Why is GaAs better than silicon for RF?

Five key advantages: (1) Higher electron mobility (8500 vs. 1500 cm2/V*s): faster transistors, higher fT and fmax. GaAs pHEMT: fT=120 GHz at 0.15um vs. Si CMOS fT=300 GHz at 16nm (but Si requires extreme scaling). (2) Semi-insulating substrate: 10^7 ohm-cm vs. 10 ohm-cm for Si. No substrate coupling, lower parasitic capacitance, better isolation between components. Enables high-Q passive elements. (3) Direct bandgap: efficient light emission (used in LEDs/lasers) and lower 1/f noise than Si. (4) Higher breakdown voltage than Si for same gate length: GaAs pHEMT at 0.25um: BV=12V vs. Si CMOS at 65nm: BV=1.2V. (5) Lower noise figure: GaAs pHEMT NFmin=0.3 dB at 10 GHz vs. Si CMOS NFmin=1.5 dB.

What is the difference between pHEMT and HBT?

pHEMT (pseudomorphic High Electron Mobility Transistor): field-effect device using an InGaAs channel on GaAs substrate. Advantages: lowest noise (NFmin=0.3 dB at 10 GHz), high frequency (fT=120 GHz at 0.15um), excellent switch performance (low Ron*Coff). Used for: LNAs, switches, attenuators, mixers. Foundries: WIN, TriQuint/Qorvo, OMMIC, UMS. HBT (Heterojunction Bipolar Transistor): vertical bipolar device with GaAs/AlGaAs or InGaP/GaAs heterostructure. Advantages: higher power density than pHEMT, better linearity (more consistent gm vs. Vbe), excellent 1/f noise (good for VCOs). Used for: smartphone PAs (dominant market), VCOs, gain blocks, driver amplifiers. Foundries: Skyworks, Qorvo, WIN.

Is GaAs being replaced?

GaAs is being challenged on three fronts: (1) GaN replaces GaAs for high-power applications. GaN has 5-10x higher power density and higher breakdown voltage. For base station PAs, radar, and defense, GaN is now preferred. (2) SiGe BiCMOS replaces GaAs for some receiver applications. SiGe offers comparable noise figure at lower cost with CMOS digital integration. Automotive radar (77 GHz) has moved largely to SiGe. (3) RF SOI CMOS replaces GaAs switches. 65nm RF SOI provides excellent switch performance at lower cost for antenna tuners and T/R switches. However, GaAs remains dominant for: smartphone PAs (HBT), low-noise amplifiers below 40 GHz, satellite receivers, and many MMIC applications. The GaAs market is still $10B+/year.

Semiconductor Materials

Gallium Arsenide (GaAs)

/gal-ee-um ar-sen-ide/

GaAs: III-V semiconductor, electron mobility 8500 cm²/V·s (5x Si). Direct bandgap 1.42 eV. Semi-insulating substrate (10⁷ Ω·cm) eliminates substrate loss. pHEMT: 0.15 μm gate, f_T=120 GHz, NF_min=0.3 dB (LNA/switch). HBT: PA, VCO, smartphone market. Being supplemented by GaN (power) and SiGe (integration) but still $10B+ market.

Mobility: 8500 cm²/V·s

Bandgap: 1.42 eV

f_T: 120 GHz

Understanding GaAs

GaAs has been the foundation of RF semiconductor technology for over 40 years. Its combination of high electron mobility, semi-insulating substrate, and direct bandgap makes it ideally suited for RF amplifiers, switches, and integrated circuits. Every smartphone contains multiple GaAs HBT power amplifier dies, making GaAs one of the highest-volume compound semiconductor technologies. The material's advantages over silicon become more pronounced at higher frequencies, where mobility determines transistor speed.

GaAs Material Properties

GaAs fundamental properties:
μ_e = 8500 cm²/V·s (6x Si)
v_sat = 1.0×10⁷ cm/s
E_g = 1.42 eV (direct bandgap)

Semi-insulating substrate:
ρ > 10⁷ Ω·cm (natural isolation)

RF Semiconductor Technology Comparison

Technology	Mobility	NF_min	Power	Cost	Primary Use
GaAs pHEMT	8500	0.3 dB	0.8 W/mm	Moderate	LNA, switch
GaAs HBT	N/A (bipolar)	2-4 dB	2 W/mm	Moderate	Phone PA, VCO
GaN HEMT	2000	0.5-2 dB	5-10 W/mm	High	Base station PA
SiGe BiCMOS	2500	0.8-2 dB	0.3 W/mm	Low	Radar, 5G mmW
RF SOI CMOS	500	1.5-3 dB	0.1 W/mm	Lowest	Switch, tuner

Key Equations

Noise Figure cascade (Friis):
NF_total = NF₁ + (NF₂−1)/G₁ + (NF₃−1)/(G₁G₂)

Gain (dB):
G = 10log(P_out/P_in) = 20log(V_out/V_in)

IP3 & dynamic range:
SFDR = 2/3(IIP3 − NF − 10log(kTB)) dB

Comparison

Property	GaAs	Si	InP	GaN
Mobility	8500	1400	5400	1500 cm²/Vs
Bandgap	1.42	1.12	1.34	3.4 eV
Substrate ρ	10⁷	10³	10⁷	10⁹ Ωcm
f_T	80–150	30–80	300–700	30–90 GHz
Cost/wafer	$$	$	$$$$	$$$

Common Questions

Frequently Asked Questions

Why better than Si?

5x mobility = faster transistors. Semi-insulating substrate = no substrate loss. Direct bandgap = lower 1/f noise. Higher BV per gate length: 12V at 0.25um vs Si 1.2V at 65nm. NFmin 0.3 dB vs Si 1.5 dB at 10 GHz.

pHEMT vs HBT?

pHEMT: field-effect, lowest noise, best switch (low Ron*Coff), LNA/switch/attenuator. HBT: bipolar, higher power, better linearity, excellent 1/f (VCOs). HBT dominates smartphone PA ($5B+ market).

Being replaced?

GaN takes high-power (base station PA, radar). SiGe takes some receiver (auto radar). RF SOI takes switches. But GaAs still dominant for phone PAs, LNAs <40 GHz, satellite Rx. Market still $10B+/year.

Related Terms

← Gain Gallium Nitride →