NICOP - Study of advanced methods for characterization of multilayer semiconductor structures

High electron mobility transistors (HEMT) are able to operate at higher frequencies thanordinary transistors, up to millimeter wave frequencies. They are used in high-frequencyreceivers and transmitters as high power switching devices, in applications such as electronicwarfare, surveillance radar, and satellite communication. The technology is based on bandgapengineering that is achieved using a stack of several semiconductor material layers havingdifferent energy bandgaps. These multi-layer heterostructures are very difficult to characterizewith present-day methods, as these methods were designed for silicon-era electronics that istypically made of a single layer of a single semiconductor.Our preliminary work shows that the manifestation of the electric-field assisted absorption (alsoknown as the Franz-Keldysh effect) in the spectral responses of such multilayer structures can beutilized to extract valuable information on each of the layers, independently, that can be used togive the main electrical parameters (built-in electric fields, 2DEG charge, and 2DEG mobility) aswell as the energy band diagram of a high-electron-mobility transistor structure as a function ofthe gate voltage.In the proposed study, we wish to study two aspects of the basic physics underlying the newmethod. One is the effect of quantum confinement, as is the case when quantum wells are part ofthe structure, on the spectral response, the manifestation and effect of excitonic absorption, thequantum confined Stark effect, and the quantum confined Franz-Keldysh effect on the measuredchannel-photocurrent spectral responses and to find whether modeling of these effects may gainfurther valuable information on the HEMT structure. We also propose to study the effect ofsurface states and deep levels on the spectral responses. To this end, we will use photovoltageand photo-Hall spectra and study and model the manifestation of the Franz-Keldysh effect inthese spectra, which are sensitive to deep levels and surface states. This aspect is especiallyimportant in the GaN HEMT which function is based on the engineering of charged surfacestates.This project will study the physics underlying advanced characterization methods to aiddevelopment of GaN high electron mobility transistors ??? the fastest high power radio frequencyswitching technology today ??? used in high power density electronic systems, compact pulsedpower systems, power electronics, RADAR, and electronic warfare. This study will be carriedout in collaboration with the Power Electronic Branch of the Naval Research Laboratory to directthe outcomes of this study to meet the needs of the developers of new power RF transistors atNRL, and will be carried out on GaN HEMT transistors supplied by NRL, tailoring our newcharacterization tools to the challenges encountered nowadays in the development of new widebandgap semiconductor-based HEMT power switches.