Two dimensional electron gas in GaN heterojunction field effect transistorsstructures with AIN spacer

摘要

Near lattice matched Al_(0.81)In_(0.19)N/GaN heterojunction structures are compared withconventional Al_(0.3)Ga_(0.7)N/GaN heterojunctions in terms of the sheet density and mobility andtheir dependence on barrier and spacer layer parameters. With the insertion of an A1N spacer, themobility of both structures is improved dramatically. Self-consistent solution of Poisson-Schrodinger equations was developed in order to determine the band structure and carrierdistribution in these GaN based heterostructures in an effort to gain insight into the experimentalobservations. Surface donor states were included to account for the origin of electrons in 2DEG,which is treated as charge neutralization conditions in the simulation. Also the change in thepiezoelectric polarization due to the electromechanical coupling effect, and shift of band gapcaused by uniaxial strain were both included in the calculations. The calculated sheet density isclose to the measured values, especially for the AlGaN samples investigated, but a notabledifference was noted in the AlInN cases. The discrepancy is confirmed to be caused by theexistence of a Ga-rich layer on the top of AlN spacer during the growth interruption, which cansplit the 2DEG into two channels with different mobilities and lower the overall sheet density.When the modifications made necessary by this GaN layer are taken into account in our modelfor the AlInN barrier case, the calculations match with the experimental data. When the spacerthickness increase from 0.3 to 3 nm, the total sheet density was found to slightly increaseexperimentally, which agreed with the theoretical prediction.