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Keywords:

  • AlGaN/GaN HEMT;
  • Ohmic contact;
  • plasma-assisted molecular beam epitaxy;
  • selective area growth

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experiment
  5. 3 Results and discussion
  6. 4 Summary
  7. Acknowledgements
  8. References

We report on AlGaN/GaN high electron mobility transistors (HEMTs) for high current operation achieved by selective area growth (SAG) technique based on plasma-assisted molecular beam epitaxy (PAMBE). Significant improvement in DC characteristics of the multiple-gate-finger HEMTs was demonstrated when SAG was employed. Furthermore, when group of HEMTs were interconnected, the resulted large-periphery device, with the total gate width of 5.2 mm, exhibited a maximum current of 1.75 A and an on-state resistance of 4.76 mΩ cm2, showing the efficacy of PAMBE-SAG to fabricate GaN-based HEMTs for high-power applications.

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experiment
  5. 3 Results and discussion
  6. 4 Summary
  7. Acknowledgements
  8. References

Recently, GaN has received much attention as a very promising candidate material for high-temperature, high-frequency, and high-power electronics, due to its higher breakdown electric field, larger electron mobility, and better thermal conductivity compared to Si [1, 2]. However, due to the resistive nature of the wide band-gap materials, it is difficult to form low-resistance, thermally stable Ohmic contacts. For decades, much effort has been placed on obtaining low Ohmic-contact resistance for GaN-based metal-semiconductor field-effect transistors (MESFETs) and high electron mobility transistors (HEMTs), utilizing various technologies like ion-implantation and surface treatments [3-5]. However, these methods usually generate surface damage and defect states at the contact regions, reducing the long-term reliability and uniformity. The high-temperature post-annealing process, aimed to repair the damage and activate the dopants after ion-implantation, further creates problems such as GaN film dissociation, Si dopant diffusion, and surface degradation [6]. By contrast, selective area growth (SAG) has been proven effective in achieving low contact resistance in a damage-free manner, since it selectively deposits highly doped GaN layers in the Ohmic contact regions which enhances the tunneling transport of electrons. While metal-organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE) have been employed to achieve SAG [7, 8], plasma-assisted molecular beam epitaxy (PAMBE) has not attracted much attention, in spite of its many advantages like ultra high vacuum and low-temperature growth conditions. Unlike MOCVD or HVPE, in which nothing grows in the mask region, the use of PAMBE gave rise to the growth of polycrystalline GaN on the mask material, due to the reactive nitrogen atoms generated by the plasma source.

In our previous studies, we were able to realize SAG by PAMBE, achieving a record low contact resistivity of 1.8 × 10−8 Ω cm2 and much enhanced peak drain currents for both GaN MESFET and HEMT [9, 10]. We also demonstrated that unlike ion implantation, the damage-free PAMBE-SAG technique is able to suppress the buffer leakage current and improve the breakdown characteristics [11-13]. Due to the simultaneous improvement of drain current and breakdown voltage, PAMBE-SAG is most suited to fabricate high power transistors. In this work, we extend our technique to realize multiple-gate-finger HEMTs and large-periphery devices targeting for high current operation. The advantage of PAMBE-SAG is most prominent for large-periphery power transistors, since the reduction of contact resistance over large area of contact regions would have a greater impact than the single device in reducing the total power loss.

2 Experiment

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experiment
  5. 3 Results and discussion
  6. 4 Summary
  7. Acknowledgements
  8. References

Figure 1a shows an optical image of the fabricated multiple-gate-finger HEMTs. Each unit has four gate fingers with a width of 100 μm. The gate length, source-to-drain and gate-to-drain distances were 5, 15, and 6 μm, respectively. The discrete units were interconnected through the bus lines by wire bonding to form large periphery devices for high current operation.

image

Figure 1. (a) Optical image of the multiple-gate-finger AlGaN/GaN HEMTs array and bus lines and (b) schematic diagram of an HEMT with 54 nm n+-GaN regrown layers in the source/drain regions.

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To fabricate the HEMTs, an Al0.26Ga0.74N (25 nm)/GaN (50 nm)/semi-insulating GaN (2.2 μm) on sapphire substrate prepared by MOCVD was used as the starting material (Fig. 1b). The electron concentration and mobility of the two-dimensional electron gas (2DEG) were measured by a Hall-effect measurement system at room temperature and were 1.3 × 1013 cm−2 and 1340 cm2 V−1 s−1, respectively. Two hundred twenty nanometer device isolation and 25 nm recessed source/drain etching were first performed with an inductively-coupled plasma reactive ion etcher (ICP-RIE) using Cl2/Ar plasma. The SAG process was then implemented as shown in Fig. 2a. A 90-nm-thick SiO2 mask was deposited using plasma-enhanced chemical vapor deposition (PECVD) at 300 °C. Photolithography and Freon-RIE etching were then performed to selectively remove SiO2 in the source and drain regions. The sample was thoroughly cleaned with degreasers and acids before being loaded into the PAMBE chamber. After 15 min growth by PAMBE, a 54-nm-thick single-crystalline n+-GaN layer (1.0 × 1019 cm−3) was grown in the unmasked region, while poly-crystalline GaN was formed on the SiO2 mask due to the lack of growth selectivity. These single-crystalline and polycrystalline structures were confirmed by atomic force microscopy (AFM) analysis (Fig. 2b), where the root-mean-square (rms) roughness for the former was <0.5 nm and that for the latter was >15 nm. Subsequently, the poly-crystalline layer was removed using a heated 15 wt% KOH solution at 75 °C and the underlying SiO2 layer was etched using a buffered oxide etchant (BOE). After removing the surface oxide by HCl:H2O (1:2) solution for 30 s, Ohmic metals of Ti/Al/Ti/Au (30 nm/90 nm/30 nm/60 nm) were deposited on n+-GaN by electron-beam evaporation, and then annealed at 850 °C for 30 s in N2 ambience by rapid thermal annealing (RTA). Finally, Ni/Au (60 nm/130 nm) were deposited as the Schottky gate electrodes.

image

Figure 2. (a) Illustration of the procedure for selectively growing n+-GaN layers in the source/drain regions by PAMBE and (b) the AFM images of each step.

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3 Results and discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experiment
  5. 3 Results and discussion
  6. 4 Summary
  7. Acknowledgements
  8. References

To examine the extent of dopant diffusion in the n+-GaN regrowth layer before and after the high temperature Ohmic annealing process, secondary ion mass spectroscopy (SIMS) was performed. As can be seen from Fig. 3, Si dopants were mostly confined within the n+-GaN layer after RTA. Thus, the high Si dopant concentration can induce significant bending of the conduction band to reduce the potential barrier thickness at the Ohmic contact interface, therefore enhancing the tunneling effect and reducing the Ohmic contact resistance.

image

Figure 3. SIMS depth profile of the Ga–N bonding and Si element in the n+-GaN regrown layer before and after Ohmic metal annealing.

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DC characteristics of the multiple-gate-finger HEMTs were measured by an Agilent 4155C semiconductor parameter analyzer. Figure 4a compares the drain current (IDS)–drain voltage (VDS) characteristics of two devices fabricated using the same procedures on the same wafer, except that one was processed with SAG (HEMT-A) and the other one was not (HEMT-B). Good pinch-off behavior was found for both devices. HEMT-A showed a maximum current (Imax) over 100 mA (limited by the semiconductor analyzer used), much higher than that of HEMT-B of 96 mA. The on-state resistance (Ron) of HEMT-A (2.07 mΩ cm2) is about 44% smaller than that of HEMT-B (3.7 mΩ cm2). The maximum transconductance (gm) is 56 mS mm−1 for the former and 48 mS mm−1 for the latter. The improved Imax, Ron, and gm are the direct result of better Ohmic contacts attributed to SAG. Moreover, it was found that the gate leakage current (Ileak) was also suppressed by SAG (Fig. 4b). Ileak for HEMT-A at the reversed gate bias (VGS) of −10 V was 1.3 μA, over an order of magnitude smaller than that of HEMT-B (14.4 μA). Due to the recessed-gate structure resulted from SAG, the leakage current between the gate and the source is lowered by the surface discontinuity. Low Ileak is highly desirable for improving the breakdown characteristics of GaN-based HEMTs for high-power applications. However, it should be noted that while SAG contributed to the simultaneous enhancement of drain current transport and leakage current suppression, a more negative VGS was required to completely pinch off the 2DEG, due to the heavily doped regrown GaN layers near the channel. With more efforts to optimize the SAG process, further improvements such as reduced threshold voltage should be expected.

image

Figure 4. (a) IDSVDS characteristics and (b) gate leakage currents for HEMT-A (HEMT with SAG) and HEMT-B (HEMT without SAG) measured by an Agilent 4155C semiconductor parameter analyzer.

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To achieve large current operation, wire-bonding was carried out to interconnect group of HEMTs in parallel, and a Tektronix 371 high power curve tracer was used for the high current measurement. Figure 5 shows the saturation drain currents of 1 unit, 5 units, and 13 units. Imax of five interconnected units was 0.68 A at VDS of 15 V, around five times of that of a single HEMT (Imax= 0.14 A). Thus, a linear relationship between the unit number and the current level was found, indicating the fabrication uniformity including the SAG process. Finally, when 13 units were interconnected, a maximum current of 1.76 A was achieved, corresponding to a total gate width of 5.2 mm. The resulting current density of 338 mA mm−1 and on-resistance of 4.7 mΩ cm2 are better than many of the previously reported values for large-periphery GaN-based HEMTs [14-17], demonstrating that PAMBE-SAG is effective to fabricate GaN-based power transistors with smaller device foot-print and lower power loss.

image

Figure 5. IDSVDS characteristics of 1 unit, 5 units, and 13 units interconnected, measured by a Tektronix 371 high power curve tracer.

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4 Summary

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experiment
  5. 3 Results and discussion
  6. 4 Summary
  7. Acknowledgements
  8. References

PAMBE-SAG was employed to improve the characteristics of AlGaN/GaN HEMTs as high power switching transistors. Significant improvement of current density and on-state resistance was observed when SAG was implemented, as well as much reduced gate leakage current. The maximum current of 1.76 A and on-state resistance of 4.76 mΩ cm2 were achieved for a large-periphery device with the total gate width of 5.2 mm. These favorable results demonstrate that PAMBE-SAG is effective in fabricating GaN-based HEMTs for high-power applications.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experiment
  5. 3 Results and discussion
  6. 4 Summary
  7. Acknowledgements
  8. References

This work was supported in part by the Grainger Center for Electric Machinery and Electromechanics of the University of Illinois. The microanalysis was carried out in the Center for Microanalysis of Materials of the University which is partially supported by the U.S. Department of Energy under Grant No. DEF02-91-ER45439.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experiment
  5. 3 Results and discussion
  6. 4 Summary
  7. Acknowledgements
  8. References