The Gunn Diode Oscillator

A comprehensive interactive study guide on the transferred electron device (TED) that revolutionized microwave generation without junctions.

Solid State Device
Negative Resistance
Microwave Frequencies
01. Fundamentals

What is a Gunn Diode?

Despite its name, the Gunn Diode is actually a two-terminal semiconductor made of n-type Gallium Arsenide (GaAs) or Indium Phosphide (InP). It is unique because it has no p-n junction.

It operates based on the Gunn Effect, discovered by J.B. Gunn in 1963. When a voltage above a certain threshold is applied, the current oscillates at microwave frequencies (1-100 GHz).

Key Characteristics

  • • No p-n junction (Bulk effect device)
  • • Operates in the microwave region (GHz)
  • • Exhibits Negative Differential Resistance (NDR)
  • • Used in Radar, Microwave relays, and Automatic door openers

I-V Characteristic Curve

Voltage (V) Current (I)

Hover over the curve to see the Negative Resistance Region.

02. The Mechanism

Two-Valley Model & Electron Transfer

The operation relies on the band structure of GaAs. It has two conduction bands: a Central Valley (low mass, high mobility) and a Satellite Valley (high mass, low mobility).

Low (Ohmic) Threshold (Eth) High (NDR)

Status:

Electrons in Central Valley (High Mobility)
Average Mobility

Step 1: At low fields, electrons reside in the Central Valley.

Step 2: As field increases, electrons gain energy.

Step 3: Above threshold, electrons scatter to the Satellite Valley, slowing down.

Step 4: Average velocity drops, causing Negative Differential Resistance.

E-Field: 0.5 kV/cm

Formation of Gunn Domain

03. Instability

The Gunn Domain

A uniform electric field is unstable in NDR materials. Any small fluctuation in carrier density grows into a High Field Domain (Gunn Domain).

  • 1

    Accumulation

    Electrons move slower in the high-field region, accumulating at the back.

  • 2

    Depletion

    Electrons move away faster at the front, leaving a positive ion core (depletion).

  • 3

    Transit

    The dipole domain drifts towards the anode. When it exits, the field drops, and a new domain forms.

Virtual Lab: Gunn Oscillator Circuit

Tune the cavity and bias voltage to observe the oscillation frequency.

0V 12.0V 20V
1µm 5.0µm 10µm
Calculated Frequency
10.0 GHz
f = v / L
CH1: OUTPUT
Time (ns)

Modes of Operation

Transit Time Mode

f ≈ v_d / L

The frequency is determined by the time it takes for the domain to travel from cathode to anode.

  • Resonant circuit is non-resonant or low Q
  • Frequency depends on bias voltage (slightly)
  • Efficiency: ~3%

Quenched Domain Mode

f > v_d / L

The RF voltage swing is large enough to drop the total field below threshold before the domain reaches the anode.

  • Domain collapses before anode
  • Higher frequencies possible
  • Requires high Q resonant circuit

Limited Spacecharge (LSA)

f >> v_d / L

The domain never forms because the RF field oscillates too fast for the dipole to accumulate.

  • Highest efficiency (~20%)
  • Very high frequencies (100GHz)
  • Requires precise RF circuit tuning