Research Projects

Gallium nitride (GaN) high electron mobility transistors (HEMTs) are fast penetrating the microwave/RF and power electronics markets due to their superior current densities, high mobility and switching speeds and unrivalled breakdown voltages. In RF, GaN HEMT is a critical technology for emerging 5G wireless, strategic sector applications such as radars & SATCOM. However, despite its market penetration and enormous promise, GaN HEMTs still haven’t reached their actual performance limit. First, the breakdown voltage trades off with current collapse under pulsed conditions, thereby limiting the output power. Secondly, GaN HEMTs are notorious for their lack of linearity because its gain (transconductance, gm) falls off with input bias i.e., GaN HEMT does not have a flat gm. This second aspect is critical for telecom applications where linearity is highly desirable by circuit designers. In this project, we propose to design, develop and demonstrate fin-channel GaN HEMTs such that the gate wraps around the ‘fins’. The advantages over conventional HEMTs which such fin-HEMTs offer are: a) Due to gate wrapping around, the channel control is superior, resulting in higher gm b) The field management can be engineered across the channel in a 3D fashion, resulting in higher breakdown voltages without compromising the ON resistance c) A flat gm is achievable unlike in regular GaN HEMTs. This is because the non-linear source access resistance which is widely believed to be a reason for gm roll off, is circumvented due to the fin in the channels d) With proper fin engineering, e-mode or normally-off devices can be achieved which is promising for various wireless applications due to single polarity supply. The challenges or disadvantage of proposed fin-HEMTs over regular HEMTs are: i. Fabricating ‘fins’ using e-beam lithography is a slow process; so, these devices could be more resource-intensive. Realizing large-area HEMTs for higher absolute power levels would be more time-consuming. ii. Fin-HEMTs give higher current density (A/mm) but slightly lower absolute current (A) for the same wafer real estate. Thus, from the wafer yield & real-estate point of view, it may not be as economical as regular HEMTs. While most of the features as outlined in advantages (a)-(d) are demonstrated in the literature including flat gm, higher breakdown voltage, and e-mode operation, there is enormous scope to demonstrate much superior performance in various aspects, given it’s still at an embryonic phase. For example, e-mode operation with flat gm is not achieved in fin-HEMTs; record RF power levels in C or X band are still held by regular GaN HEMTs, not by fin-channel devices. Thus, in this project, we propose to bring in novel concepts in gate-drain field engineering such as high-k and p-type materials as passivation, field-plate dielectrics and fin-fillers which are expected to boost the performance of fin-channel GaN HEMTs to a level unprecedented in GaN RF technology.