Threshold voltage control of InAlN/GaN heterostructure field-effect transistors for depletion- and enhancement-mode operation

摘要

We describe a method to change the threshold voltage of heterostructure field-effect transistors (HFETs) using In_xAl_(1-x)N/GaN heterostructures by using polarization and strain modification in the InAIN barrier layer to realize enhancement-mode operation. The threshold voltage and electronic band structure of the heterostructures were calculated for different indium compositions in the InAlN layer. Band structure calculations predict the enhancement-mode operation of compressively strained InAlN/GaN HFETs with an indium composition higher than 0.25. Studies of InAIN/GaN HFETs with different In alloy compositions show that the sheet resistance increases and the carrier concentration decreases for the heterostructures with increasing indium composition due to changes in the compressive strain and polarization in the InAIN barrier layer. Fabricated HFETs show threshold voltages of -2.5, -0.75, and +0.2 V for In_(～0.18)Al_(～0.82)N/GaN, In_(～0.22)Al_(～0.78)N/GaN, and In_(～0.25)Al_(～0.75)N/GaN HFETs, respectively, corresponding to a shift from depletion-mode to enhancement-mode operation.