P5. A polarization-induced 2D hole gas in undoped gallium nitride quantum wells

Science (2019)

Featured in Cornell Chronicle, Semiconductor Today, EurekaAlert!, ScienceDaily, Phys.org

R. Chaudhuri, S. Bader, Z. Chen, D. Muller, H. Xing, D. Jena

A high-conductivity two-dimensional (2D) hole gas, analogous to the ubiquitous 2D electron gas, is desirable in nitride semiconductors for wide-bandgap p-channel transistors. We report the observation of a polarization-induced high-density 2D hole gas in epitaxially grown gallium nitride on aluminium nitride and show that such hole gases can form without acceptor dopants. The measured high 2D hole gas densities of about 5 × 10^13 per square centimeters remain unchanged down to cryogenic temperatures and allow some of the lowest p-type sheet resistances among all wide-bandgap semiconductors.The observed results provide a probe for studying the valence band structure and transport properties of wide-bandgap nitride interfaces.

P4. High Breakdown Voltage in RF AlN/GaN/AlN Quantum Well HEMTs

IEEE Electronic Device Letters (2019)

A. Hickman, R. Chaudhuri, S.J. Bade, K. Nomoto, K. Lee, H. Xing, D. Jena

In evaluating GaN HEMTs for high-power applications, it is crucial to consider the device-level breakdown characteristics. This work replaces the conventional AlGaN barrier and common AlGaN backbarrier with unstrained AlN, and it assesses the breakdown voltage of AlN/GaN/AlN quantum well HEMTs for gate-drain spacings in the range of 0.27 to 5.1 microns. Results are highlighted by a high breakdown voltage of 78 V for a gate-drain spacing of 390 nm, among the best reported for submicron-channel devices. Additionally, small-signal RF measurements showed record performance for HEMTs on the AlN platform, with ft=fmax = 161/70 GHz. Cutoff frequency and corresponding drain bias are benchmarked against stateof-the-art GaN HEMTs using the Johnson figure of merit, with measured devices highlighted by a JFoM value of 2.2 THzV. These results illustrate the potential for AlN/GaN/AlN quantum well HEMTs as a future platform for high-power RF transistors.

P3. Wurtzite phonons and the mobility of a GaN/AlN 2D hole gas

Appl. Phys. Lett. 114, 253501 (2019)

S.J. Bader, R. Chaudhuri, M. Schubert, H. Then, H. Xing, D. Jena

To make complementary GaN electronics a desirable technology, it is essential to understand the low mobility of 2D hole gases in III-Nitride heterostructures. This work derives both the acoustic and optical phonon spectra present in one of the most prominent p-channel heterostructures (the all-binary GaN/AlN stack) and computes the interactions of these spectra with the 2D hole gas, capturing the temperature dependence of its intrinsic mobility. Finally, the effects of strain on the electronic structure of the confined 2D hole gas are examined and a means is proposed to engineer the strain to improve the 2D hole mobility for enhanced p-channel device performance, with the goal of enabling wide-bandgap CMOS.

P2. Gate-recessed E-mode p-channel HFET with high on-current based on GaN/AlN 2D hole gas

IEEE Electron Device Letters, vol. 39, no. 12 (2018)

S.J. Bader, R. Chaudhuri, K. Nomoto, A. Hickman, H. Then, Z. Chen, D. Muller, H. Xing, D. Jena

High-performance p-channel transistors are crucial to implementing efficient complementary circuits in wide-bandgap electronics, but progress on such devices has lagged far behind their powerful electron-based counterparts due to the inherent challenges of manipulating holes in wide-gap semiconductors. Building on recent advances in materials growth, this work sets simultaneous records in both on-current (10 mA/mm) and on-off modulation (four orders) for the GaN/AlN wide-bandgap p-FET structure. A compact analytical pFET model is derived, and the results are benchmarked against the various alternatives in the literature, clarifying the heterostructure trade-offs to enable integrated wide-bandgap CMOS for next-generation compact high-power devices.

P1. Terahertz spectroscopy of an electron-hole bilayer system in AlN/GaN/AlN

Appl. Phys. Lett. 111, 073102 (2017)

H. Condori Quispe, S. M. Islam, S.J. Bader, A. Chanana, K. Lee, R. Chaudhuri, A. Nahata, H. Xing, D. Jena and B. Sensale-Rodriguez

We report studies on the nanoscale transport dynamics of carriers in strained AlN/GaN/AlN quantum wells: an electron-hole bilayer charge system with a large difference in transport properties between charge layers. From electronic band diagram analysis, the presence of spatially separated two-dimensional electron and hole charge layers is predicted at opposite interfaces. Since these charge layers exhibit distinct spectral signatures at terahertz frequencies, a combination of terahertz and far-infrared spectroscopy enables us to extract (a) individual contributions to the total conductivity and (b) effective scattering rates for charge-carriers in each layer. Furthermore, by comparing direct-current and THz-extracted conductivity levels, we are able to determine the extent to which structural defects affect charge transport. Our results evidence that (i) a non-unity Hall-factor and (ii) the considerable contribution of holes to the overall conductivity lead to a lower apparent mobility in Hall-effect measurements. Overall, our work demonstrates that terahertz spectroscopy is a suitable technique for studying bilayer charge systems with large differences in transport properties between layers such as quantum wells in III-nitride semiconductors.

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