AlGaN/GaN High Electron Mobility Transistors Employing Multiple Al2O3/Ga2O3 stacks

Ogyun Seok1), Woojin Ahn1), Young-Shil Kim1), Min-Woo Ha2), and Min-Koo Han1)

1)Department of Electrical Engineering, Seoul National University, Seoul 151-744, Korea

2)Korea Electronics Technology Institute, Seongnam, Gyeonggi-do 463-816, Korea

E mail: mkh@snu.ac.kr, Phone: +82-2-880-7254, Fax: +82-2-875-7254

Keywords: GaN, HEMT, Sputter, Al2O3, Ga2O3, stack Abstract

We have proposed and fabricated AlGaN/GaN HEMTs employing a new structure of multiple Al2O3/Ga2O3 stacks under gate metal in order to increase of the breakdown voltage and shift VTH in positive direction. 10 nm thick rf-sputtered Al2O3/Ga2O3 stacks, which consist of 2 nm thick Al2O3 and Ga2O3, induce electrons and holes accumulation during the switching operation of AlGaN/GaN HEMTs. The proposed device shows a high breakdown voltage of 1100 V and drain leakage current of 33 nA/mm at VGS of -10 V and VDS of 100 V while those of the conventional device are 380 V and 654 μA/mm respectively. The VTH of the proposed device employing the multiple Al2O3/Ga2O3 stacks is -1.4 V while that of the conventional device is -2 V due to the electrons accumulation in the multiple Al2O3/Ga2O3 stacks. INTRODUCTION

AlGaN/GaN HEMTs have a considerable attention for high power applications due to wide bandgap properties such as a high critical electric field and a low intrinsic carrier concentration [1]. Also they exhibit a very low on-resistance and a fast switching speed due to the 2DEG induced by discontinuity of conduction band in the AlGaN/GaN heterostructure and its piezoelectric polarization [2]. However, the soft breakdown characteristics of the

AlGaN/GaN HEMTs should be investigated for reduction of off-state power loss and stable blocking operation besides. It has been reported that the leakage current of AlGaN/GaN HEMT is attributed to surface traps, defects, and dislocations which is formed by the lattice mismatch between GaN and substrate [3].

Various surface passivations such as SiO2, SiNX, MgO, Al2O3 and AlN have been reported in order to suppress the surface related problems and improve reverse blocking characteristics of AlGaN/GaN HEMTs [4-8]. The edge termination structure such as filed plate, floating metal and formation of LDD region using fluorine treatment are also reported for suppression of electron trapping from gate into

the surface states by reducing electric field at drain-side gate edge and increase of breakdown voltage [9-11].

It is important to study an electron trapping mechanism through surface traps for investigation of the reverse blocking operation of the AlGaN/GaN HEMTs. Electron trapping through shallow traps would induce surface leakage current. It is well known that shallow trap is originated from the unintentionally formed N vacancies due to dislocations, plasma and thermal damage during the process. An electron trapping through deep traps suppresses the leakage current due to a low probability of electron de-trapping from traps site into conduction band. However, an electron trapping through deep traps causes a transient delay which is called current collapse [12]. We reported a low drain leakage current of 20 nA/mm and

a high breakdown voltage of 1140 V by employing O2 annealing on AlGaN/GaN HEMTs on SiC substrate [13]. Electron trapping through the deep trap site such as Ga vacancies which are generated by O2 annealing suppresses the surface leakage current.

The purpose of our paper is to propose the AlGaN/GaN HEMTs employing a new structure of the multiple Al2O3/Ga2O3 stacks under the gate in order to increase the breakdown voltage and shift VTH in positive direction. The devices were fabricated by using AlGaN/GaN heterostructure (1.7 μm thick GaN buffer) on Si substrate. Al2O3 and Ga2O3 layer were deposited by rf-sputtering at room temperature. Ga2O3 layer which is sandwiched by Al2O3 acts as charge accumulation center due to the unintentionally formed Ga vacancies. The accumulated electrons when VGS<VTH deplete the electrons in bulk and 2DEG so that breakdown voltage and VTH are increased. The proposed device employing multiple Al2O3/Ga2O3 stacks shows a high breakdown voltage of 1100 V and a low drain leakage current of 33 nA/mm with a large hysteresis while AlGaN/GaN HEMT employing Al2O3 only shows 1050 V and 1.8 µA/mm with a small hysteresis. It means that a large hysteresis and suppression of leakage current are originated from charge accumulation in Ga2O3 layer.

CS MANTECH Conference, April 23rd - 26th, 2012, Boston, Massachusetts, USA

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CS MANTECH Conference, April 23rd - 26th, 2012, Boston, Massachusetts, USA

Our experimental results show that breakdown voltage of the AlGaN/GaN HEMTs with longer gate-drain distance than 20 μm is limited to 1100 V.

Output characteristics (Figure. 4) are measured with sweeping VGS from 2 V to -4 V at -2 V increment. The AlGaN/GaN HEMT employing the multiple Al2O3/Ga2O3 stacks shows the increased drain current. Drain current of the AlGaN/GaN HEMT employing the multiple stacks, Al2O3 only, and the conventional device when VGS: 2V and VDS: 20 V are 305 mA/mm, 221 mA/mm, and 224 mA/mm respectively. It is due to the injected holes in Ga2O3 layer of the device employing the multiple Al2O3/Ga2O3 stacks at forward bias accumulate more electrons in quantum well of 2DEG than conventional Ni/GaN Schottky contact. However, the charging and de-charging process of electron/hole from gate into Ga2O3 layer causes the increase of on-resistance.

Fig. 5 shows the transfer curve of the fabricated devices. The VTH of the AlGaN/GaN HEMT employing Al2O3 only is shifted from -2 V to -2.2 V (determined by the 1 mA/mm) by longer distance from gate to 2DEG than that of the conventional device while the VTH of the AlGaN/GaN HEMT employing Al2O3/Ga2O3 stacks is -1.4 V. It indicates that the electron accumulation at reverse bias under gate extends the depletion region in vertical direction.

Fig. 4: Output characteristics

Fig. 5: Transfer characteristics

Fig. 6: Capacitance-voltage characteristics

Fig. 7: Expanded capacitance-voltage characteristics

Charge accumulation of the proposed devices is investigated by capacitance-voltage measurement in Fig. 6, 7. The capacitance between gate and drain is measured in double direction with frequency of 1 MHz. The large capacitance of the AlGaN/GaN HEMT employing Al2O3 only is attributed to the MIS structure. Charge accumulation in Ga2O3 of the device employing the multiple Al2O3/Ga2O3 stacks also induces increase of maximum capacitance. This maximum capacitance agrees well with the increase of drain current shown in Fig. 4. The AlGaN/GaN HEMT employing Al2O3/Ga2O3 stacks show a large hysteresis. The large hysteresis induces the positive shift of VTH, suppression of leakage current, and increase of output current.

A small hysteresis of the device employing Al2O3 only indicates that the hysteresis of the device employing the multiple Al2O3/Ga2O3 stack is not attributed to Al2O3/GaN interface and that originated from charge accumulation in Ga2O3 sandwiched by Al2O3.

Fig. 8 shows the conductance modulation with gate-drain voltage sweep and frequency of 1 MHz. The AlGaN/GaN HEMT employing the multiple Al2O3/Ga2O3 stack has a

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CS MANTECH Conference, April 23rd - 26th, 2012, Boston, Massachusetts, USA

clearly separated two peak with high peak value. It indicates that the both of on-state and those of off-state electrical properties can be improved by hole/electron accumulation in the Al2O3/Ga2O3 stacks.

Fig. 8: Conductance characteristics

CONCLUSIONS We proposed and successfully fabricated the AlGaN/GaN HEMTs employing the 10 nm thick Al2O3/Ga2O3 stacks, which consist of 2 nm thick Al2O3 and Ga2O3 for increase of breakdown voltage and positive shift of VTH. We achieved a high breakdown voltage of 1100 V and VTH of -1.4 V while those of the conventional device had 380 V and -2 V. The unintentionally formed Ga vacancy site in multiple Al2O3/Ga2O3 stacks formed by rf-sputtering at room temperature acted as accumulation center under gate. The charge accumulation induced improvement of reverse blocking characteristics such as leakage current and breakdown voltage. The large hysteresis of the AlGaN/GaN HEMT employing the Al2O3/Ga2O3 stacks and small hysteresis of the device employing Al2O3 only revealed that the improved electrical properties were attributed to charge accumulation in Ga2O3 sandwiched Al2O3. The proposed multiple Al2O3/Ga2O3 stacks may be suitable structure for a high voltage AlGaN/GaN HEMTs.

ACKNOWLEDGEMENTS This work was supported by the ‘Power Generation &

Electricity Delivery’ of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge Economy.

REFERENCES [1] T. Nomura, et al., Proc. Int. Symp. Power Semiconductor Device and

ICs, pp. 313-316, June 2006 [2] O. Ambacher, et al., JOURNAL OF APPLIED PHYSICS 85, 6 15

(1999) [3] B. Gil, “Group III Nitride Semiconductor Compounds – Physics and

Applications,” Clatendon Press Oxford, 1998, Chapter 2 [4] M.-W. Ha, et al., Proc. Int. Symp. Power Semiconductor Devices & ICs, pp. 169, June 2006 [5] A. V. Vertiatchikh, et al., Electron. Lett, 38, 8 (2002) [6] B. Luo, et al., Appl. Phys. Lett, 80, 9 (2002) [7] Y.-S Lin, et al., ELECTRON DEVICE LETTERS, 31, 2 (2010) [8] Y. Uemoto, et al., IEDM Tech. Dig, pp. 861, December, 2007 [9] H. Xing, et al., ELECTRON DEVICE LETTERS, 25, 4 (2004) [10] S.-C. Lee, et al., Proc. int. Symp. Power Semiconductor Devices & ICs, pp. 247, May 2005 [11] D. Son, et al., ELECTRON DEVICE LETTERS, 28, 3 (2007) [12] H. Kim, et al., Appl. Phys. Lett. 86, 143505 (2005) [13] Y.-H. Choi, et al., Proc. int. Symp. Power Semiconductor Devices & ICs, pp. 233, June 2010 [14] L.-C. Tien, et al., J. Am. Ceram. Soc., 94, 9 (2011)

ACRONYMS

HEMT: High Electron Mobility Transistor 2DEG: 2Dimensional Electron Gas LDD: Lightly Doped Drain VTH: Threshold voltage UID: UnIntentionally Doped MOCVD: Metal-Organic Chemical Vapor Deposition ICP-RIE: Inductively Coupled Plasma Reactive Ion Etching BOE: Buffered Oxide Etchant

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1.0x10-6

1.5x10-6

2.0x10-6

2.5x10-6

3.0x10-6

3.5x10-6

Conventional HHEMTs AlGaN/GaN HEMTs with Al

2O

3

AlGaN/GaN HEMTs with Al2O

3/Ga

2O

3 stack

Con

duc

tan

ce (

mho

)

Gate-Drain Voltage (V)

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