GaInAsAnP composite channel HEMTs

Abstract

GalnAs/AllnAs on InP HEMTs have demonstrated great potential for microwave and millimeter wave power applications [1,2]. By using Al,lnl-,As Schottky layer with x>0.48 we have been able to improve the gate-to-drain breakdown voltage of these HEMTs. But the remaining drawback has been their low value of the drain-to-source breakdown voltage due to the low breakdown field of the Ga0.47ln0.53As channel. Using InP as the channel material should improve the drain-to-source breakdown voltage, but due to the difficulty in achieving very low contact resis?ances-as well as the higher fields necessary to achieve velocity saturation-InP channel HEMTs have a higher knee voltage in their current-voltage characteristics and lower fT's than typical Gao.47lno.53As channel HEMTs. By using a combination of a thin layer of GalnAs and InP as the channel material it is theoretically possible to use the advantages of both materials [3]. Previously reported composite channel structures did not demonstrate any improvement in the drain-to-source breakdown voltage, probably due to the limitation of the gate-to-drain breakdown voltage of the devices. In this study we have combined AIo.60h0.40As Schottky layers with GalnAsAnP composite channels. We investigated the effect of changing the thickness of the Ga0.471n0.53As channel on the DC and RF characteristics of the HEMTs. The wafers were grown using a Varian Gen I I gas-source MBE on InP substrates. The channel consisted of a 100 A InP layer doped 2X1018cm-3, followed by a 50 A undoped InP layer. The Ga0.47ln0.53As channel thicknesses were varied between the three wafers grown and they were 100 A, 50 A, and 30 A for wafers A, 6, and C respectively. A 250 A Alo.6oino.40As undoped layer was used as the Schottky layer. HEMTs with gate-length of 0.15 pm, drain-to-source spacing of 2 pm, and total gate-widths ranging from 50 to 900 pm were fabricated on these three wafers. Preliminary measured results on the wafers are very encouraging. The maximum transconductance (measured at Vds=l.5 V) for wafers A, 6, and C, were 795, 71 1, and 613 mS/mm. Wafers A, 6, and C, had full channel currents of 733, 747, and 560 mA/mm, and drain-to-source breakdown voltages of 6.8, 8.3, and 10.6 V respectively. The knee voltage on all the wafers were comparable to Ga0.471n0.53As channel HEMTs. Based on the full channel currents and the breakdown voltages, power densities of 480, 627, and 633 mW/mm can be calculated for wafers A, B, and C. The preliminary results indicate that GalnA$/lnP composite channel HEMTs with a Gao.47lno.53As channel thickness of 50 A or less show improvement in the drain-to-source breakdown voltage without sacrificing performance.