Semiconductor and Device Technology III-V Semiconductors Informational

How does the gate length of a transistor affect its noise figure and gain at millimeter wave frequencies?

Q: What gate length do I need for 28 GHz?

For 28 GHz (5G mmWave): LNA: 100-150 nm gate length provides adequate gain (10-15 dB) and good NF (0.7-1.5 dB). 100 nm is preferred for best NF. PA: 100-150 nm provides 8-14 dB gain per stage at 28 GHz. Two stages = 16-28 dB total. 150 nm is a common choice for PA (balance of gain, breakdown voltage, and cost). For GaN PA: 150 nm provides Vbr > 60 V and fmax > 100 GHz (adequate for 28 GHz PA with good efficiency).

The gate length (Lg) of an RF transistor is the single most important geometric parameter determining its noise figure (NF) and gain at millimeter-wave frequencies: (1) Gain: the maximum available gain (MAG) at a given frequency is related to fmax: MAG(f) ≈ (fmax/f)² (in linear, for f < fmax). fmax scales approximately as 1/Lg: shorter gates = higher fmax = more gain at mmWave. For a GaAs pHEMT: Lg = 250 nm: fmax ≈ 150 GHz, MAG at 60 GHz ≈ (150/60)² = 6.25 (8 dB). Lg = 100 nm: fmax ≈ 300 GHz, MAG at 60 GHz ≈ (300/60)² = 25 (14 dB). Lg = 50 nm (InP HEMT): fmax ≈ 600 GHz, MAG at 60 GHz ≈ (600/60)² = 100 (20 dB). Each 2× reduction in gate length approximately doubles fmax and increases the MAG by approximately 6 dB at a given frequency. (2) Noise figure: the minimum noise figure scales with the gate length: NF_min ≈ 1 + 2×pi×f × sqrt(2×Cgs/(gm × fT)), where Cgs = gate-source capacitance and gm = transconductance. Both Cgs and gm scale with gate width, so NF_min depends primarily on f/fT: NF_min ≈ 1 + k × f/fT (simplified). Since fT ∝ 1/Lg: NF_min improves (decreases) as Lg decreases. At 28 GHz: Lg = 250 nm GaAs (fT ≈ 70 GHz): NF_min ≈ 1.5 dB. Lg = 100 nm GaAs (fT ≈ 150 GHz): NF_min ≈ 0.7 dB. Lg = 100 nm InP (fT ≈ 300 GHz): NF_min ≈ 0.4 dB. At 77 GHz: Lg = 100 nm GaAs: NF_min ≈ 2.0 dB. Lg = 50 nm InP: NF_min ≈ 0.8 dB. (3) Practical limits: gate length cannot be reduced indefinitely. Below approximately 30-50 nm: short-channel effects degrade the transistor performance (reduced gain, increased leakage, reduced breakdown voltage). The gate resistance (R_g ∝ 1/Lg for a given cross-section) increases, which limits fmax and increases NF. T-gate or mushroom-gate structures are used to reduce R_g while maintaining a short footprint.

Category: Semiconductor and Device Technology

Updated: April 2026

Product Tie-In: Transistors, MMICs, Evaluation Boards

Gate Length vs mmWave Performance

Gate length scaling has been the primary driver of mmWave transistor performance for decades, following a trajectory parallel to Moore law in digital CMOS.

Performance verification: confirm specifications against the application requirements before finalizing the design
Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture

Common Questions

Frequently Asked Questions

What gate length do I need for 28 GHz?

For 28 GHz (5G mmWave): LNA: 100-150 nm gate length provides adequate gain (10-15 dB) and good NF (0.7-1.5 dB). 100 nm is preferred for best NF. PA: 100-150 nm provides 8-14 dB gain per stage at 28 GHz. Two stages = 16-28 dB total. 150 nm is a common choice for PA (balance of gain, breakdown voltage, and cost). For GaN PA: 150 nm provides Vbr > 60 V and fmax > 100 GHz (adequate for 28 GHz PA with good efficiency).

Does gate length matter for a switch?

For RF switches (SPDT, SPNT): the gate length affects the switch on-resistance (R_on) and off-capacitance (C_off). Shorter gates have lower R_on (the channel resistance is proportional to Lg). But: shorter gates have higher C_off (the gate capacitance per unit width is similar, and the off-state capacitance scales with the junction area). The figure of merit for a switch: FOM = R_on × C_off (lower is better). The FOM is relatively constant vs gate length (R_on and C_off trade off). Gate length affects the maximum frequency: f_max_switch ≈ 1/(2×pi×R_on×C_off). For GaAs pHEMT switches: 100-250 nm gates provide f_max > 40 GHz. For SOI CMOS switches: 45-130 nm nodes provide f_max > 30 GHz.

What is the state-of-the-art gate length for GaN?

For commercial GaN HEMT (available from foundries): 100 nm: Wolfspeed, Qorvo (0.1 um GaN HEMT process). fT ≈ 80-120 GHz. Suitable for 28-40 GHz PAs. 90 nm: WIN Semiconductors (NP15 process). fT ≈ 100-130 GHz. 70 nm: Qorvo (0.07 um GaN). fT ≈ 120-150 GHz. Suitable for V-band PAs. For research/military: 40-50 nm GaN HEMT: fT > 200 GHz, fmax > 400 GHz. Demonstrated by HRL, UCSB, and others. Goal: GaN performance competitive with InP at mmWave while retaining GaN power handling advantage. 20 nm GaN: fT > 300 GHz (research, not yet production). The trend: GaN gate lengths are scaling 10-15 years behind GaAs and InP, following a similar trajectory but with the added challenge of managing the higher operating voltages and electric fields.

Keep Reading

How does the gate length of a transistor affect its noise figure and gain at millimeter wave frequencies?

Gate Length vs mmWave Performance

Frequently Asked Questions

What gate length do I need for 28 GHz?

Does gate length matter for a switch?

What is the state-of-the-art gate length for GaN?

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