HIGH CURRENT BULK GaN SCHOTTKY RECTIFIERS

 

K. P. Ip (1), K. H. Baik (1), B. Luo (2), F. Ren (2),  S.J. Pearton (1) S. S. Park (3), Y. J. Park (3), and A. P. Zhang (4)

 

(1)   Department of Materials Science and Engineering, University of Florida, Gainesville FL 32611, USA

(2)   Department of Chemical Engineering, University of Florida, Gainesville FL 32611, USA

(3)   Samsung Advanced Institute of Technology, PO Box 111, Suwon 440-600, South Korea

(4)   General Electric, Corporate R&D Center, Niskayuna, NY12309

 

ABSTRACT

GaN Schottky rectifiers employing guard-ring and SiO2 edge termination show almost ideal forward current characteristics, with ideality factor 1.08 and specific on-state resistance as low as 2.6x10-3 W cm2. A maximum forward current of 1.72A at 6.28V was achieved under pulsed (10% duty cycle) conditions. The reverse breakdown voltage was inversely dependent on rectifier area. The presence of defects in the GaN still dominates the reverse leakage, with both field emission and thermionic field emission contributions present. The parallel-plane breakdown voltage is never reached, even with the use of multiple edge termination methods, but the results show the promise of GaN rectifiers for power conditioning and electric utility applications.


INTRODUCTION

            GaN-based electronic devices are attractive for high power, high temperature applications, such as motor drives in military and hybrid electric vehicles, in utility power flow control systems, in advanced radar systems and in satellite and rf-communication and data transmission.(1-5) GaN Schottky rectifiers are of interest because of their fast switching times and minimal reverse recovery current. Numerous reports have shown excellent blocking voltage characteristics for lateral GaN and AlGaN Schottky rectifiers grown on sapphire substrates.(6-9) However these devices exhibit poor forward characteristics, with low on-state currents. The forward currents are improved in vertical geometry GaN and AlGaN rectifiers, but the blocking voltages are limited to <750V by the available epi thickness.(10-12) The best forward current characteristics have been reported with rectifiers fabricated on free-standing GaN substrates.(13-15) It is clear that these types of bulk GaN templates offer the best path for progress in making large-area devices which retain high breakdown voltages and follow thermionic emission theory in their forward characteristics.

            In this paper we report on the on-state characteristics of edge-terminated GaN bulk Schottky rectifiers and on the area dependence of the reverse breakdown voltage. The results are compared to simulation of the expected rectifier performance.

 

Experimental

            The GaN substates were ~1x1 cm2 and ~200 mm thick. They were separated from the sapphire substrates on which they were grown by vapor phase epitaxy to produce a free-standing GaN layer with excellent mechanical stability.(16) A key feature in the design of power rectifiers is to avoid the electric field crowding that occurs at the edge of the Schottky metal contact, as shown in Figure 1. Since the impact ionization coefficient of electrons, an, depends exponentially on electric field strength, E, through the relation (17)

where a0 and b0 are material-dependent constants, then it is necessary to mitigate the field-crowding in order to avoid high reverse leakage current and soft breakdown at voltages well below the calculated value.

            In our devices we have employed two edge termination techniques, which are shown schematically in Figure 2. In the first, the metal contact is extended over a surface dielectric layer (eg. SiO2), which extends the depletion boundary and reduces electric field crowding. In the second, the formation of a p+ floating ring or guard ring also reduces field crowding. The potential of the floating field ring  VFFR, is given by

where NA is the acceptor concentration, e the GaN permittivity, WS the field ring spacing and VA the applied Schottky bias. The p+ regions were formed by Mg+ implantation (50eV, 5x1014cm-2) and annealing under N2 at 1050°C for 30s. The 2000Ĺ thick SiO2 dielectric was deposited by plasma enhanced chemical vapor deposition at 250°C using SiH4 and N2O presursors. Windows were opened in the dielectric by wet etching. Full area backside ohmic contacts were formed by e-beam evaporation of Ti/Al/Pt/Au, annealed at 750°C in under N2. Schottky contacts of different diameters (54mm-7mm) were placed on the front surface by e-beam evaporation and lift-off of Pt/Ti/Au. Schematics of the completed devices are shown in Figure 3. Some of the large rectifiers were packaged for pulsed forward current-voltage (I-V) measurements. The experimental details have been given previously(15). Simulations of the structure were performed using the MEDICI TM software package.

 

RESULT AND DISCUSSION

Figure 4 shows the measured and simulated forward I-V characteristics at 25°C for 7 mm diameter rectifiers. The experimental data was obtained at 10% duty cycle and was independent of measurement frequency in the range examined (100-10000 Hz). A maximum current of 1.72A at 6.28V was achieved, a record for GaN devices. The on-state resistance was 2.6 mWcm2 for 75mm diameter rectifiers, which is comparable to the expected value from the relation.(18)

where WD is the depletion layer thickness, e is the electronic charge, mn the electron mobility and ND the background n-type doping level of the GaN substrate. In our case, we have a non-punchthrough drift region condition. The forward voltage drop, VF is related to RON through the equation (19)

where n is the diode ideality factor (1.08 in this case), T is the measurement temperature, k is Boltzmann’s constant, JF the forward current density (usually taken as 100 Acm-2) and fB the barrier height. We obtain a value for VF of ~1.8V in our rectifiers. From the close match of experimental and simulated forward I-V characteristics, we conclude that the dominant current transport mechanism is thermionic emission.

By contrast, the reverse bias characteristics show a strong dependence on contact area. The breakdown voltage VB for a Schottky rectifier is given by (17)

where e is the GaN dielectric constant, EC the critical field, e the electron charge and ND the background doping in the GaN. The simulation shows that a 200mm thick layer doped at 8x1016 cm-3 has an ideal parallel plate breakdown of ~600V.This value is limited by the depletion layer thickness at this doping. Even with the use of the edge termination techniques, we observe experimental VB’s of ~160V for 75mm diameter devices and ~6V for 7mm diameter rectifiers (Figure 5). An area-dependence to breakdown voltage is typical of wide bandgap rectifiers such as SiC in which crystal defects such as micropipes and dislocations act as initiation sites for carrier multiplication.(20)

Plan view transmission electron micrographs in the GaN substrates showed defect densities well below those observed in conventional heteroepitaxial layers grown to a few mm thick on sapphire by methods such as metalorganic chemical vapor deposition or molecular beam epitaxy. Figure 6 shows a representative area from our free-standing GaN samples, in which the total defect density determined by TEM is of the order of  106 cm-2. At this density, it is virtually certain that a large area rectifier will contain many defects within its active region and consequently that the reverse breakdown voltage will be compromised. However, the availability of large-area, free-standing GaN substrates and their excellent on-state current performance show the promise of the technology for power switching applications.

 

 

SUMMARY AND CONCLUSION

The forward characteristics of GaN Schottky rectifiers fabricated on free-standing substrates diplay almost ideal behavior and the current trasport is dominated by thermionic emission. A maximum current of 1.72A was measured under pulsed conditions. The reverse characteristics were dependent on rectifier area, which indicates the behavior is still a function of the defect density.

 

ACKNOWLEDGEMENTS

The work at UF is partially supported by NSF (CTS-991173, DMR-0101438)

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Figure Captions

 

Figure 1. Schematic representation of field crowding at the edge of the rectifying contact.

 

Figure 2. Schematic representation of two different edge termination techniques, namely metal overlap onto a dielectric (top) and floating field rings (bottom).

 

Figure 3. Schematic of Schottky rectifiers fabricated on free-standing GaN substrates.

 

Figure 4. Measured and simulated forward I-V characteristics at 25°C from GaN bulk rectifiers.

 

Figure 5. Measured (from 75mm diameter or 7mm diameter rectifiers) and simulated reverse I-V characteristics at 25°C from GaN bulk rectifiers.

 

Figure 6. Plan view TEM micrograph of free-standing GaN substrate.