What makes GaN superior to GaAs and LDMOS for RF power?

GaN's wide bandgap (3.4 eV) provides three fundamental advantages. First, high breakdown electric field (3.3 MV/cm, 10x silicon) enables high operating voltage: GaN HEMTs operate at 28 to 65 V drain voltage vs 12 V for GaAs and 28 to 32 V for LDMOS. Higher voltage means higher output power from a given device size, since power equals V^2/(2*R_opt). Second, high electron saturation velocity (2.5 times 10^7 cm/s, 2x GaAs) enables high current density and high frequency operation (f_T greater than 100 GHz for 0.15 um gate length). Third, high thermal conductivity of SiC substrates (490 W/m-K, 3x GaAs, 13x sapphire) enables efficient heat extraction, allowing sustained high power density operation. The combined result: a single GaN transistor on SiC delivers 5 to 10 W/mm gate periphery, compared to 1 to 1.5 W/mm for GaAs pHEMT and 1 to 2 W/mm for LDMOS. A 100 W PA that requires 100 mm of GaAs gate periphery needs only 10 to 20 mm of GaN.

What substrate options exist for GaN?

GaN-on-SiC is the premium option for RF applications. SiC's high thermal conductivity (490 W/m-K) enables the highest power density because heat is efficiently extracted from the device. SiC substrates are expensive (4-inch wafer cost is 5 to 10x silicon) but provide the best performance. GaN-on-Si uses standard silicon substrates, reducing wafer cost by 5 to 10x compared to SiC. However, silicon's lower thermal conductivity (150 W/m-K) limits power density to 3 to 5 W/mm, and the lattice mismatch between GaN and Si introduces more defects, reducing reliability. GaN-on-Si is gaining traction for 5G small cell PAs (1 to 10 W) where the cost advantage outweighs the performance penalty. GaN-on-diamond is an emerging technology using diamond substrates (2000 W/m-K thermal conductivity) for ultimate heat spreading, achieving power densities above 15 W/mm in research demonstrations. GaN-on-sapphire is primarily used for LEDs and low-power RF due to sapphire's poor thermal conductivity (35 W/m-K).

How are GaN HEMTs used in 5G base stations?

In macro base stations, GaN HEMTs are used in Doherty PA configurations delivering 60 to 200 W average output power per sector at sub-6 GHz frequencies (1.8 to 3.5 GHz). A typical implementation uses a 0.25 um GaN-on-SiC process with 28 to 50 V drain supply. The Doherty architecture achieves 45 to 55 percent average efficiency for LTE and 5G NR signals with 8 to 12 dB PAPR, combined with DPD for linearity. For massive MIMO active antenna systems (64T64R), each antenna element has its own GaN PA, typically 5 to 10 W per element using 0.15 um GaN-on-SiC. The total array power is 320 to 640 W from 64 elements, with beamforming gain compensating for the lower per-element power. At mmWave (28 and 39 GHz), GaN competes with SiGe and InP for PA applications, offering higher power per element (1 to 2 W vs 100 to 200 mW) but at higher cost.

Semiconductor Technology

GaN HEMT

/gan hemt/ — Gallium Nitride HEMT

A wide-bandgap (3.4 eV) AlGaN/GaN heterojunction FET forming a 2DEG channel with high electron mobility and saturation velocity (2.5×10⁷ cm/s). Breakdown: 100-650 V enabling 28-65 V drain operation. Power density: 5-10 W/mm (vs. 1-1.5 for GaAs, 1-2 for LDMOS). On SiC substrates (490 W/m-K) for optimal thermal management. Dominant for 5G macro Doherty PAs, radar transmitters, and EW systems.

Bandgap: 3.4 eV

Power: 5-10 W/mm

V_DS: 28-65 V

Understanding GaN HEMTs

GaN HEMT technology has revolutionized RF power amplification over the past two decades. The AlGaN/GaN heterojunction creates a two-dimensional electron gas (2DEG) at the interface, providing high sheet charge density (~10¹³ cm^-2) and high electron mobility (~2000 cm²/V-s). Combined with GaN's wide bandgap enabling high-voltage operation, the result is transistors that deliver an order of magnitude more power per unit gate width than conventional GaAs or silicon technologies.

The practical impact is transformative. A base station PA that previously required multiple large LDMOS transistors in complex power-combining networks can now use a single GaN die. Radar transmitters that required traveling-wave tubes (TWTs) for high peak power can be replaced with solid-state GaN amplifiers, improving reliability and reducing maintenance. Electronic warfare jammers benefit from GaN's wide bandwidth, covering multiple octaves with a single amplifier design.

GaN Material Properties

Key material parameters:
Bandgap: E_g = 3.4 eV
Breakdown field: E_br = 3.3 MV/cm
Sat. velocity: v_sat = 2.5×10⁷ cm/s
2DEG density: n_s = 1×10¹³ cm^-2
Mobility: μ = 2000 cm²/V-s

Power density:
P_max = V_DS² / (2 × R_opt × W_g)
@ V_DS=50 V: P = 8-10 W/mm
@ V_DS=28 V: P = 3-5 W/mm

Johnson FOM (freq × voltage):
JM = E_br × v_sat / (2π)
GaN: 27.5 THz-V
GaAs: 2.7 THz-V (10x lower)

RF Transistor Technology Comparison

Technology	V_DS	W/mm	f_T	Substrate	Application
GaN-on-SiC	28-65 V	5-10	40-150 GHz	SiC (490 W/m-K)	5G macro, radar
GaN-on-Si	28-48 V	3-5	40-100 GHz	Si (150 W/m-K)	5G small cell
GaAs pHEMT	5-12 V	1-1.5	60-150 GHz	GaAs (46 W/m-K)	Handset PA
LDMOS	28-32 V	1-2	5-20 GHz	Si (150 W/m-K)	Legacy BTS, ISM
SiGe HBT	1.5-3.3 V	0.1-0.3	200-350 GHz	Si (150 W/m-K)	mmWave, LNA

Common Questions

Frequently Asked Questions

Why is GaN superior for RF power?

Three advantages from wide bandgap (3.4 eV): high breakdown (3.3 MV/cm, 10x Si) enables 28-65 V operation for high power density (5-10 W/mm vs 1-1.5 GaAs). High v_sat (2.5e7 cm/s, 2x GaAs) enables high current and frequency. SiC substrate thermal conductivity (490 W/m-K) enables sustained high power. 100 W PA: 10-20 mm GaN vs 100 mm GaAs gate periphery.

GaN-on-SiC vs. GaN-on-Si?

SiC: premium, 490 W/m-K thermal, 5-10 W/mm, 5-10x wafer cost. Best for macro BTS (60-200 W), radar, EW. Si: 150 W/m-K, 3-5 W/mm, much cheaper wafers. Growing for 5G small cells (1-10 W). Diamond substrate (2000 W/m-K) is emerging research for >15 W/mm. Sapphire: poor thermal, mainly LEDs.

How is GaN used in 5G?

Macro BTS: 0.25 μm GaN-on-SiC Doherty PA, 60-200 W at 1.8-3.5 GHz, 28-50 V, 45-55% efficiency with DPD. Massive MIMO (64T64R): 0.15 μm GaN, 5-10 W per element, 64 elements = 320-640 W total. mmWave (28/39 GHz): GaN competes with SiGe/InP, offering 1-2 W/element vs 100-200 mW but at higher cost.

Related Terms

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