About

Elaheh Ahmadi is an Associate Professor and Vice Chair of Graduate Studies in the Department of Electrical and Computer Engineering at UC Santa Barbara. She holds the Mehrabian Endowed Career Development Chair. Her research interests include the design, fabrication, and characterization of novel (ultra)wide bandgap materials and devices for high power and high frequency applications, as well as quantum applications. She is involved in advancing the development of materials and devices that are critical for high-performance electronic and quantum technologies.

Research topics

• Electrical engineering
• Optoelectronics
• Materials science
• Nanotechnology
• Condensed matter physics
• Engineering physics
• Engineering

Selected publications

• High-Performance MBE-Grown N-Polar GaN HEMTs on Bulk GaN with Simplified Backbarrier

  IEEE Electron Device Letters · 2026-01-01

  articleSenior author

  In this letter, we demonstrate N-polar GaN HEMTs with a simplified backbarrier structure grown on low-dislocation bulk GaN by plasma assisted molecular beam epitaxy. Standard N-polar GaN HEMT epistructures grown by MOCVD are relatively complex, requiring a combination of Si doping and graded AlGaN as the backbarrier to eliminate dispersion due to hole traps. On the contrary, our devices employ a simplified undoped AlGaN as the backbarrier. Devices with 100–400 nm GaN buffers showed >1.6 A/mm drain current, nA-scale leakage, fT > 60 GHz, and f<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAX</sub> up to 206 GHz with a gate length of 150 nm. Pulsed-IV measurements confirmed reduced dispersion with thicker buffers, and the 400 nm device achieved 7.2 W/mm output power and 48% PAE at 10 GHz. These results show that MBE-grown N-polar GaN HEMTs on bulk GaN can simultaneously deliver high frequency and power performance while using a simplified structure, thereby improving the manufacturability of the technology.

  PublisherDOI

• N-polar GaN HEMT with Deep Wet Etched Recess Demonstrating Record G-band Performance at 170 GHz

  IEEE Electron Device Letters · 2026-01-01

  article

  In this paper, we report on record large signal performance at G-band (170 GHz) from novel N-polar GaN high-electron-mobility transistors (HEMTs) with Schottky-barrier TiN/Ru gate with enhanced gate aspect ratio. Unlike traditional N-polar GaN Schottky-gate HEMTs, this work establishes the Schottky barrier directly on GaN channel, as opposed to on AlGaN cap, by wet removal of AlGaN cap using citric acid. The developed novel process combines the wet selective GaN cap etch and wet AlGaN cap etch for a fully-wet deep recess etching. As a result, a HEMT with a gate length (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L_G</i>) of 67 nm demonstrated a peak DC transconductance of 924 mS/mm with ~0.8 A/mm saturation current. The device demonstrated a near enhancement-mode (E-mode) behavior with +75 mV threshold voltage. The same transistor showed a cut-off frequency (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_T</i>) of 145 GHz with maximum oscillation frequency (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_MAX</i>) of 330 GHz. Large signal characterization at 170 GHz revealed a record performance of 1.1 W/mm with an associated power-added efficiency (PAE) of 11.9%, and 1 W/mm with 13.3% PAE on sapphire substrate, which demonstrates the best performance among the previously reported >150 GHz GaN results.

  PublisherDOI

• Engineering the future with ultrawide-bandgap semiconductors

  APL Materials · 2026-05-01

  articleOpen access
  PublisherDOI

• Investigation of SiO ₂ :HfO ₂ Composition Effects on ALD HfSiO _x as Gate Dielectric on <i>β</i> -Ga ₂ O ₃ (001)

  IEEE Transactions on Electron Devices · 2026-01-01

  articleSenior author

  This study investigated the dielectric properties of HfSiO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> thin films deposited on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $</tex-math> </inline-formula>-Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>, with a focus on the effects of SiO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>:HfO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composition ratio and post-deposition annealing (PDA) environment using deep ultraviolet (DUV)-assisted capacitance–voltage (C–V) and current–voltage (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I$</tex-math> </inline-formula>–<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}\text {)}$</tex-math> </inline-formula> measurements. Initial results showed no clear trend between composition and performance, attributed to HfSiO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub>/Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> interfacial layer formation. To minimize interfacial layer effects, a thicker initial SiO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer was introduced, enabling a clear correlation between composition ratio and dielectric behavior. Transmission electron microscopy (TEM) and corresponding energy-dispersive X-ray spectroscopy (EDS) further confirmed a well-defined interface between the dielectric and the Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate. Oxygen PDA improved dielectric constant (~10.6), and significantly reduced interface trap density (~<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ 6.1\times 10^{12}$</tex-math> </inline-formula> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> eV<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>) compared to air annealing (~<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ 11.9\times 10^{12}$</tex-math> </inline-formula> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> eV<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup>).

  PublisherDOI

• First demonstration of an N-polar high Al-content AlGaN channel HEMT grown by plasma-assisted molecular beam epitaxy

  APL Electronic Devices · 2026-03-01

  articleOpen accessSenior author

  An N-polar Al0.65Ga0.35N/Al0.85Ga0.15N high electron mobility transistor (HEMT) was demonstrated on an AlN substrate. The epitaxial layers were grown by plasma-assisted molecular beam epitaxy (PAMBE), and a 3 nm in situ GaN cap was employed to prevent surface oxidation of the AlGaN channel. To reduce contact resistance, highly Si-doped, compositionally graded AlGaN was selectively regrown by PAMBE in the contact regions. Peak drain currents of 0.12 mA/mm at room temperature and 0.8 mA/mm at 300 °C were measured. Transconductance also increased from 53 μS/mm at room temperature to 262 μS/mm at 300 °C. Temperature-dependent Hall measurements indicated a reduction in channel resistance with increasing temperature, attributed to enhanced carrier density and mobility. A breakdown voltage of 95.8 V was obtained, along with a corresponding critical electric field of 2.74 MV/cm. While further improvements in current density are necessary for practical applications, this paper reports the first N-polar AlGaN channel HEMTs grown by PAMBE.

  PublisherDOI

• Study of crystalline N-Polar Aluminum Nitride growth on both on-axis and off-axis Si (111) using plasma assisted molecular beam epitaxy

  Journal of Crystal Growth · 2026-02-05

  articleSenior author
  PublisherDOI

• Dual-atom catalysts based on PN-co-doped graphene: a comparative study of iron, cobalt, and nickel for ORR

  Structural Chemistry · 2025-09-17

  articleSenior author
  PublisherDOI

• Recent Advancements in N-polar GaN HEMT Technology

  Crystals · 2025-09-22 · 2 citations

  articleOpen access

  N-polar GaN HEMT technology has emerged as a disruptive technology that outperforms Ga-polar GaN HEMTs in terms of high-frequency power amplification capability. In this paper, the authors present a comprehensive review of the evolution of N-polar GaN HEMT technology from the perspective of crystal growth, dielectrics, and metals on N-polar GaN, transistor design, and performance. Specifically, the authors discuss the progress of the N-polar GaN HEMTs toward high-frequency, high-power, and high-efficiency applications with recent record-level performances, demonstrated by the authors, at mmWave frequencies.

  PublisherOA PDFDOI

• DFT insights into synergistic interactions and ORR mechanisms of BN-supported dual-atom catalysts

  Journal of Molecular Graphics and Modelling · 2025-12-31

  article1st author
  PublisherDOI

• Enhancement Mode N-Polar Deep Recess GaN HEMT With Record Small Signal Performance

  IEEE Electron Device Letters · 2025-07-03 · 9 citations

  articleSenior author

  In this letter, we report a novel enhancement mode N-polar Deep Recess (NPDR) Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT). Enhancement mode operation was achieved by recess etching with a combination of atomic layer etching (ALE) and wet etching. A high-k and high breakdown field HfSiO gate dielectric was employed. The epi-structure was grown on a low dislocation density on-axis N-polar GaN substrate by plasma-assisted molecular beam epitaxy (PAMBE). As a result, true normally-off operation with +0.8V threshold voltage, 1.5 A/mm peak saturation drain current, and 0.55 S/mm transconductance was achieved with 75 nm gate length (L<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub>). A cutoff frequency (f<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub>) of 122 GHz was measured, which resulted in a record f<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub>*L<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> of 9.1 GHz ⋅μm for E-mode AlGaN/GaN HEMTs. Load pull measurements demonstrated an output power of 2.7 W/mm and a power-added efficiency of 46% at 10 GHz.

  PublisherDOI

Recent grants

• CAREER: A novel Gallium Oxide based transistor for low-waste power conversion applications

  NSF · $500k · 2021–2025

Frequent coauthors

• Umesh K. Mishra

  50 shared

• S. Keller

  University of California, Santa Barbara

  42 shared

• Kamruzzaman Khan

  University of California, Santa Barbara

  30 shared

• Brian Romanczyk

  University of California, Santa Barbara

  26 shared

• Matthew Guidry

  University of Padua

  24 shared

• Steven Wienecke

  Transphorm (United States)

  22 shared

• Xun Zheng

  21 shared

• James S. Speck

  University of California, Santa Barbara

  21 shared