© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

p s scurrent topics in solid state physics

c

sta

tus

so

lid

i

www.pss-c.comph

ysic

aPhys. Status Solidi C 8, No. 7–8, 2219–2222 (2011) / DOI 10.1002/pssc.201001089

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 0.5 1 1.5 2 2.5 3

I F(A)

VF(V)

High-low temperature performance of GaN 600 V Schottky rectifiers Harsh Naik*, Tom Marron, and T. Paul Chow

Centre for Integrated Electronics, Rensselaer Polytechnic Institute, 110 8th Street, Troy NY 12180, USA

Received 17 September 2010, revised 22 November 2010, accepted 8 January 2011 Published online 17 June 2011

Keywords GaN, Schottky rectifier, cryogenic operation, Schottky barrier inhomogeneities * Corresponding author: e-mail naik@rpi.edu

The performance of a 600 V gallium nitride Schottky rectifier at high temperatures up to 125 °C and cryogenic operation has been reported. A 600 V, 4 A GaN Schottky rectifier from Velox semiconductors has been used for the characterization. Forward conduction and reverse blocking performance was

measured down to 77 K. Two Schottky barrier heights have been noticed at low temperatures and a tunnelling limited re-verse leakage current was observed. The SPICE model pa-rameters are also extracted for circuit simulation purposes.

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction In power electronic applications, there has been a continuous trend toward higher operating frequency especially in motor control and switch mode power supplies. The silicon power devices are reaching their theoretical performance limits, hence other wide bandgap materials such as SiC and GaN have been ex-plored for power electronic applications. Schottky diodes have been one of the first wide bandgap power devices to be commercialized. SiC Schottky diodes have demon-strated their ability to have low on-state voltage drop along with excellent reverse recovery and reverse blocking leak-age characteristics even at high operating temperatures, which makes them ideal for use in high-voltage power electronic circuits. Their operation at cryogenic tempera-tures has also been investigated [1].

The high substrate costs of SiC have made GaN Schot-tky rectifiers made on sapphire substrate commercially at-tractive alternative for SiC Schottky rectifiers. Their per-formance in switch mode power supplies has already been demonstrated to be at par with SiC Schottky rectifiers up to 70 °C [2]. The temperature dependence of I-V characteris-tics of GaN Schottky diodes Ga-face and N-face has been studied elsewhere [3]. Thermal effects of GaN Schottky diodes on I-V characteristics has also been studied [4]. In this paper we report high temperature characterization up to 125 °C and the cryogenic operation of a 600 V, 4 A GaN Schottky rectifiers from Velox Semiconductors.

2 Experimental results 2.1 High temperature performance The forward

I-V characteristics were first measured at high tempera-tures up to 125 °C, shown in Fig. 1. It is seen that at lower currents the forward drop decreases with temperature but at higher currents the conduction is limited by series resis-

Figure 1 High temperature forward characteristics. tance and hence the forward drop increases with tempera-ture due to reduction in bulk mobility with temperature. The series resistance is 0.35 Ω at room temperature and in-creases linearly with temperature at the rate of 1.5 mΩ/°C. An ideality factor of 1.16 was extracted indicating close to ideal thermionic emission limited current. From the tem-perature dependence of saturation current (Js) the Schottky

25C to 125C

2220 H. Naik et al.: High-low temperature performance of GaN 600 V Schottky rectifiers

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

ph

ysic

ap s sstat

us

solid

i c

10

100

1000

10000

250 300 350 400 450

Reve

rse

Curr

ent @

600

V(µA

)

Temperature(K)

SiC

GaN

1

10

100

1000

10000

0 200 400 600 800 1000

I R(µ

A)

VR(V)

barrier height can be calculated from the Richardson plot. This value was extracted as 0.95 eV. The reverse leakage current was also measured up to 125 °C, shown in Fig. 2. The measured reverse leakage current at 600 V increased from 150 μA at room tempera-ture to 660 μA at 125 °C. Or for the same leakage current the device blocks ~400 V at 125 °C. The leakage current at room temperature and the increase in reverse current with temperature is much more compared to 40 μA at room temperature to 65 μA at 125 °C measured for similarly rated commercial SiC Schottky rectifiers from Cree Inc. as shown in Fig. 3. Figure 2 Reverse leakage current at high temperatures. Figure 3 Reverse leakage comparison of GaN vs. SiC Schottky rectifier. The higher leakage current at higher temperatures for GaN Schottky diode is attributed to the quasi-vertical structure of the GaN diode which provides additional sidewall sur-faces as leakage current conduction path. Based on the forward, reverse and capacitance measurement the SPICE model parameters were also extracted. The SPICE parame-ters extracted are summarized in Table.1 2.2 Low temperature performance The forward I-V characteristics and the reverse leakage current of the recti-fier were also measured at 77 K by cooling the device in liquid nitrogen. The device was then allowed to heat up to

room temperature and the measurements were performed at intermediate temperatures. The diode is stable at 77 K with a higher forward drop and returns back to the room temperature curve after it heats up, Fig. 4. At low tempera-tures the diode displays an anomalous behaviour, with a plateau in the forward characteristics observed. This is similar to what has been previously observed in SiC and

Table 1 SPICE model parameters.

Name Parameter Value IS Saturation Current 7x10-11 A N Emission Coefficient 1.16

RS Parasitic Resistance 0.35 Ω VJ Junction Potential 0.8 V

XTI IS Temperature Exponent 2 EG Activation Energy 0.95 eV BV Breakdown Voltage 600 V IBV Current at Breakdown 1.5x10-4 A CJ0 Zero Bias Junction Capacitance 2.1x10-10 F TT Minority Carrier Transit Time 0 s

GaN Schottky diodes [1, 3, 5]. This can be attributed to the barrier inhomogeneities in the active area of the rectifier. The structure can be thought of as two Schottky rectifiers with different barrier heights, as shown in Fig. 5.

Figure 4 Forward characteristics at low temperatures.

Anode

Cathode

n+ Substrate GaN

Sapphire

n- epitaxial GaN Cathode

High SBH High SBHLow SBH

Figure 5 Schematic of two Schottky barrier model.

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

1.E+00

0 0.5 1 1.5 2

I F(A)

VF(V)

77KT1T2T3T4RT

77K<T1<T2<T3<T4<25°C

25C to 125C

Phys. Status Solidi C 8, No. 7–8 (2011) 2221

www.pss-c.com © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Contributed

Article

Various phenomena have been proposed to be responsible for this behaviour. Mixture of different metallic phases with different Schottky barrier heights, doping non-uniformities, dopant clustering or contamination are some of the sources which could lead to barrier inhomogeneities in a Schottky diode [6]. At higher temperatures high SBH region limits the current conduction. However, there exists a temperature threshold below which these inhomogenei-ties come into effect and hence higher currents are ob-served due to conduction through the low SBH region at low temperatures.

Figure 6 Forward drop and on-resistance vs. temperature. The variation of forward drop and on-resistance of the recti-fiers from 77 K to 125 °C is shown in Fig. 6. The forward drop increases by about 0.2 V at 77 K compared to room temperature. The measurement was also done in isothermal bath maintained at -90 °C with dry ice in isopropyl alcohol. At low temperatures due to increase in carrier mobility the on resistance decreases. However at 77 K the Fermi level approaches conduction band and mobile carriers begin to “freeze out” on donor impurities and corresponding de-crease in electrical conductivity in bulk material is observed and hence an increase in on resistance is observed [7]. The ideality factor and the high and the low Schottky bar-rier height are extracted from the Richardson plot and summarized in Table 2. From the ideality factor it can be seen that the conduction is thermionic emission limited at room temperature but diffusion becomes the limiting me-chanism at 77 K as indicated by higher ideality factor [7]. The barrier height also decreases at lower temperatures in-dicating that barrier inhomogeneities come into picture when the temperature is reduced. Table 2 Extracted Ideality factor and barrier heights.

Temp. Ideality Factor

High Barrier Height

Low Barrier Height

77 K 2.4 0.59 eV 0.35 eV 300 K 1.02 0.89 eV -

The reverse leakage current was also measured at low tem-peratures up to 600 V (Fig. 7). It is observed that the re-verse leakage current is independent of the temperature for VR > 300 V. This is a signature of tunneling limited current. Barrier height of 0.93 eV was extracted from the Fowler-Nordheim plot, using Eqs. (1)–(3), by assuming a barrier thickness (tunneling distance) of 4 nm, which matches fairly well with the barrier height calculated from the Richardson plot.

2

3

** (1)

E= (2)

4 2 *B= (3)

3

BE

B

J A E eVd

qm φ

−=

Here E is the electric field, V is the applied voltage and d is the tunneling distance.

Figure 7 Reverse current at low temperatures.

3 Summary In summary the high and low tempera-ture performance of GaN Schottky rectifier was tested. SPICE model was developed based on parameters ex-tracted from the high temperature characterization. At cryogenic temperatures certain anomalies similar to those observed for the SiC Schottky rectifiers [1, 3, 4] were ob-served and the possible reasons for the anomalies has been modeled. A two Schottky barrier height effect was ob-served for forward conduction at low temperatures. Tun-neling was observed to be the limiting mechanism for re-verse conduction.

Acknowledgements The authors would like to thank The Boeing Company for the project support and Velox Semiconduc-tors for providing us with the devices.

References [1] M. Shanbhag and T. P. Chow, in: Proc. Int. Symp. Power

Semiconductor Devices and ICs (2002), pp. 129-132. [2] L. Liu, T. Zhu, M. Murphy, M. Pabisz, M. Pophristic, B.

Peres, and T. Hierl, Mater. Sci. Forum 600-603, 1251 (2009).

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

0 100 200 300 400 500 600 700

I R(A

)

VR(V)

77K

T1

T2

T3

RT

77K<T1<T2<T3<T4<25°C

2222 H. Naik et al.: High-low temperature performance of GaN 600 V Schottky rectifiers

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

ph

ysic

ap s sstat

us

solid

i c

[3] J. Osvald, J. Kuzmik, G. Konstantinidis, P. Lobotka, and A. Georgakilas, Microelectron. Eng. 81, 181 (2005).

[4] S. Chung, W. Hwang, C. Lee, and M. Shin, J. Cryst. Growth 268, 607 (2004).

[5] D. Defives, O. Noblanc, C. Dua, C. Brylinski, M. Barthula, V. Aubry-Fortuna, and F. Meyer, IEEE Trans. Electron De-vices 46(3), 449 (1999).

[6] J. P. Sullivan, R. T. Tung, M. R. Pinto, and W. R. Graham, J. Appl. Phys. 70(12), 7403 (1991).

[7] S. M. Sze, Physics of Semiconductor Devices, 2nd ed. (Wiley-Interscience, 1981).