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Novel High Drain Breakdown Voltage AlGaN/GaN HFETsusing Selective Thermal Oxidation Process

Hiroyuki Masato, Yoshito Ikeda, Toshinobu Matsuno,Kaoru Inoue and Katsunori NishiiSemiconductor Device Research Center, Semiconductor Company

Matsushita Electronics Corporation1-1 Saiwai-cho, Takatsuki, Osaka 569-1 193, Japan

AbstractWe have developed novel selective thermal oxidationprocess for AlGaN/GaN Heterostructure Field EffectTransistors (HFETs) and realized extremely high deviceisolation and high drain breakdown voltage device of over1OOV The leakage current of device isolation between twoactive islands exhibited drastic reduction of 5 order ofmagnitude smaller than that of conventional mesa isolationprocess. Moreover, the fabricated 1.3um-gatelengthAlGaN/GaN HFETs exhibited maximum transconductance( g m d of 130mS/mm, maximum drain current (I,-) ofSOOmA/mm and excellent pinch off characteristics at highdrain voltage of over 120V.

heterostructure is that the density of 2DEG is extremelyhigh, more than 1X lOI3 by the piezoelectric effect andspontaneous polarization effects(3). One-order magnitudehigher output power can be expected by simply changingthe material. In this respect, GaN-based HeterostructureField Effect Transistors (HFETs) is promising material formicrowave or milliwave high power(4)-(7). However, thereare several issues to be solved about process technology.Device isolation is one of the most important issues torealize high power device.In this work, we present extremely high drain breakdownvoltage AlGaN/GaN HFETs by using newly developedselective thermal oxidation process.

Layer structureand fabricationprocessIntroduction

With the rapid expanding of recent wireless communicationmarkets, high-fkquency power devices operating at higherdrain voltage are strongly demanded for high-power solid-state amplifiers. Although GaAs-based FETs have beenused as a high-power microwave devices, they are comingclose to their performance limit. The increase of matchingloss due to lower device impedance and phase shift of theinput signal to each unit device becomes significant. Thematerial with higher breakdown field and higher currentdensity require to solve this problem without increasing thedevice size. In this respect, GaN is very attractive materialbecause of one-order magnitude higher breakdown fieldsand high electron saturation velocities of 2.5 X lo7cm/s(l),(2). The most striking fact in the GaN-based

Figure 1shows a schematic cross-sectional view of AlGaN/GaN epi-structure. The epi-layers were grown on sapphireby MOCVD. A newly developed selective isolation processflow by thermal oxidation is shown in Figure 2.To protectan active region during thermal oxidation, Si mask layerwas selectively deposited on active region(a). The layerthickness of Si was 1OOnm . Next, the thermal oxidation forthe surface of epi-layers was performed in dry O2 ambientat 900C(b). SiO,/Si layer was formed on an active region,and Ga,03 layer was formed on a device isolation region.And then, Si02/Si layer was removed by HF based solutionafter thermal oxidation(c). Finally, FET fabrication processwas performed(d). Ohmic contacts were formed with TdA1by annealing at 550C for 60sec. Schottky gate electrodewas used PdSiINilAu.

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Thermal Oxid ized Area Step Height

I

t J Epi LayerI i - AlGaN (x=0.25) 2nm I

900C, D ry 02, 4 h r s -I I I

HapphireFig. 1 Schematic cross-section of

AlGaN /GaN epi-structure

Si mask layerAIGaN/GaN

epi-layer(a) Formation of Si mask

Si02/Si (Act ive region)I 0 2 1 1 1 / I

(b) Thermal Oxidationz( c ) Remove S i O 2 / S i layerSi3N4

(d) FET fabricationFig. 2 Selective oxidized isolation process flow

It was fmt studied by measuring the step height betweenthe masked and oxidized regions. Figure 3shows schematicdiagram of oxidized layer thickness (a), oxidation rate (stepheight) as a hct ion of time (b) and a function ofA N molefraction (c). The true oxide thickness are about twice thestep height. The oxidation rates are almost constant withtime and become smaller with the increaseof A1 content.

IubstrateI(a) Schematic of thermal oxidized ar ea

10000

3 00053e 1003i

1010 100 1000

Time (min)(b) Oxidation r at e a s a function of time

0 5 10 15 20 25 30AIN mole fiaction (%)

(c) Oxidation ra te as a function ofAlN mole fractionFig. 3 Preliminary Result of thermal oxidation process.

The true oxide thickness is about twice the step height.

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Figure 4shows cross-sectional TEM photograph of athermal oxidized region and an active region in dry 0,ambient at 900C for four hours. Oxidized layer of lOOnm isselectively formed on the isolation region, the compositionof oxidized layer was revealed to G%03 by X-raydiEmction analysis. Oxidized layer of WO3 formed onGaN epi-layer was quite chemically stable. Table 1 showsthe electrical properties before and after thermal oxidationprocess. The degradation of epi-layer quality and electricproperties such as electron mobility and sheet carrierdensity in the active region was not observed even afterthermal oxidation process.

Mobility (cm2Ns)

Oxidized

L( pdsiEpi layer

Before thermal After thermaloxidation oxidation744 767

(a) Active device region (b) Oxidized isolation region

1.4x 0'3heet carrier density(cm-')

Fig. 4 Cross-sectional E M photograph of an activedevice region (a) and a thermal oxidized region (b)which performed in dry9 mbient at 900C for fourhours

1.5x 10'3

Table 1. Comparison of electrical properties before andafter thermal oxidation process.

Device performance

The overview of device isolation between two activeislands is shown in Figure 5. Two 150um-width electrodeislands separated by 6um. Figure 6 shows the comparisonof current-voltage characteristics of thermal oxidizedisolation and mesa isolation by using ECR plasma dryetching. The leakage current for thermal oxidized isolationis 65pA at applied voltage of 30V, and drastically reduced 5

order of magnitude smaller than that fix mesa isolation.Moreover, quite high breakdown voltage of 300V was alsoconfiied. The superior isolation characteristics forthermally oxidized isolation samples despite the relativelythnoxide layer suggest that the GaN buffer layer under theoxidized region has become highly resistive after thermaloxidation. The mechanisms for this high resistivity are notclear, but either d i h i o n s of oxygens or stressesin the GaNbuffer layer generated by the grown oxide will beresponsible.

Isolation Region

I II II I1 1

6I . lmFig. 5 Overview of device isolation between two active

islands

30h AIGaN/GaN HFET structure on sapphire

20

10

noxidation isolation 65pA

- 0 10 20 30 40Voltage (V)

Fig. 6 Current-voltage characteristics between two activeregion islands separated by 6um for thermaloxidized isolation and mesa isolation

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I-V characteristics of AlGaN/GaN HFET is shown inFigure 7. Gate-length and gate-width are 1.3um and 1OOum ,respectively. The GaN HFET exhibited a maximum The au thor s would like to thank Mr. Masarutransconductanc(& of 13OmS/mm, a maximum drain Kazumura, Dr. Osamu Ishikawa fo r theircurrent (I-)f 5 0 0 d m m and excellent pinch off encouragements and helpful discussions in thischaracteristics at high drain voltage of over 120V. These work. This work wa s partly supported by he NED0results indicate that AlGaN/GaN HFETs using thermal reg ional Consortium Project.oxidation isolation is quite suitable for high frequencypower applications.

Acknowledgements

References

n4EWv)IE!

60 Lg=l.3um, W p lOOum50403020100 0 40 80 120

Vd s (V)Fig. 7 I-V characteristics of AlGaN/GaN HFET

ConclusionsWe have developed novel selective thermal oxidationprocess. We have realized extremely high resistance fordevice isolation. The leakage current of device isolationbetween two active islands separated by 6um exhibiteddrastic reduction of 5 order of magnitude smaller than thatof conventional mesa isolation process. Moreover, thefabricated 1.3~1-gatelengthAlGaN/GaN WETS exhibitedmaximum transconductance (gm,,x> of 13OmS/mm, amaximum drain current (I ,- ) of 5 O O d m m and excellentpinch off characteristics at high drain voltage of over 120V.

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