Next Generation High Electron Mobility Transistor based on InGaN Quantum Well Channel

Abstract

In past two decades and more, III-N based high electron mobility transistors (HEMTs) have generally used GaN as the channel layer. During this period, the quest for operating it at higher frequencies has resulted in the evolution of different heterostructures viz. Ga-polar AlGaN/GaN, AlGaN/AlN/GaN, InAlN/AlN/GaN, InAlGaN/AlN/GaN etc. as well as its N-polar variants. As a result of technological developments related to epitaxial growth and device processing, the performance of such HEMTs have shown significant progress in recent years and are approaching their performance limits. Such high frequency devices are the key components of many communication systems, defence systems and sensors operating at mm- and sub-mm wave frequencies. To meet such requirements, the operating frequencies can be increased even more by addressing the intrinsic delay (τ=Lgate/vsat) via increase in the electron velocity (vsat). In this context, InGaN is a promising material for the channel layer due to an increase in electron saturation velocity with InN fraction. As part of the present work, theoretical and experimental aspects required for the development of InxGa1-xN QW channel HEMTs with 0<x<0.4 were studied. Analytical models were first developed to understand the transport properties of 2DEG viz. sheet carrier density (ns) and mobility (μ) in the InxGa1-xN QW channel HEMTs. As a step towards realization of InGaN QW channel HEMTs using Metal Organic Vapor Phase Epitaxy (MOVPE), indium incorporation in InxGa1-xN was studied as a function of different growth parameters. The results of characterization confirmed the growth of InxGa1-xN with ‘x’ up to 0.23. The growth conditions optimized for InGaN were utilized for growth of InGaN QW channel HEMTs on different substrates viz. Si(111), Sapphire and SiC. The epitaxial issues like integration of low temperature AlN spacer and the barrier layer were addressed experimentally. Finally, InxGa1-xN QW channel HEMT structures with composition 0.02≤x≤0.20 were successfully grown using MOVPE. The carrier concentration and mobility of the 2DEG measured at room temperature in In0.12Ga0.88N QW channel HEMT grown on SiC were ns ~ 1.84×1013 cm-2 and μ ~ 471 cm2/V∙s respectively. The corresponding values for In0.20Ga0.80N QW channel HEMT grown on Sapphire were ns ~ 2.17×1013 cm-2 and μ ~ 265 cm2/V∙s. The trends of measured values of transport properties of 2DEG in the grown InGaN channel HEMT structures were found to be in good correlation with the respective models proposed in this work