Surface stabilization for higher performance AlGaN/GaN hemt with in-situ movpe SiN
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SURFACE STABILIZATION FOR HIGHER PERFORMANCE ALGAN/GAN HEMT WITH IN-SITU MOVPE SIN M. Germain, M. Leys, J. Derluyn, S. Boeykens, S. Degroote, W. Ruythooren, J. Das, R. Vandersmissen, D.P. Xiao, W. Wang, and G. Borghs IMEC, Microsystems, Components and Packaging, Kapeldreef 75, B-3001 Leuven, Belgium.
ABSTRACT Passivation of AlGaN/GaN HEMT by SiN has been shown by several authors to improve device performance. In this paper we demonstrate a new method of passivating the top AlGaN layer, that leads to a drastic improvement of the device characteristics. On top of the AlGaN/GaN HEMT layer structures, a thin SiN layer is grown at high temperature in-situ, i.e. in the MOCVD reactor prior to unloading the HEMT epiwafers. The quality of the SiN is shown to be very high. As the AlGaN surface is protected against any air contamination, ohmic contacts deposited directly on the SiN layer exhibit better properties than on uncapped AlGaN layers. Transistor performance increases impressively: the source-drain dc current, for a positive gate voltage of 2 V, increases from 0.5 A/mm to 1.2 A/mm for similar AlGaN/GaN HEMT structures by depositing SiN in-situ. RF characteristics also show some improvement. The positive impact of in-situ SiN layer has different origins, some being intrinsically linked: surface protection against oxide formation, better ohmic contact formation, passivation of the surface states, reduction of AlGaN relaxation. All this leads to an increase of the 2DEG carrier concentration. INTRODUCTION The importance of surface-related phenomena in nitride semiconductors remains a major concern towards the development of high-power high-frequency devices. In AlGaN/GaN High Electron Mobility Transistors (HEMTs), spontaneous and piezoelectric polarization fields in these wurtzite materials account for the formation of charge sheets of opposite polarity in the AlGaN layer [1]: the bottom one at the AlGaN/GaN interface, positive, results in the spontaneous formation of the two-dimensional electron gas (2DEG) in the GaN. The carrier density in the 2DEG can be tuned by varying the Al content. For too high Al content at a given AlGaN thickness, relaxation of the stress in the top AlGaN layer sets an upper limit to the 2DEG carrier density, leading to reduced values compared to those predicted by theory [1]. At the top AlGaN surface, in response to the negative polarization charges, overall charge neutrality requires that positive surface charges build up on the free surface, originating from ionized donor-like surface states [2]. Would the surface charge filling not be sufficient to balance the polarization charges, the 2DEG would deplete accordingly. Trapping of charges by surface states has been shown by several authors to be responsible for current slump at high voltages, gate lag effect and RF dispersion (see e.g. [3]). To minimize the impact of those surface states on device operation, passivation of the top AlGaN surface with SiO2 or Si3N4 deposited by Plasma-enhanced Chemical Vapour Deposition (PECVD) after (oh
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