Emerging RF Technology

CMOS THz

Q: How can CMOS generate THz signals when f_max is below 1 THz?

CMOS transistors in 65 to 28 nm nodes have f_max of 200 to 400 GHz, well below the THz range. Generation uses harmonic extraction: a fundamental oscillator runs at 100 to 200 GHz (within the transistor's gain bandwidth), and nonlinear device characteristics produce harmonic content at 2x, 3x, and 4x the fundamental frequency. A push-push oscillator architecture naturally cancels the fundamental and odd harmonics, extracting the second harmonic. A triple-push topology extracts the third harmonic. At 300 GHz (third harmonic of 100 GHz), a single CMOS source produces -25 to -35 dBm. Large-scale power combining using 16 to 64 coherently locked sources on a single chip achieves -5 to 0 dBm total radiated power, sufficient for imaging at 0.5 to 2 meters range. At 28 nm CMOS, sources up to 1 THz have been demonstrated with output power of -40 to -50 dBm.

Q: How do CMOS THz detectors work?

CMOS THz detectors exploit the plasma-wave rectification effect in FET channels, first predicted by Dyakonov and Shur in 1993. When THz radiation couples to the gate-source terminals, it excites plasma oscillations in the 2D electron gas of the FET channel. Due to the asymmetric boundary conditions (source grounded, drain open or loaded), these oscillations are rectified, producing a DC voltage proportional to the THz power. This mechanism works even when the signal frequency far exceeds f_max because it does not rely on conventional transistor gain. In 65 nm CMOS, single-pixel detectors achieve noise equivalent power (NEP) of 10 to 30 pW/sqrt(Hz) at 300 GHz and 30 to 100 pW/sqrt(Hz) at 1 THz, with responsivity of 100 to 500 V/W. Arrays of 32x32 to 1024 pixels have been demonstrated for real-time THz imaging at video frame rates (30 fps).

Q: What are the applications of CMOS THz technology?

Security screening uses THz imaging to detect concealed weapons and contraband through clothing and packaging without ionizing radiation, at standoff distances of 1 to 10 meters. Non-destructive testing inspects pharmaceutical tablet coating thickness (10 to 500 micrometer resolution), IC package defects, and composite material delamination. Short-range wireless communication at 300 GHz provides 100+ Gbps data rates for chip-to-chip links, kiosk download stations, and data center rack-to-rack interconnects using 50+ GHz of available bandwidth. Gas spectroscopy identifies molecular species through their rotational absorption lines in the 0.1 to 3 THz range, enabling breath analysis for medical diagnostics and environmental gas monitoring. CMOS integration makes these applications affordable: a THz imaging chip costs 5 to 50 USD in volume versus 10,000+ USD for III-V or Schottky diode alternatives.

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CMOS THz generates and detects terahertz signals (0.1 to 10 THz) using standard silicon CMOS. Harmonic generation from 100 to 200 GHz fundamental oscillators produces -20 to -40 dBm at 300 GHz to 1 THz per element. Detection uses plasma-wave rectification in FET channels, achieving NEP of 10 to 100 pW/√Hz. Enables low-cost imaging arrays (1,024+ pixels), 100+ Gbps short-range communication at 300 GHz, and spectroscopy-on-chip for pharmaceutical and security applications.

Category: Emerging RF Technology

Source power: -20 to -40 dBm/element

Detector NEP: 10 to 100 pW/√Hz

Understanding CMOS THz

The terahertz gap, the frequency range between 0.1 and 10 THz, has historically been difficult to access electronically because conventional transistors run out of gain (f_max limits) while optical devices lack efficiency at such long wavelengths. CMOS THz technology bridges this gap by exploiting two key insights. For generation, nonlinear harmonic extraction from fundamental oscillators operating near f_max produces useful power at 2x to 4x the fundamental, reaching frequencies well beyond the transistor's unity-gain frequency. For detection, plasma-wave physics in FET channels provides a rectification mechanism that works at frequencies far above f_max because it relies on collective electron oscillations rather than individual carrier transit.

The economic impact of CMOS THz is transformative. A THz imaging sensor fabricated in 65 nm CMOS costs $5 to $50 per chip in volume production, compared to $10,000 or more for equivalent InP Schottky diode or III-V based detectors. This cost reduction, enabled by existing CMOS foundry infrastructure, makes mass-market THz applications viable for the first time. A 32×32 pixel CMOS THz camera demonstrated at 300 GHz achieves video-rate imaging (30 fps) with sufficient sensitivity to detect concealed objects through clothing at 1 to 3 meters standoff distance. The integration density of CMOS allows on-chip signal processing (amplification, filtering, ADC) alongside the THz front-end, creating complete system-on-chip solutions that III-V technologies cannot match.

CMOS THz Equations

Harmonic Output Power:
P_n ≈ P_fund × (f_max / (n × f_fund))² (simplified rolloff)

Plasma-Wave Detector Responsivity:
R_v = ΔV_DC / P_THz (V/W) ; typical 100 to 500 V/W

Noise Equivalent Power:
NEP = V_noise / R_v (W/√Hz)

Where n = harmonic number, f_fund = fundamental frequency, P_fund = fundamental output power. In 65 nm CMOS: f_max ≈ 250 GHz, P_fund at 125 GHz ≈ 0 dBm, P₂ at 250 GHz ≈ -10 dBm, P₃ at 375 GHz ≈ -25 dBm.

CMOS THz Technology Comparison

Technology	Frequency Range	Source Power	Detector NEP	Cost (Volume)
65 nm CMOS	200 GHz to 1 THz	-20 to -40 dBm/elem	10 to 30 pW/√Hz	$5 to $50
28 nm CMOS	300 GHz to 1.5 THz	-15 to -35 dBm/elem	5 to 20 pW/√Hz	$10 to $80
SiGe BiCMOS	200 GHz to 800 GHz	-5 to -20 dBm	3 to 15 pW/√Hz	$50 to $200
InP Schottky diode	100 GHz to 3 THz	-10 to -30 dBm	1 to 10 pW/√Hz	$1,000 to $10,000
QCL (quantum cascade)	1 to 5 THz	+10 to +20 dBm	N/A (source only)	$5,000+

Common Questions

Frequently Asked Questions

How can CMOS generate THz signals when f_max is below 1 THz?

Harmonic extraction from fundamental oscillators at 100 to 200 GHz produces power at 2x to 4x. Push-push architecture extracts 2nd harmonic; triple-push extracts 3rd. A single source at 300 GHz produces -25 to -35 dBm. Coherent power combining of 16 to 64 sources achieves -5 to 0 dBm total radiated power, sufficient for imaging at 0.5 to 2 m range.

How do CMOS THz detectors work?

Plasma-wave rectification in FET channels (Dyakonov-Shur, 1993) works above f_max by exploiting 2D electron gas oscillations under asymmetric boundary conditions. In 65 nm CMOS: NEP of 10 to 30 pW/√Hz at 300 GHz, responsivity 100 to 500 V/W. Arrays of 32×32 to 1,024 pixels demonstrated for real-time THz imaging at 30 fps.

What are the applications of CMOS THz technology?

Security screening (concealed object detection at 1 to 10 m standoff), pharmaceutical inspection (coating thickness at 10 to 500 μm resolution), 100+ Gbps short-range wireless (300 GHz, 50+ GHz bandwidth), and gas spectroscopy (rotational lines at 0.1 to 3 THz). CMOS brings cost to $5 to $50 vs $10,000+ for III-V alternatives.

Related Terms

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