How does the Gunn effect work?

In GaAs, the conduction band has two valleys: the central gamma valley (high mobility, 8500 cm2/V*s) and satellite L-valleys (low mobility, ~300 cm2/V*s). At low electric fields, electrons occupy the gamma valley. Above the threshold field (~3.2 kV/cm for GaAs), electrons gain enough energy to scatter into the L-valley. Since velocity = mobility * field, and the L-valley has much lower mobility, the current actually decreases with increasing voltage (negative differential resistance). This NDR causes charge accumulation (domains) that travel through the device and dissipate at the anode, creating current pulses at a frequency determined by the device length: f = v_domain / L. For L = 10 um: f approximately 10 GHz.

GaAs vs InP Gunn diodes?

GaAs Gunn: operates to approximately 100 GHz. Power: 100 mW at 30 GHz, 10 mW at 94 GHz. Threshold field: 3.2 kV/cm. Most mature technology. InP Gunn: operates to approximately 200+ GHz. Higher power at millimeter-wave frequencies because InP has larger intervalley energy separation (0.53 eV vs 0.31 eV for GaAs) and higher peak-to-valley current ratio. Power: 300 mW at 30 GHz, 50 mW at 94 GHz, 10 mW at 200 GHz. More expensive. Both are being replaced by MMIC oscillators (transistor-based VCOs) for applications below 100 GHz, but Gunn diodes remain relevant above 100 GHz where transistor gain is limited.

What are the applications of Gunn diodes?

Automotive radar (being replaced by MMIC): 77 GHz CW source for FMCW radar. Gunn diodes powered the first generation of automotive radar sensors. Now replaced by SiGe MMIC transceivers. Speed measurement: police radar guns at 10.525 GHz (X-band), 24.15 GHz (K-band), and 34.7 GHz (Ka-band). Simple, low-cost CW oscillator. Intrusion detection: 10.525 GHz motion sensors for security and door openers. Low cost, simple circuit (Gunn diode + mixer diode + horn antenna). Laboratory sources: tunable mmWave sources to 200+ GHz for spectroscopy and material characterization. Scientific instruments: radio astronomy local oscillators at frequencies above 100 GHz where transistor oscillators have insufficient power.

Microwave Sources

Gunn Diode

/gun die-ode/

A Gunn diode oscillates via the transferred electron effect: above ~3.2 kV/cm in GaAs, electrons transfer to a low-mobility valley creating negative differential resistance. Frequency = v_domain/L. GaAs: to 100 GHz, 100 mW @ 30 GHz. InP: to 200+ GHz, 300 mW @ 30 GHz. Used in radar, speed guns, motion sensors. Being replaced by MMIC VCOs below 100 GHz.

GaAs: to 100 GHz

InP: to 200+ GHz

Power: 10 mW-1W

Understanding Gunn Diodes

The Gunn diode, discovered by J.B. Gunn at IBM in 1963, is one of the simplest microwave oscillators. A single piece of semiconductor with two contacts, placed in a resonant cavity and biased above a threshold voltage, produces CW microwave power. No transistor, no feedback network, no complex circuit design. This simplicity made Gunn diodes the technology of choice for low-cost microwave applications like speed radar and motion sensors for decades.

Gunn Effect Parameters

Gunn oscillation:
f₀ ≈ v_domain/L (transit-time mode)
v_domain ≈ 10⁷ cm/s (GaAs)

Threshold field:
E_th = 3.2 kV/cm (GaAs)
E_th = 4.0 kV/cm (InP)

Output power:
P ∝ V²/R ∝ (E×L)² (bias dependent)

Microwave Source Comparison

Source	Frequency	Power	Tuning	Phase Noise	Application
GaAs Gunn	10-100 GHz	10-200 mW	Varactor/mech	Moderate	Radar, sensors
InP Gunn	10-200 GHz	10-300 mW	Varactor/mech	Moderate	mmWave source
MMIC VCO	DC-150 GHz	1-100 mW	Electronic	Good	PLL, transceivers
DRO	3-30 GHz	1-50 mW	Limited	Excellent	Low PN source
YIG oscillator	2-20 GHz	10-100 mW	Octave+	Good	Test equipment

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

Material	f range	P_out (CW)	Efficiency	Application
GaAs	1–100 GHz	10–500 mW	1–5%	X/Ku LO
InP	50–300 GHz	5–100 mW	2–8%	mmW/THz source
GaN (theory)	>100 GHz	Higher	Predicted better	Research
2nd harmonic	100–600 GHz	0.1–10 mW	0.5–3%	THz source
Varactor-tuned	2–50 GHz	10–100 mW	1–4%	VCO/swept source

Common Questions

Frequently Asked Questions

Gunn effect?

Above 3.2 kV/cm, GaAs electrons transfer from high-mobility gamma valley to low-mobility L-valley. Current decreases with voltage = NDR. Domains form and travel through device creating oscillating current at f = v/L.

GaAs vs InP?

InP: higher frequency (200+ GHz vs 100), higher power at mmWave (3x), larger intervalley separation (0.53 vs 0.31 eV). GaAs: lower cost, more mature. Both being replaced by MMIC VCOs below 100 GHz.

Applications?

Police radar (10.5/24/34.7 GHz). Motion sensors (10.5 GHz door openers). First-gen auto radar (77 GHz, now MMIC). Lab mmWave sources >100 GHz. Simple, low-cost, two-terminal oscillator.

Related Terms

← Guard Band Harmonic →