A channel layer substitution of a wider bandgap AlGaN for conventional GaN in high electron mobility transistors (HEMTs) is one possible method of enhancing the breakdown voltage for higher power operation. Wider bandgap AlGaN, however, should also increase the ohmic contact resistance. We utilized a Si ion implantation doping technique to achieve sufficiently low resistive source/drain contacts, and realized the first HEMT operation with an AlGaN channel layer. This result is very promising for the further higher power operation of high-frequency HEMTs.

For further improvements in AlGaN/GaN heterojunction field-effect transistor performance, it is necessary to reduce the buffer leakage current of the GaN layer. We studied the correlation between the buffer leakage current and the thickness of GaN layers. Unintentionally doped GaN layers with different thicknesses from 0.2 µm to 2 µm were grown by metal organic chemical vapor deposition on SiC substrates. The buffer leakage current of the GaN layer was measured after deposition of ohmic metal contact and the dislocations were observed using a transmission electron microscopy. The thinner samples had a smaller buffer leakage current although a higher density of threading dislocations. We discussed the origin for the reduction in the buffer leakage current for the thinner GaN layers by assuming that the residual carrier in the GaN layer was compensated by dislocations.

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The surface potential of cleaved cross sections of AlGaN∕GaN high electron mobility transistors was measured by Kelvin probe force microscopy. For the bias conditions of Vgs=−5V and Vds=20V, the electric field was concentrated near the GaN∕SiC interface under the gate and between the gate and drain electrodes. A negative potential that decreased over time was observed in the GaN layer beginning 10min after the bias stress was removed. The transient surface potential was found to be well described by an exponential dependence with two time constants: 11 and 55s.