Spectroscopic Investigation of Traps Producing Current Collapse In AlGaN/GaN Hemt Structures
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SPECTROSCOPIC INVESTIGATION OF TRAPS PRODUCING CURRENT COLLAPSE IN AlGaN/GaN HEMT STRUCTURES P.B. Klein, S.C. Binari, K. Ikossi, A.E. Wickenden1 , D.D. Koleske and R.L. Henry, Naval Research Laboratory, Washington, DC 20375-5347, U.S.A. 1 Army Research Laboratory, Adelphi, MD 20783-1197, U.S.A. ABSTRACT Photoionization spectroscopy (PS) measurements, previously carried out for the GaN MESFET, have been extended to the more complicated AlGaN/GaN HEMT structures. In all cases, the spectra revealed that the same two traps causing current collapse in the high resistivity (HR) GaN buffer layer of the MESFET were also responsible for current collapse in the HEMT structures. The HR buffer layers supporting the HEMT structures were prepared by MOVPE at varying growth pressures, in order to vary the incorporation of deep trapping centers. Lower growth pressures were observed to correlate with more severe current collapse and with an enhanced incorporation of carbon. Detailed analysis of the PS data suggests that one of the two responsible traps is related to carbon, while the other may be associated with structural defects in the material.
INTRODUCTION The effort to fabricate nitride-based field effect transistors (FETs) for high-power microwave applications has made significant recent advances. Currently, devices with continuous wave output powers up to 9.8 W/mm [1] and high frequency operation up to fT=100 GHz and fmax= 155 GHz [2] have been reported. Good device characteristics are not always reproducible, however, due to the presence of trapping centers in some regions of the device structure. Current collapse is a process resulting from the presence of these trapping centers that causes a reduction in the drain current after the device has been exposed to a large drain-source voltage. The severity of this effect can vary considerably between wafers, and is sensitive to the conditions of materials growth. In this report we investigate the traps causing current collapse in device structures grown by metalorganic vapor phase epitaxy (MOVPE). Current collapse occurs when the applied drain-source voltage is high enough to produce hot carriers in the conducting channel. The hot carriers can be injected into regions of the device structure adjacent to the channel that may contain deep traps. These carriers can be trapped, and can remain trapped after the high voltage is removed, thus resulting in a reduced drain current. Current collapse in nitride-based devices has been investigated in GaN junction FETs (JFETs) [3], in AlGaN/GaN heterostructure insulated gate FETs (HIGFETs) [4] and in GaN metalsemiconductor FETs (MESFETs) [5-8]. For the case of MOVPE-grown MESFETs, the responsible traps were found to be located in the high-resistivity (HR) GaN buffer layer [5-8]. The characteristics of the traps causing current collapse in the GaN MESFETs were studied [7,8] by photoionization spectroscopy (PS). In these measurements, the absorption spectrum associated with the optical emission of trapped carriers from the deep traps is deter
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