In GaN-based high-electron mobility transistors, although excellent electron confinement has been demonstrated using a graded AlGaN buffer with linearly decreasing Al-content along [0001] direction, guidelines for graded buffer design are still lacking. To obtain overall pictures of the carrier distribution and energy-band profile in AlGaN/GaN/graded AlGaN buffer heterostructures, the influences of the related physical parameters of the buffer are studied by one-dimensional self-consistent simulation. The results show that the negative polarization charge over the buffer can induce free holes in the depths of the buffer, resulting in the coexistence of electrons and holes. By adjusting the related physical parameters of the buffer, it is even possible to form two-dimensional holes gas (2DHG) at the channel/graded buffer interface. The cause of the coexistence of electrons and holes and the formation condition of 2DHG are analyzed. In addition, in the course of the variation of the related physical parameters, the characteristics of the two-dimensional electron gas density are also exhibited. This study can provide a reference for graded AlGaN buffer design in GaN-based field-effect transistors.
In this work, we use a 3-nm-thick Al 0.64 In 0.36 N back-barrier layer in In 0.17 Al 0.83 N/GaN high-electron mobility transistor (HEMT) to enhance electron confinement. Based on two-dimensional device simulations, the influences of Al 0.64 In 0.36 N back-barrier on the direct-current (DC) and radio-frequency (RF) characteristics of InAlN/GaN HEMT are investigated, theoretically. It is shown that an effective conduction band discontinuity of approximately 0.5 eV is created by the 3-nm-thick Al 0.64 In 0.36 N back-barrier and no parasitic electron channel is formed. Comparing with the conventional InAlN/GaN HEMT, the electron confinement of the back-barrier HEMT is significantly improved, which allows a good immunity to short-channel effect (SCE) for gate length decreasing down to 60 nm (9-nm top barrier). For a 70-nm gate length, the peak current gain cut-off frequency ( f T ) and power gain cut-off frequency ( f max ) of the back-barrier HEMT are 172 GHz and 217 GHz, respectively, which are higher than those of the conventional HEMT with the same gate length.
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