Principal Investigator Tomas Palacios
Gallium nitride (GaN)-based transistors are very promising candidates for high power applications due to their high electron mobility and high electric break- down field. Compared to conventional Si or GaAs based devices, wide bandgap GaN also has fundamental advantages for high-temperature applications thanks to their very low thermal carrier generation below 1000 degrees C. However, in spite of the excellent performance shown by early high-temperature prototypes, several issues in traditional lateral AlGaN/GaN HEMTs could cause early degradation and failure under high-temperature operation (over 300 degrees C). These include ohmic degradation, gate leakage, buffer leakage and poor passivation. In addition, to enable digital circuits, it is critical to have enhancement-mode HEMTs, while two-dimensional electron gas induced by AlGaN/GaN heterostructure makes HEMTs be natural depletion-mode devices.
In this work, we are developing a new GaN technology for high-temperature applications (>300 degrees C). For this, we are first increasing the temperature stability of the ohmic contacts in GaN HEMTs, by combining a refractory metal such as tungsten (W) with Si-ion implantation, which locally dopes the material n-type and reduces the contact resistance. The schematic cross section is shown in Figure 1. An R c of 0.8 omega mm, I max of 700 mA/mm were obtained with the W ohmic contacts in a transistor with a gate length of 4 µm. The W ohmic contacts were stable at least up to 300 degrees C in air for at least 30 min, while conventional alloyed Ti/Al/Ni/Au ohmic contacts showed a strong temperature dependence and their contact resistance increased from 0.47 omega mm (RT) to 2.15 omega mm (300 degrees C). Gate injection transistors (GIT) have also been studied for enhancement-mode HEMTs. The structure used in this work had a 110nm extra p-GaN layer on 15nm Al0.2Ga0.8N barrier layer to fully deplete 2DEG under gate area. As shown in Ids-Vgs, a positive VT around 3V was achieved, and their high- temperature stability is currently under investigation.