Understanding the Gunn Diode: Negative Resistance Oscillator

A Gunn diode is a semiconductor device distinguished by its “negative differential resistance.” This means that as the voltage across the diode increases, the current through it decreases – the opposite of what you’d typically expect in most electronic components. It’s also known as a transferred electron device (TED).

Gunn diodes are used to generate microwave signals in the frequency range of a few GHz up to terahertz. This makes them valuable in high-frequency electronic circuits.

Key Features of Gunn Diodes

Unlike standard diodes which have both N and P-doped regions, a Gunn diode is made only of N-type semiconductor material.

Gunn diode symbols

As the image shows, a typical Gunn diode has three layers: two heavily doped (N+) and one lightly doped (N-). When voltage is applied, a large electrical gradient develops across the middle (N-) layer. However, at high potential, the conductive properties of this middle layer can change, reducing the gradient across the device. This eventually prevents conduction, causing the current to decrease.

Here are some key points to remember about Gunn diodes:

• Negative Differential Resistance: Gunn diodes operate in a negative differential resistance region, commonly found in semiconductor materials like GaAs (Gallium Arsenide).
• Gunn Effect: When biased with DC voltage, they can spontaneously start oscillating at microwave frequencies due to this characteristic. This is known as the “Gunn Effect.”
• Microwave Signal Generation: They can generate continuous wave (CW) or pulsed microwave signals from a few GHz to terahertz, making them useful as microwave signal sources in radar, communication systems, and electronic warfare systems.
• Two-Terminal Device: They are two-terminal devices, with one terminal acting as the anode and the other as the cathode.
• Limitations: Gunn diodes generally produce relatively low output power and have a narrower tuning range compared to other microwave signal sources (like klystrons and magnetrons).

Gunn Diode Oscillator Circuit

Gunn diodes leverage their negative differential resistance to generate microwave oscillations. The circuit provides the necessary bias and feedback to sustain oscillations at the desired frequency.

Gunn diode oscillator

The image above shows the layers of a Gunn diode, its transfer characteristics, and a circuit diagram of a Gunn diode oscillator.

The Gunn diode diagram illustrates the negative resistance region. This negative resistance, combined with lead inductance and stray capacitance, leads to oscillations. Without proper protection, this can lead to relaxation-type oscillations with large amplitudes that can damage the diode. A large capacitor is often connected across the Gunn diode to prevent this failure. This characteristic allows oscillators to be built for higher frequencies in the GHz and THz bands. A resonator can be added to precisely control the oscillation frequency.

Conclusion

Tuning a Gunn diode oscillator circuit can be complex due to the sensitivity of various circuit parameters such as the diode’s bias voltage, feedback path, and resonator dimensions. Advanced designs often incorporate extra components for modulation, frequency stabilization, and power control.

Modern microwave systems increasingly use FETs (Field-Effect Transistors) and VCOs (Voltage-Controlled Oscillators) due to their improved performance and reliability compared to Gunn diode oscillators. While Gunn diodes have played an important role, newer technologies are often preferred for demanding applications.