Digbijoy N. Nath

Assistant Professor
Digbijoy’s research is primarily on wide band gap materials and heterostructure devices for high-power, high-speed electronics and deep-UV opto-electronics. The work is highly collaborative & inter-disciplinary in nature and involves engineering materials as well as energy band landscape toward high-performance devices and/or trying to understand the science behind processes which prevent wide band gap devices from reaching their full potential. Our work is in close collaboration with Prof Srinivasan Raghavan (Vasu) and Dr R Muralidharan (Emeritus Professor)

Gallium nitride transistors for high power, high-speed applications:

We work on developing the process & device know-how toward realizing large periphery AlGaN/GaN HEMTs on silicon for high voltage & high current applications. This has been under a multi-disciplinary umbrella for a vertically integrated platform which starts with material growth and goes on to device packaging while encompassing device design, fabrication, measurement and analysis en route. Our group works on device design, processing and measurements of these HEMTs. Several attempts are underway toward realizing 600 V devices, exploring different transistor topologies & architectures. This effort is primarily directed at creating indigenous GaN power device technology.

In parallel, we also investigate the science behind GaN power devices vis-à-vis some of the interesting problems to solve in the area of GaN HEMTs. We have worked on dielectrics, interfaces, passivation, and more importantly, on trying to understand the correlation between carbon doping (to reduce leakage) and dislocation density. We study the interplay of C-doping with both vertical and lateral leakage, correlating it with material growth and device design. We also look at Au-free contacts to HEMTs (for CMOS compatibility) and novel designs to boost the breakdown voltages.

GaN HEMTs for RF applications are typically on SiC substrates for superior performance and robustness. However, RF GaN HEMT on silicon presents a highly challenging but promising platform for enabling low-cost and scalable RF devices for possible use in mobile base stations and even handsets in the foreseeable future. With the advent of 5G communication and rapid growth in wireless communication across different IoT platforms, RF devices are staring at a large market. We are presently working on making AlGaN/GaN and InAlN/GaN HEMTs on silicon for possible use in C-band and Ku-band applications respectively. We are investigating different gate & device architectures, passivation schemes and trying to work on novel Ohmic contact schemes in our effort to understand and solve the problems associated with GaN on silicon operating at GHz.

Ultrawide band gap deep-UV photodetectors:

A major research thrust in our group has been the study & development of ultrawide band gap deep-UV photodetectors for solar blind applications. In particular, we look at AlGaN and emerging gallium oxide (Ga2O3) to design, explore, understand and enable high-performance deep-UV detectors. We also look at hybrid oxide/nitride heterostructures for broadband UV sensing.

We have worked on metal semiconductor metal (MSM) AlGaN UV detectors on silicon as well as on sapphire substrates and looked at the interplay of their optical performance such as responsivity, gain, dark current etc. with material quality and growth conditions. We also study AlGaN p-i-n detectors on sapphire, correlating material-to-device performance while demonstrating self-powered devices with record-high zero-bias efficiency. We use a combination of different characterization and fabrication tools to gain deeper insight into the factors that degrade device performance, and engineer ways to improve the functionality and output. Aspects of reliability of photodetectors, packaging, linear arrays and stress measurements are also investigated.

We have worked on MSM and Schottky solar blind detectors on MBE-grown β-Ga2O3, reporting the first demonstration of self-powered MSM devices as well as linear array on bulk Ga2O3 crystal. Presently, we are working on developing an in-house mist CVD setup wherein we are able to stabilize single phase α-Ga2O3 and ε-Ga2O3 on sapphire substrates. Optical properties of these different phases of gallium oxide are being studied.

Devices with 2D layered materials

Layered 2D semiconductors are attracting attention of the device community for their promise of enabling novel, high-performance electronic and optoelectronic devices including those on flexible/transparent substrates. These materials also enable extreme band gap engineering and are apt for heterogenous integration onto dissimilar platforms.

One aspect of 2D materials which we look at is to integrate them with wide band gap semiconductor such as GaN, and seek to explore photodetectors which can work in both near-IR (owing to band gap of 2D material) and near-UV (owing to GaN band gap). In this regard, we study the properties of α- and β-In2Se3, and also have been working on MoS2/GaN and In2Se3/GaN dual-band photodetectors in both vertical and lateral geometry.

Another aspect of 2D materials which we have recently started to explore is to realize artificial synaptic devices that can mimic various brain-inspired functionalities like long term & short-term potentiation, spike timing dependent plasticity and pulsed paired facilitation. In this context, we are particularly looking at ferroelectric α-In2Se3 in conjunction with high-k dielectric for realizing low-power

© 2020 Digbijoy N. Nath