the m–E curve withthe results of Monte Carlo simulation [22]. Traps introduced atthe HfO2/AlGaN interface to evaluate the effects of the interfacestates were assumed to be donor-type based on reports from theMishra [20] and Hashizume [21] groups. The energy of the trap le-vel was defined as a depth from the conduction band minimum EC

of AlGaN, EC�ET, as shown in Fig. 1b. Simulations were carried outwith various energy levels and trap concentrations, NT. As a typicalvalue, EC�ET = 1.0 eV was used based on Refs. [20] and [23], and atrap density NT of 6 � 1012 cm�2 was used considering the thresh-old voltage shift of about 5 V during the measurement of gate volt-age sweep. Even though the device simulation was performed forthe device with uniform distribution of the deep level at the inter-face, the effects of non-uniform distribution of the trapped elec-trons in a direction parallel to the interface was taken intoaccount in the device simulation because the current continuityequation and Poisson equation are solved self-consistently.

In the present device simulation, no gate leakage currentthroughout the MOS system was assumed. This assumption iswidely used in the standard device simulation of the MOSFETs. Itis expected that the assumption will not cause serious error be-cause a small gate leakage current of 10�10–10�4 A/mm was con-firmed in the experiment [24], even though some modificationmight become necessary at large forward gate voltage.

3. Simulation results

Fig. 2 shows the drain current–gate voltage (ID–VGS) (dottedline) and transconductance gm–VGS (solid line) characteristics at adrain voltage VDS of 4 V. The densities of the 2DEG at the AlGaN/

Fig. 2. ID, gm, nGaN, and nAlGaN as functions of VGS for the device without traps.VDS = 4 V.

Y. Hayashi et al. / Solid-State Electronics 54 (2010) 1367–1371 1369

GaN heterointerface (nGaN) and the HfO2/AlGaN interface (nAlGaN)are also shown by a long-dash line and short-dash line, respec-tively. The device exhibited normally-off operation with a thresh-old voltage Vth of 0.1 V and maximum gm of 191 mS/mm. The gm

increased rapidly at first, saturated at a VGS of about 5 V, and de-creased at VGS = 15 V, resulting in a plateau structure.

In order to examine the reason for the gm decrease, nGaN andnAlGaN were calculated as a function of VGS. The value of nGaN wasfound to build up at VGS = 0 V and linearly increased as shown bythe long-dash line in Fig. 2. For VGS larger than 15 V, the nGaN sat-urated and an increase in nAlGaN was induced at the HfO2/AlGaNinterface, as shown by short-dash line in Fig. 2. The nGaN saturationat VGS = 15 V is explained by the shielding effect of VGS by the nAlGaN

induced at the HfO2/AlGaN interface. This is consistent with Ref.[18] in which electron accumulation was observed at the oxide/Al-GaN interface. Since the electron velocity in AlGaN is lower thanthat in GaN, gm decreases rapidly when nAlGaN builds up. In theexperiments, however, gate voltage dependence of gm showed apeak structure with no plateau [17], which will be discussed later.

We next studied the effects of traps at HfO2/AlGaN interface.Fig. 3 shows ID–VGS (dotted line) and gm–VGS (solid line) character-istics when the trap (EC�ET = 1 eV, NT = 6 � 1012 cm�2) is intro-duced at the HfO2/AlGaN interface. In addition to the thresholdvoltage shift in the negative direction (Vth = �4.5 V) compared tothe case without the trap, another gm decrease and its recoverywere observed at VGS between 6 and 15 V prior to the onset of

Fig. 3. ID, gm, nGaN, nAlGaN, and nT as functions of VGS for the device with traps(EC�ET = 1 eV, NT = 6 � 1012 cm�2). VDS = 4 V.

the previously-mentioned gm decrease due to the channel forma-tion at the HfO2/AlGaN interface.

In order to understand these behaviors, electron density wascalculated as a function of VGS. Electrons captured by the traps,nT, were found to build up at almost the same voltage of 6 V wheregm (solid line) decreases and nGaN (long-dash line) saturates asshown by the dot-dash line in Fig. 3. The above results are ex-plained as follows.

Fig. 4 shows the EC distribution at the center of the gate in adirection perpendicular to the device surface for VGS from �6 V(top) to 20 V (bottom) in 1 V steps at VDS = 0 V forNT = 6 � 1012 cm�2. In the off-state between VGS = �6 V and �4 V,EC shifted downward in the whole region under the gate with in-crease in VGS. For VGS > �4 V on the other hand, the channel wasformed at the AlGaN/GaN interface and nGaN increased as shownin Fig. 3. Then, EC of AlGaN/GaN interface was almost pinned atEF and the VGS increase was consumed by the EC down shift ofHfO2 and AlGaN. For VGS > 6 V, ET reached EF and electrons werecaptured by the trap. Then the increase of VGS was consumed bythe increase of the trapped electrons nT and ET was almost pinnedat EF. Consequently the electron density at the AlGaN/GaN inter-face, nGaN, increased very little, resulting in a decrease in gm. ForVGS > 11 V, most of the traps were filled with electrons and thenEC of HfO2 and AlGaN shifted downward again with increasingVGS, so that nGaN increased again resulting in a recovery of the gm

as shown in Fig. 3. For VGS > 15 V, another channel was formed atthe HfO2/AlGaN interface and EC of HfO2/AlGaN interface was pin-ned at EF. Then, the gm decreased as discussed for the device with-out interface traps.

Fig. 5 shows gm–VGS characteristics at VDS = 4 V for different ET

values (EC�ET = 0.5, 1.0, 1.5 eV) with a trap concentration ofNT = 6 � 1012 cm�2. All devices showed the same threshold voltageof about �5 V since the traps were completely ionized (ET > EF)independent of the energy level of the trap. On the contrary, theVGS at which the gm started to decrease was small for the deep trapresulting in a decrease of ID. This is because the deeper the ET is, theearlier it reaches EF. It is notable that the plateau structure of gm

obtained for the shallow interface trap disappeared when theinterface trap was deep, resulting in a peak structure of gm. Thisis consistent with the experimental results with the gm peak, asshown in Fig. 6. This suggests that the gm decrease obtained inRef. [17] was not due to the channel formation at the HfO2/AlGaNinterface but to electron capture at the interface states. If theAlGaN/GaN MOSHFETs have interface states, the drain current doesnot increase so much as that in Fig. 2 in which no interface trap

Fig. 4. EC profiles for various VGS from �6 to 20 V (in 1 V steps) at the center of thegate. VDS = 0 V. NT = 6 � 1012 cm�2.

Fig. 5. gm–VGS characteristic at VDS = 4 V for different EC�ET values of 0.5, 1.0, and1.5 eV. NT = 6 � 1012 cm�2.

0

50

100

150

200

0

100

200

300

400

500

600

700

800

-2 0 2 4 6 8 10

VDS

= 8V

Gate voltage (V)

Dra

in c

urre

nt (m

A/m

m)

Tran

scon

duct

ance

(mS/

mm

)

Fig. 6. ID–VGS and gm–VGS characteristics of the fabricated AlGaN/GaN MOSHFETswith HfO2 gate oxide. VDS = 8 V.

Fig. 7. gm–VGS characteristics for different NT values of 1, 6, and 10 � 1012 cm�2.EC�ET = 1 eV. VDS = 4 V.

Fig. 8. Amounts of the threshold voltage shift and the gm decrease as functions ofNT. VDS = 4 V.

1370 Y. Hayashi et al. / Solid-State Electronics 54 (2010) 1367–1371

was assumed. The gm in the experiments showed monotonicdecrease in contrast with the double peak structure in the devicesimulation. One possibility of this difference is that the VGS is notsufficiently large in the experiment and the other possibility isthe continuum distribution of the interface states. It is difficult atpresent to determine which is dominant. Drain current deep leveltransient spectroscopy [25] is effective in clarifying this point

because it is possible to determine the energy distribution of thedeep level by measuring the temperature dependence of the draincurrent transient. This is now under study.

Fig. 7 shows simulated gm–VGS characteristics for a different trapconcentration (NT = 1, 6, 10 � 1012 cm�2). Here, EC�ET was assumedto be 1 eV. The larger concentration led to a larger threshold voltageshift and a greater gm decrease. Fig. 8 shows NT dependences of thethreshold voltage shift and the gm decrease. The threshold voltageshifted in proportion to the NT and gm decreased linearly at smallNT, saturating at large NT. If the trap concentration was less than4 � 1011 cm�2, the threshold voltage shift was less than 0.3 V andthe gm decrease was less than 10%.

4. Conclusion

Two-dimensional device simulations of HfO2/AlGaN/GaNMOSHFET have been carried out to investigate the operation mech-anism and the effects of interface traps on the device characteris-tics. First, devices without the traps were studied. Although thetransconductance increased with the gate voltage for small VGS, itdecreased for larger VGS, resulting in a plateau structure. This is ex-plained by the channel formation at the HfO2/AlGaN interfacewhere the electron velocity was rather low. Next, simulations fordevices with donor-type traps at HfO2/AlGaN interface were per-formed. Two effects were observed: the shift of the threshold volt-age in the negative direction, and the decrease of gm at large VGS

which was different from the previous gm decrease. The gm de-crease was due to the increase of VGS being expended to increasethe electrons captured by the traps at the HfO2/AlGaN interface,and was therefore unavailable to increase the density of the2DEG at the AlGaN/GaN interface. The gm decrease occurred at asmaller gate voltage than the onset of the channel formation atthe HfO2/AlGaN interface, resulting in a peak structure of the gm.The gm peak structure obtained in the simulation was consistentwith the experimental results. The magnitude of the thresholdvoltage shift and the gm decrease were dependent on the trap con-centration. If the trap concentration was less than 4 � 1011 cm�2,the threshold voltage shift was less than 0.3 V and the gm decreasewas less than 10%.

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