Heterostructure designs for enhanced performance and reliability in GaN HFETs: camelback channels

Abstract

The electric fields present in a heterostructure field effect transistor (HFET) give rise to large densities of hot electrons and hot phonons not in equilibrium with the lattice (thus the term "hot"). Hot electrons give rise to an accumulation of hot phonons, which play a deleterious role in device performance and reliability, particularly as 2DEG densities increase. Thus, it is important that hot phonon effects be mitigated. The hot phonon effect arises due to the fact that LO phonon (longitudinal optical phonon) scattering is the primary scattering mechanism at high fields in GaN, and that LO phonons tend to accumulate in the channel due to long lifetimes and low group velocity; minimizing the existence in time of hot phonons in the channel is the overall goal. That said, the lifetime of the LO phonons decaying into propagating LA modes is not constant, and it is through the exploitation of this fact that one gains the ability to enhance the performance and reliability of HFETs. This has been evidenced by operating HFETs at particular bias conditions which tend to lend themselves to relatively short LO phonon lifetimes. Alternatively, minimization of hot phonon lifetimes may be achieved through novel device designs. In this work, we outline designs for minimizing hot phonon effects in HFET devices with sheet densities >2 x 10¹³ cm^-2. We propose "camel-back" structures with wavefunction "spreading layers" to maximize 2DEG charge while mitigating hot phonon effects arising from high density 2DEGs.