Microwave diode (e.g. Gunn diode or TED – Transferred-Electron Device) – semiconductor or vacuum form of diode designed to operate in the range of microwave frequencies (from single GHz to single THz). In 1963 John Battiscombe Gunn (J.B. Gunn) as a first person has observed that in the wafers of gallium arsenide with a very small thickness, after supplying them with a sufficiently large voltage, very high oscillation frequencies were generated. They are usually made of gallium arsenide (GaAs) and their maximum operating frequency is about 200 GHz. However, Gunn diodes made from gallium nitride (GaN) materials can reach up to 3 THz.
Despite the fact that the Gunn diode is called a “diode”, it doesn’t have a p-n junction, so the structure of it is different than in normal semiconductor diode. That makes this diode unable to conduct in only one direction and work as a rectifying diode. Instead, it consists of three areas: two highly n-doped areas and a thin area between them with low dopant concentration.
When we join resonator to a diode, we can obtain sinusoidal voltage. Just to let you know, in case of this article author will use “Gunn diode” name most of the time.
On a daily basis Gunn diodes are used in high-frequency electronics as a source of great output power and high frequency.
For several years progress has been made on indium phosphide (InP) diodes, however their operation principles weren’t fully investigated yet. They are mostly used in generational, mixing and detection systems. Semiconductor microwave diodes are manufactured in special environment (from lead because of the sensitivity to electromagnetic pulses) with very low inductance and capacitance that enable placing them in the microwave circuit. One of the most popular devices where this component can be found is gunn diode oscillator that are used to generate microwaves or control frequency, also in microwave technology applications like relays, radars or automatic door openers.
Gunn diode oscillator operating principle – after biasing the diode into it’s negative resistance region (as shown on the characteristic below) with DC voltage it will produce self-generated oscillations. Value of the frequency of this phenomena depends mostly on the type of the middle, thin diode area mentioned above. However, this parameter can be further adjusted by other, external factors. Gunn diodes are used to build oscillators in the frequency range of GHz – THz.
Microwave diodes are usually used as a substitute for germanium diodes when low threshold voltage VT is required (approx. 0.3-0.4 V). Gunn diodes have very short switching times due to their construction and operating principles (see: semiconductor diode). They are used in technology as e.g. detectors, radar speed guns, relays or microwave trackers. Characteristic of Gunn diode is shown below.
Operating principles of the Gunn diode:
Gunn diode’s operating principle is based on Gunn effect. In some materials (such as GaAs and InP), after reaching a threshold level by an electric field in the material, the electrons mobility decreases simultaneously with increasing electric field in effect producing negative resistance. When the electric field intensity of gallium arsenide crystal reaches its critical value at the negative “electrode”, an area with low electron mobility is created (domain of a strong electric field). Area moves with the average speed of electrons towards the positive “electrode”. When area contacts with the positive “electrode” at the negative electrode, cyclic formation of the area of low electron mobility and high electric field starts to re-create. Due to cyclical phenomenon vibrations are generated, which frequency can reach up to 100 GHz. After passing that frequency, the vibration efficiency is abruptly corrupted.
The example of typical Gunn diode datasheet is presented in gunn diode pdf file. http://911electronic.com/wp-content/uploads/2015/07/gunn.pdf
Interesting presentation worth checking out as file gunn diode oscillator ppt: http://www.slideserve.com/thadine/the-gunn-diode
Gunn Diode application – Microwave energy detector project
This project allows us to build a wireless energy collection system that captures the radiation from the microwave oven (2.5 GHz frequency) and then transforms it into electricity that will supply the red LED. That system also allows to capture radiation with other wavelengths such as AM / FM waves from the radio, telephony and other signals. The red LED lights up on the assumption that the system collects 1-10mW of energy. In case of obtaining waves from the microwave oven, we will be able to detect from which side of the casing is the radiation coming from.
List of components needed to construct the detector:
1x RFD102 module (can be replaced with Gunn diode),
1x APT1608EC Kingbright diode,
Antenna made of two wires 28.6 mm long each.
Step 1: Mounting the system
Antenna can be built from power resistor’s leads (they have the right length). Thanks to these we can construct a dipole antenna of 2.5GHz frequency.
Installation of the system starts with applying the solder paste to the RFD102 module’s 1,4,5 and 8 pins. We solder antenna is to pins 4 and 5, which is the input of the RF module. We should solder at the lowest possible temperature. The red LED’s anode should be soldered to the pin 1 and cathode to pin 8 at the voltage output of the module. This is all we need to do in step one.
Step 2: Device testing by using microwaves
During the test, the cup filled with water will be used. Put it inside microwave oven and switch it on for 2 minutes. By moving the module around the microwave oven casing we are trying to detect where the is the strongest field value. When the right spot is found, module should be glued to the casing to prevent it from moving. During the microwave oven operating cycle we can observe changes in the intensity of diode’s lighting depending on the position of the mug inside the oven. The attached photos show the operation of the system.
If you want to detect signals with other frequencies, just experiment with the length of the antenna. At the current length you can also detect signals from WiFi devices that operate on the 2.5GHz band.
A few tips from the project’s author, which will solve some doubts:
Q: What is the maximum current that can be supplied by the system?
About 0.5 – 5 mA, but the maximum achieved was 18 mA.
Q: What is the maximum output voltage?
37 V, at 0.5 W RF module and 915 MHz frequency.
Q: Is it possible to charge the cellphone using this chip?
If you use 4x RFD1-2A modules with antennas then you can reach the needed current. The problem is with the way of delivering such energy from the microwave field. A device that would achieve the right values would not be healthy for people.