PhD Defense: "Ph.D. Defense: Design of III-N Hot Electron Transistors"

Geetak Gupta

September 25th (Friday), 1:00pm
Elings Hall (CNSI), Rm 1601

The III-Nitride material system has shown excellent properties for various applications like LED and LASER lighting, high frequency and high power amplifiers, and power switching. As the technology and theoretical understanding of the III-N system matures, the limitations on further development are based on very basic electronic properties of the material, one of which is electron scattering. A good understanding of electron scattering (and ballistic electron effects) in the III-N system can potentially offer solutions to many problems encountered in these fields. In the field of high frequency transistors, III-N based high electron mobility transistors (HEMTs) have shown excellent performance, however, due to low saturation velocities and parasitic delays, the pathway to THz amplification is unclear. Vertical heterojunction bipolar transistors (HBTs) in the III-N system, unlike their counterparts in the III-As/III-P system, have not had much success in the high-frequency domain due to the high activation energy of the primary p-type dopant, Mg. Ballistic electron effects in transistors can be used to overcome drift/diffusion limits and reduce device delays for very high-frequency operation. Towards this goal, we explore III-N
based vertical unipolar hot-electron transistors (HETs).

The HET consists of three primary regions, a high-energy electron injector (emitter), a thin transit region (base), and a high-energy filter (collector). The injected high-energy electrons (hot-electrons) travel across the base in a quasi-ballistic manner. The collector allows only hot-electrons to pass through and quantum mechanically reflects scattered electrons. The HET designs explored here use AlGaN and InGaN based polarization-dipoles to form potential barriers in the conduction band. Using such a structure, transistor operation was demonstrated in III-N HETs for the first time. The current gain (β) in these first devices was, however, very low (β ~ 0.1 using a 25nm base). Upon further investigation, it was experimentally determined that the ballistic mean free path of hot-electrons is 6nm in the HET base. The III-N HET base was then scaled to 4nm and a β of 3.5 was achieved, which is the highest reported for III-N HETs. This order of magnitude improvement in β was enabled by the use of novel design, growth and process techniques which will be discussed in this talk. We believe that this work provides a clear pathway towards realizing the high-frequency potential of III-N HETs.

About Geetak Gupta:

photo of geetak gupta Geetak Gupta completed his bachelors degree in Electrical Engineering from Indian Institute of Technology, Kanpur in 2010. He received his M.S. degree in Electrical and Computer Engineering from UCSB in 2013. He is currently a Ph.D. candidate in the ECE department at UCSB under Prof. Umesh Mishra. His work involves the investigation of hot-electron effects in the III-Nitride material system to build a ballistic electron transistor.

Hosted by: Professor Umesh Mishra