AlGaN/GaN microwave transistors for wireless communication systems and advanced nanostructures for high-speed sensor applications

Abstract

I. Introduction. The AIGaN/GaN heterostructures are a relatively young class of materials, which have already found their way into many commercial applications and established themselves as key devices for the next generation of wireless communication systems [1]. GaN-based material systems possess fundamental properties that allow high-power operation in the millimeter-wave range. The piezoelectric effect is three times stronger in nitride-based materials than in GaAs, resulting in the formation of high sheet electron density (5xl0 1 3 cm-2 ) at the AIGaN/GaN interface without any intentional doping. To improve the high-frequency (HF) performance, transistor gate length and barr ier layer thickness should be decreased. However, this usually results in a decreasing density of the two-dimensional (2D) gas. One innovative approach for a high electron mobility transistor (HEMT) design was presented by M.Higashiwaki [2], who proposed the use of a thin and high-AI content barrier layer to keep sheet channel charge concentration high enough with a decreasing barrier layer thickness to reach high-power operation with a current-gain cutoff frequency of 152 GHz. Recently, novel processes for recessed 70-nm gate-length AIGaN/GaN HEMTs for gate-footprint definitions [3] were reported. The highest reported value today for any nitride transistor power-gain cutoff frequency is 300 GHz and it was achieved by combining a low-damage gate-recess technology and recessed source/drain ohmic contacts to enable minimum short channel effects and very low parasitic resistance [4]. However, material quality still limits the HF performance due to a lack of a suitable lattice-matched substrate. Therefore, epitaxial films contain a high density of dislocations. The substrates of choice are sapphire, silicon carbide and silicon. In spite of considerably improved thermal removing properties, the SiC substrate is still very expensive and Si substrates still demonstrate a comparatively high level of leakage currents. The latter results in decreasing output power densities from 12 W/mm (sapphire substrate) [5] to 5.1 W/mm (Si substrate) [6].One of most promising approaches for the fabrication of low-cost HEMTs involves the use of a sapphire substrate. The low-temperature (500° C) buffer layer of AIN proposed by Akasaki [7] serves to relax the strain between the GaN film and the sapphire, which in turn allows improving the surface morphology and crystalline quality of GaN epilayers. In spite of recent achievements, growth technology needs still to be improved in order to reduce the large concentration of structural defects with dislocation densities (DD) of the order of;:::; lO S + 10 10 cm-2 and to reduce significant residual deformations [8,9]. In addition, the limitation for CW power performance is the self-heating effect [10], which increases the knee voltage by (T/300K) LS for a definite channel temperature, T, of the HEMT. Recently, increasing interest has been shown to resonant tunneling diodes (RTD) [11,12], which represent the most promis...