GaN HEMT

A sliver of gallium nitride thinner than a human hair that pumps kilowatts of radio-frequency energy with the efficiency of a well-tuned engine.

Takes a small microwave signal and amplifies it to the high power levels needed to drive 5G base-station antennas, radar transmitters, satellite uplinks, and electronic-warfare systems. Because gallium nitride has a much wider energy bandgap than silicon or gallium arsenide, a GaN transistor can operate at drain voltages of 28–50 V while sustaining breakdown fields around 3.3 MV/cm — roughly ten times the breakdown field of silicon — enabling power densities of 5–10 W per millimetre of gate width at X-band. The same transistor that replaced bulky travelling-wave-tube amplifiers in many satellite payloads is now the dominant active device in sub-6 GHz and millimetre-wave 5G base stations.

The key is a heterojunction: a thin layer of aluminium-gallium-nitride (AlGaN) is grown on top of a gallium-nitride (GaN) buffer. Because the two materials have different crystal polarisations (spontaneous and piezoelectric), electrons pile up spontaneously at the interface without any intentional doping, forming a two-dimensional electron gas (2DEG) — a sheet of extremely mobile electrons trapped in a potential well just a few nanometres thick. Electron mobility in this 2DEG reaches 1,500–2,000 cm²/V·s, and because the electrons are not surrounded by donor atoms that would scatter them, they travel fast and reliably. A metal gate electrode positioned over this 2DEG controls the current like a valve: applying a negative voltage depletes the 2DEG and pinches off the transistor. The GaN-on-SiC variant is dominant for RF power because silicon carbide substrates have a thermal conductivity of about 490 W/m·K — roughly three times that of silicon — allowing the transistor to shed heat fast enough to sustain continuous kilowatt-level outputs.

In plain terms

Imagine a garden hose. A regular transistor is like a thin hose — squeeze lots of water through it and it bursts. A GaN HEMT is like a fire-hose made of titanium. You can force enormous amounts of electrical 'water' (RF power) through it very fast without it breaking, which is why it powers the radio towers that connect your phone to the internet.

Also called: gallium-nitride HEMT, GaN power transistor, AlGaN/GaN HEMT, wide-bandgap RF transistor.

Why it matters

Enables 5G massive-MIMO base stations to radiate average powers of tens to hundreds of watts per antenna element from transistors the size of a postage stamp.
Replaced travelling-wave-tube amplifiers in many satellite and radar applications, cutting size, mass, and warm-up time from minutes to microseconds.
Power density of 5–10 W/mm at X-band is 5–10× higher than GaAs pHEMT, allowing significantly smaller transistor die for a given power level.
Wide bandgap (3.4 eV vs 1.4 eV for GaAs) permits higher operating temperature and voltage, directly reducing cooling overhead in fieldable systems.

Where it lives

Wolfspeed CGHV-series GaN-on-SiC transistors (X-band radar, 5G sub-6 GHz)
Qorvo TGM2543-series MMIC power amplifiers (Ka-band satellite)
Macom MAGX-series GaN transistors (L/S/C-band radar)
EPC eGaN FETs (power conversion, 48 V bus architectures)
Ericsson and Nokia 5G Massive-MIMO radio units (sub-6 GHz, 64T64R)
Airborne X-band fire-control radars (APG-77, APG-81 variants)

Key numbers

Bandgap: 3.4 eV (vs 1.4 eV for GaAs and 1.1 eV for Si — drives all other performance advantages)
Breakdown field: ~3.3 MV/cm (~10× silicon, ~8× GaAs — enables high-voltage, high-power operation)
Power density (X-band, production): 5–10 W/mm (Gate-width-normalised continuous-wave output; 10+ W/mm demonstrated in advanced research devices)
Typical drain voltage: 28–50 V (vs 4–12 V for GaAs pHEMT — fundamental to high power density)
SiC substrate thermal conductivity: ~490 W/m·K (~3× silicon; essential for heat removal at high power densities)
PAE (power-added efficiency, L/S-band class-AB): 50–65% (State-of-the-art; efficiency falls at mm-wave frequencies)
2DEG electron mobility: 1,500–2,000 cm²/V·s (At room temperature, AlGaN/GaN heterojunction)

Surprising facts

A single 10 mm-wide GaN-on-SiC transistor die can deliver over 100 W of continuous RF power at X-band — the same power as a household incandescent light bulb squeezed through an area smaller than a fingernail.
The 2DEG channel where all the current flows is only 5–10 nanometres thick, yet it carries carrier densities of 10¹³ electrons per square centimetre — about ten times denser than a comparable silicon transistor channel.
GaN HEMTs have cut-off frequencies (f_T) exceeding 300 GHz in research devices, meaning the transistor switches its current on and off 300 billion times per second.
The first RF power demonstration of a GaN HEMT, by Mishra and colleagues at UCSB in 1996, produced just 1.1 W/mm — power densities exceeding 10 W/mm at the same frequency are now routinely achieved in production.

Open questions

Current collapse and trap-induced dispersion: Under RF drive, GaN HEMTs often produce less output power and higher on-resistance than DC measurements predict — a phenomenon called current collapse or dispersion. Electrons become temporarily trapped at surface states or buffer defects during large-signal operation and create a virtual gate that pinches the channel. Despite decades of study and numerous passivation strategies (silicon-nitride, field plates, carbon-doped buffers), the exact density, energy distribution, and identity of the dominant trap species in any given device remain difficult to predict from first principles. The effect can shift the apparent knee voltage by several volts and reduce PAE unpredictably under pulsed radar drive conditions.

A short history

The concept of a high-electron-mobility transistor using a semiconductor heterojunction dates to Takashi Mimura at Fujitsu in 1979, using GaAs/AlGaAs. The AlGaN/GaN version arrived in 1993 when Asif Khan and colleagues at the University of South Carolina demonstrated the first AlGaN/GaN HEMT grown by low-pressure metalorganic chemical vapour deposition, achieving electron mobilities around 600 cm²/V·s. The jump to practical RF power came in 1996 when Umesh Mishra and Steve DenBaars at UC Santa Barbara demonstrated that GaN HEMTs could produce useful microwave output power — 1.1 W/mm — for the first time. By 1999, Sheppard and colleagues showed that GaN-on-SiC substrates dramatically improved heat removal, establishing the commercial platform used today. Cree (now Wolfspeed) acquired the first spin-off commercialising RF GaN transistors (Nitres) in 2000, and the first commercial GaN-on-SiC RF transistors entered the market in the early 2000s. By the early 2020s, GaN HEMTs had displaced GaAs in virtually every new high-power microwave design above 1 GHz.

Sources

Khan et al. (1993) — first AlGaN/GaN HEMT demonstration (Appl. Phys. Lett.) (https://pubs.aip.org/aip/apl/article-abstract/63/9/1214/532536)
Wolfspeed GaN-on-SiC RF HEMT product family — technical overview (https://www.wolfspeed.com/products/rf/gan-hemt)
Mishra et al. (2008) — 'GaN-Based RF Power Devices and Amplifiers', Proc. IEEE 96(2), pp. 287–305. DOI: 10.1109/JPROC.2007.911060 (https://doi.org/10.1109/JPROC.2007.911060)
Meneghesso et al., 'GaN-based power devices: Physics, reliability, and perspectives', J. Appl. Phys. 130 (2021) (https://pubs.aip.org/aip/jap/article/130/18/181101/1025623)
Wolfspeed application note — Wolfspeed 150-nm Ka-band GaN HEMT: >5 W/mm at 10 GHz, PAE 45% (https://csmantech.org/wp-content/acfrcwduploads/field_5e8cddf5ddd10/post_1324/012.3_S12_P3_KyleBothe.pdf)