Thin body InAlN/GaN HEMT technology on sapphire: special focus on high-temperature and electrochemical applications

Abstract

This thesis is dedicated to the investigation of lattice-matched InAlN/GaN high electron-mobility transistors (HEMTs) for their application in high-temperature electronics and in electrochemical sensor systems. In this work, this refers to ambient temperatures as high as 600 °C or to the application in corrosive liquid media, like strong acidic (pH = 1) or alkaline (pH = 13) solutions. These two environments cannot be addressed with conventional silicon (Si) or gallium arsenide (GaAs) devices, but can be accessed using a more robust, more thermally stable material like the lattice-matched InAlN/GaN heterostructure. However, to further enhance performance and stability the proposed electrochemical sensor comprises an InAlN/GaN HEMT and, as the actual chemical sensor, an inert boron-doped diamond (BDD) electrode applicable for pH measurements or redox signal detection. The integration with diamond and, apparently, the operation at elevated temperatures require a thermally stable HEMT. Additionally, both applications also require HEMTs exhibiting low overall device leakage characteristics. Therefore, in this work a new GaN-on-Sapphire (GOS) HEMT technology comparable to the Silicon-on-Insulator (SOI) technology was developed, capable of reducing parasitic leakage currents through the gate diode, through the buffer, and between separate contact pads (also between separate devices). Here, GOS implies the oxidation of the InAlN barrier, employment of a thin body (as thin as 50 nm GaN buffer + 50 nm AlN nucleation layer) material stack on sapphire, and the confinement of the active device mesa by dry-etching to small islands on the sapphire substrate. At room temperature, 0.25 µm gate length devices prepared in this GOS technology exhibit an output current density of IDS = 0.4 A/mm and a threshold voltage of Vth = or less than 1.4 V. The residual of buffer-state current is 1 pA nearly independent of the gate width and the subthreshold swing is 73 mV/dec.