UC Santa Barbara

Gallium Nitride has gained prominence in the field of power electronics due to its high bandgap, high critical electric field, high mobility and high saturation-drift velocity. This means that GaN can be used to make devices that have a low on-resistance along with a high breakdown voltage. High frequency GaN HEMTs have been commercially available since 2006 and still being improved. The high power market is just starting to tap into GaN devices. Vertical transistors are especially attractive for high power applications, as they provide the possibility of a high breakdown voltage at a low chip size, thus, a low cost price. In addition to this, GaN devices can also use the two-dimensional electron gas formed at the GaN/AlGaN heterojunction to obtain a higher current.

Several GaN based power devices are being investigated - both vertical and lateral. Our group has been focusing on the Oxide GaN-interlayer Field Effect Transistor (OGFET) and the Current Aperture Vertical Electron Transistor (CAVET). On the lateral side, power HEMTs are being developed. This work focuses on materials development for the various types of power devices. A primary motive for this thesis is for it to serve as a reference manual for researchers working on GaN power devices.

Vertical power devices are united by a common feature – a thick drift region with a low n-type carrier concentration. Through simulations, it has been shown that the optimal carrier concentration to simultaneously achieve a low Ron and a high breakdown voltage is about 1 x 1016 cm-3. Through our experiments, it was demonstrated that it is also the carrier concentration range where the electron mobility peaks. Achieving such a low doping can be problematic as it is in the range of impurity dopants such as Carbon and Oxygen. A low doping and record mobility was achieved for vertical devices by MOCVD and applied to the OGFET and the CAVET. Devices were grown both, on sapphire and on bulk GaN and high breakdown voltages were achieved.

Power devices require an elaborate growth and fabrication process, including regrowth on p-GaN doped by Magnesium and regrowth within trenches. Magnesium is known to diffuse into subsequent layers during regrowth, lowering device performance. A novel approach to circumvent this issue is presented, which can eliminate the need to remove the sample from the MOCVD chamber altogether. A low temperature GaN layer grown via flow modulation epitaxy successfully suppressed the Mg penetration at a rate of 5 nm/dec, the lowest ever by MOCVD, leading to the formation of an abrupt p-GaN:Mg/GaN junction.

Regrowth within trenches was optimized for different applications. GaN was grown conformally within narrow trenches by exploring various growth conditions. A complete filling of trench was also achieved for a different set of applications. Finally, conformal regrowth of high composition, low temperature AlGaN was optimized for deep-recessed gate HEMTs to lower gate leakage. All of these were planar regrowths and no masks were used.