How do different RF FET technologies compare?

GaAs pHEMT (pseudomorphic high electron mobility transistor) uses an InGaAs channel with high electron mobility (8000 cm^2/V*s) on a GaAs substrate. It achieves the lowest noise figure (0.3 to 0.5 dB at 2 GHz) and high f_T (60 to 150 GHz), making it the dominant technology for low-noise amplifiers and millimeter-wave circuits. GaN HEMT uses an AlGaN/GaN heterostructure with high breakdown voltage (100 to 200 V) and high power density (5 to 10 W/mm), ideal for power amplifiers and radar transmitters. The wide bandgap (3.4 eV) enables operation at junction temperatures exceeding 200 degrees C. LDMOS (laterally diffused MOSFET) on silicon achieves high power (1 to 2 W/mm) at low cost, dominating the cellular base station PA market below 4 GHz. RF CMOS (65 nm to 5 nm nodes) enables fully integrated transceivers at low cost and high volume, though with higher noise figure and lower power compared to III-V technologies.

What is transconductance and why does it matter?

Transconductance (g_m) is the ratio of drain current change to gate voltage change: g_m = delta_I_D / delta_V_GS, measured in siemens (S) or millisiemens (mS). It quantifies how effectively the gate voltage controls the drain current, which directly determines the device's voltage gain capability: A_v = g_m times Z_load. Higher g_m means more gain per unit gate width, which translates to higher f_T (since f_T = g_m / (2*pi*C_gs)) and lower minimum noise figure. Typical g_m values: GaAs pHEMT achieves 400 to 600 mS/mm of gate width, GaN HEMT achieves 300 to 400 mS/mm, and LDMOS achieves 100 to 200 mS/mm. The g_m also determines the device's linearity: the variation of g_m with gate voltage (g_m compression) is the primary source of intermodulation distortion in Class A amplifiers. A device with flat g_m over a wide V_GS range produces lower IMD3 for the same output power.

Active Devices

Field-Effect Transistor

Q: What are f_T and f_max?

f_T (transition frequency or current-gain cutoff frequency) is the frequency at which the short-circuit current gain |h21| equals unity (0 dB). It is determined by the intrinsic device speed: f_T = g_m / (2*pi*C_gs) for an idealized FET, where g_m is the transconductance and C_gs is the gate-source capacitance. Shorter gate lengths increase f_T by reducing C_gs and increasing g_m (higher electron velocity). A 0.15 micrometer GaAs pHEMT has f_T of approximately 100 GHz. f_max (maximum oscillation frequency or power-gain cutoff frequency) is where the unilateral power gain equals unity. f_max depends on f_T but also on parasitic resistances: f_max = f_T / (2*sqrt(R_g*G_ds + R_g*g_m*(tau/tau_transit))), where R_g is gate resistance and G_ds is output conductance. A device with high f_T but high gate resistance will have reduced f_max. Generally, f_max greater than 2 times f_T is achievable with good layout optimization. Practical amplifier design requires operating well below f_max, typically below f_max/3.

/eff-ee-tee/ — FET

Controls channel current via gate electric field. RF types: GaAs pHEMT (NF 0.3 dB, f_T 100 GHz, LNA/mmWave), GaN HEMT (5-10 W/mm, 100-200 V breakdown, PA/radar), LDMOS (1-2 W/mm, BTS PA), CMOS (integrated transceivers). Key metrics: f_T = g_m/(2πC_gs), f_max (power-gain cutoff), g_m (transconductance, 300-600 mS/mm), V_breakdown.

f_T: 60-300 GHz

g_m: 100-600 mS/mm

Power: 0.5-10 W/mm

Understanding RF FETs

The field-effect transistor is the foundation of modern RF electronics. Unlike bipolar junction transistors (BJTs) that are controlled by base current, FETs are voltage-controlled devices with essentially infinite input impedance at DC. This high input impedance simplifies biasing and matching, and the majority-carrier-only operation eliminates minority carrier storage time, enabling very high-frequency operation.

The evolution of RF FET technology has followed a clear trajectory: from GaAs MESFETs in the 1970s to pHEMTs in the 1990s to GaN HEMTs in the 2010s. Each generation brought higher frequency capability, more power, better efficiency, or lower noise. Today, the choice of FET technology depends primarily on the application: GaAs pHEMT for lowest noise, GaN HEMT for highest power, LDMOS for lowest cost at moderate power, and CMOS for highest integration.

FET Performance Equations

Cutoff frequency:
f_T = g_m/(2πC_gs)
≈ v_sat/(2πL_g)
0.15 μm GaAs: f_T ≈ 100 GHz

Maximum oscillation frequency:
f_max = f_T/(2√(R_gG_ds))
f_max > 2×f_T with good layout
Design below f_max/3

Transconductance:
g_m = ΔI_D/ΔV_GS (S or mS/mm)
Voltage gain: A_v = g_m×Z_load
pHEMT: 400-600 mS/mm
GaN: 300-400 mS/mm

Power density:
P_max ≈ (V_break−V_knee)×I_max/8
GaN (120V): ~10 W/mm
LDMOS (28V): ~2 W/mm

RF FET Technology Comparison

Technology	f_T	NF_min	Power	V_DS	Application
GaAs pHEMT	60-150 GHz	0.3-0.5 dB	1-1.5 W/mm	3-5 V	LNA, mmWave
GaN HEMT	30-90 GHz	0.5-1.5 dB	5-10 W/mm	28-50 V	PA, radar, EW
Si LDMOS	5-15 GHz	1-2 dB	1-2 W/mm	28-50 V	BTS PA (<4 GHz)
SiGe HBT*	200-400 GHz	0.5-1 dB	0.1-0.3 W/mm	1.5-3 V	5G mmWave
RF CMOS	200-350 GHz	1-3 dB	0.05-0.2 W/mm	1-1.8 V	WiFi, BLE, IoT

Common Questions

Frequently Asked Questions

What are f_T and f_max?

f_T: frequency where |h21|=1 (0 dB). f_T = g_m/(2πC_gs). Shorter gate = higher f_T (less C_gs, faster electrons). 0.15 μm GaAs: f_T~100 GHz. f_max: where unilateral power gain = 1. Depends on f_T AND gate resistance. f_max = f_T/(2√(R_g×G_ds)). f_max > 2f_T possible with optimized layout. Design below f_max/3.

How do RF FET technologies compare?

GaAs pHEMT: InGaAs channel, mobility 8000 cm²/Vs, lowest NF (0.3 dB), dominates LNA/mmWave. GaN HEMT: wide bandgap (3.4 eV), V_break 100-200 V, 5-10 W/mm, radar/PA. LDMOS: Si, low cost, 1-2 W/mm, BTS PA <4 GHz. CMOS: fully integrated transceivers, highest volume, higher NF/lower power than III-V.

Why does g_m matter?

g_m = ΔI_D/ΔV_GS. Determines voltage gain (A_v = g_m×Z_L), f_T (g_m/2πC_gs), and NF_min. pHEMT: 400-600 mS/mm. GaN: 300-400 mS/mm. LDMOS: 100-200 mS/mm. g_m variation with V_GS causes IMD3 in Class A: flatter g_m = lower distortion for same output power.

Related Terms

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