This invention is related generally to homoepitaxial gallium nitride (GaN) based electronic devices and specifically to homoepitaxial GaN based transistors, rectifiers, thyristors, and cascode switches.
Gallium nitride (GaN) based electronic devices offer superior high voltage, high power, high temperature, and high frequency operation, as compared to analogous devices fabricated on silicon, gallium arsenide (GaAs) or indium phosphide (InP) substrates due to GaN's wide bandgap, high breakdown field, and high saturation velocity. A variety of types of GaN-based devices are of interest for microwave power amplifier and low-noise amplifier applications, including metal semiconductor field effect transistors (MESFETs), metal oxide field effect transistors (MOSFETs), metal insulator field effect transistors (MISFETs), bipolar junction transistors (BJTs). Heterojunction bipolar transistors (HBTs) and high electron mobility transistors (HEMTs), also known as heterojunction field-effect transistors (HFETs), modulation-doped field effect transistors (MODFETs), two-dimensional electron gas field effect transistors (TEGFETs), or selectively-doped heterostructure transistors (SDHTs), which take advantage of the bandgap engineering possible with III-V heterojunctions to provide considerably higher electron mobilities than analogous MESFETs. Additional GaN-based devices are of interest for power electronic applications, including thyristors, Schottky rectifiers, p-i-n diodes, power vertical MOSFETs, power vertical junction field effect transistors (JFETs), and cascode switches, which take advantage of GaN's wide bandgap, high breakdown field, high thermal conductivity, and high electron mobility.
Typically, GaN-based electronic devices have employed heteroepitaxial growth of GaN and AlGaN on sapphire or SiC substrates. A thin low-temperature nucleation layer, AlN or GaN, also referred to as a buffer layer, is typically used in order to accommodate the lattice mismatch between GaN and the substrate and maintain an epitaxial relationship to the substrate. This approach suffers from a number of drawbacks, including: (i) generation of about 1010 threading dislocations per cm2 due to lattice mismatch, degrading device performance; (ii) excess strain in the device structure, due to thermal expansion mismatch, resulting in degraded performance, device yield, and reliability; and (iii) in the case of sapphire substrates, poor heat dissipation. Heteroepitaxial GaN-based electronic devices have been able to demonstrate performance levels that are satisfactory for some applications, but do not have the requisite level of reliability.
At least one homoepitaxial GaN-based electronic device design, a HEMT, has been reported to date. Khan et al. [Appl. Phys. Lett. 76, 3807 (2000)] disclose the fabrication of an AlGaN/GaN HEMT on a bulk GaN substrate that was grown in a liquid Mg/Ga alloy at temperatures of 1300-1500° C. and N2 pressures of 15-20 kbar. These substrates, however, have several disadvantages including: (i) a high concentration of Mg and O atoms, approximately 1019 cm−3 each [J. I. Pankove et al., Appl. Phys. Lett. 74, 416 (1999)], which could potentially diffuse into device structures during high temperature processing; and (ii) relatively poor thermal conductivity. Dopants may diffuse into the undoped GaN buffer layer, in which transport by the two-dimensional electron gas is designed to occur, degrading carrier mobility. In addition, the presence of the point defects scatters phonons in the bulk GaN substrate and degrades thermal conductivity, which is detrimental to achieving theoretical performance levels in GaN-based HEMTs. In fact, the homoepitaxial HEMT reported by Khan et al. actually had a slightly inferior performance to a similar device fabricated on a SiC substrate.