AIGaN/GaN MICROWAVE TRANSISTORS FOR WIRELESS COMMUNICATIONSYSTEMS AND ADVANCED NANOSTRUCTURES FOR HIGH-SPEED SENSORAPPLICATIONS

摘要

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 (5x10~(13)cm~(-2)) at the AIGaN/GaN interface without any intentional doping. To improve the high-frequency (HF) performance, transistor gate length and barrier 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-Alcontent 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 apphire, 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].