Fabrication of AlGaN/GaN heterostructure field effect transistor using room-temperature ohmic contact

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Abstract

AlGaN/GaN heterostructure field effect transistor (HFET) with the room-temperature ohmic contact (1.0×10−4Ω cm2) was demonstrated through the surface treatment using N2 inductively coupled plasma. The N2 plasma produced N vacancies on the surface of undoped AlGaN, leading to ohmic contact at as-deposited state. The fabricated HFET exhibited the saturation drain current density of 736 mA/mm and transconductance of 148 mS/mm. This room-temperature ohmic contact was suitable for fabrication of AlGaN/GaN HFET.

Introduction

GaN based heterostructure field effect transistors (HFETs) are an excellent candidate for future applications in high power electronics. Ohmic contacts on AlGaN/GaN heterostructure with a low contact resistivity attract continuously great interest in improving microwave gain for power applications. For AlGaN/GaN heterostructure, ohmic contacts with a low contact resistivity (<10−5Ω cm2) have been achieved [1] using Ti/Al metallization after the high temperature annealing (>700 °C). It was suggested that the formation of ohmic contact was due to a high concentration of N vacancies, VN, below the contact, produced by the formation of a TiN and AlN layers, causing the AlGaN to be heavily doped n-type [2]. The Ti/Al-based metal scheme has been widely used [3], [4], but it has some drawbacks, such as surface roughness, because of the high temperature annealing.

It is important to reduce gate length and specific contact resistivity simultaneously for improving microwave gain of HFETs. As gate length is reduced, the space between source and drain becomes narrow, which makes it difficult to align and produce a fine gate pattern between the ohmic pads because of the limited depth of focus in optical lithography. If the ohmic contact forming even at room temperatures is developed, a fine gate electrode could be produced in advance, followed by the formation of source and drain electrodes using the room-temperature ohmic contacts. This promises more precise alignment of a fine gate between source and drain electrodes as well as improvement in uniformity of electrical properties.

The reactive ion etching of n-type GaN using SiCl4 and Ar was found to be effective in reducing contact resistivity [5]. An order of magnitude improvement in the contact resistivity was observed on as-deposited Ti/Al contacts on n-type GaN. However, no work has been conducted on AlGaN/GaN heterostructure yet. Using plasma with the higher density such as the inductively coupled plasma (ICP) could cause the contact resistivity to decrease further.

In this paper, we developed an AlGaN/GaN HFET with a room-temperature ohmic contact. The HFET sample was treated with ICP with N2, on which source and drain electrodes with a low contact resistivity were successfully demonstrated. The fabricated HFET exhibited the saturation drain current density of 736 mA/mm and transconductance of 148 mS/mm.

Section snippets

Sample preparation

Undoped AlGaN/GaN heterostructure used in this work was grown by metal-organic chemical vapor deposition on (0 0 0 1) sapphire substrate. An undoped GaN buffer layer with a thickness of 1.2 μm was grown, followed by the growth of 270-Å-thick undoped AlGaN with the surface of Ga phase. The Al composition in AlGaN was set to be 25%. The electron mobility and sheet concentration were respectively 510 cm2/V s and 3×1013 cm−3, determined by Hall measurements.

In the fabrication of AlGaN/GaN HFET, an

Experimental results and discussion

Fig. 1 showed resistances measured at 0 V with the spacing between ohmic pads in order to evaluate specific contact resistivity. The IV curve in the as-grown sample is nonlinear, showing a completely rectifying contact characteristic over the whole range of voltages, as displayed in the inset of Fig. 1. However, the curve from the ICP-treated sample is linear, showing a good ohmic characteristic. The specific contact resistivity for the ICP-treated sample is evaluated to be 1.0×10−4Ω cm2.

The

Conclusions

In conclusion, we applied the newly developed ohmic contacts to fabricate AlGaN/GaN HFET. The N2 plasma produced N vacancies on the surface of undoped AlGaN, resulting in the ohmic contact formation with the contact resistivity of 1.0×10−4Ω cm2 even in as-deposited state. Transconductance as high as 148 mS/mm and a high saturation drain current density of 736 mA/mm were demonstrated in the 1.0-μm-gate HFET fabricated using the room-temperature ohmic contacts.

Acknowledgements

This work was performed through the project for “National Research Laboratory” sponsored by the Korea Institute of Science and Technology Evaluation and Planning (KISTEP).

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