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Improved inverted AlInGa/GaInAs two-dimensional electron gas structures for high quality pseudomorphic double heterojunction AlInAs/GaInAs high electron mobility transistor devices
Molecular beam epitaxy grown AlInAs/GaInAs single quantum well high electron mobility transistor structures (SQW-HEMT) on InP were developed for transistor applications with high current drive capability. Use of low growth temperatures for the layers below the GaInAs channel in case of the inverted interface proved to be essential to achieve simultaneously high electron concentrations in the channel region and mobilities equal to those of normal single heterojunction HEMT structures. The mobilities obtained in SQW-HEMT structures which employed Si delta -doping on both sides of the SQW channel were found to be only weakly dependent on the channel thickness down to 16 nm whereas below the mobility tended to degrade. Based on theoretical calculations an optimum spatial distribution of the carriers is deduced aiming at high channel electron density and low parallel concentration in the lower supply region by optimizing the thickness of the spacers and the asymmetric distribution ofthe donors above and below the channel. Further improvements of the SQW-HEMT structures were obtained by incorporating elastically strained In-rich channels. In this way, increased mobilities and concomitantly enhanced electron concentrations have been achieved. Unsurpassed 77 K mobilities amounting up to 55.000 cm2/V s in conjunction with a Hall carrier density of 6.0*1012 cm-2, which compares with a simulated channel density of 5.4*1012 cm-2, were attained. 0.6 mu m gate length devices fabricated on the optimized SQW-HEMT layer structures clearly demonstrate the superior performance of the SQW design in terms of saturation current without compromising the pinch-off behavior.