Microwave diode (e.g. Gunn diode or TED – Transferred-Electron Device) – type of the semiconductor or vacuum form of diode, which is 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) elements can reach up to 3 THz. On a daily basis, Gunn diodes are used in high-frequency electronics as a source of great output power and high frequency. After joining resonator to a diode, we can obtain sinusoidal voltage. Just to let you know, in case of this article the author will use “Gunn diode” name most of the time. 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 fast switching times due to their construction and operating principles. They are used in detecting technologies, radar speed guns, relays or microwave trackers.
Gunn Diode – Internal structure
Despite the fact that the Gunn diode is called a “diode”, it doesn’t have a p-n junction in its structure, so it is different than in normal semiconductor diode. That makes this diode unable to conduct in only one direction and work as a rectifier diode. Instead, Gunn Diode structure consists of three areas: two highly n-doped areas and a thin area between them with low concentration of dopants.
For several years progress has been made in development of Indium Phosphide (InP) diodes, however their principles of operation weren’t fully investigated yet. They are mostly used in generation, frequency-mixing and detection systems. Semiconductor microwave diodes are manufactured in the special environment (from lead, because it negates the influence of the very damaging 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 is applied is Gunn diode oscillator, that is used to generate microwaves or control frequency. In addition, it is also used in microwave technology applications like relays, radars or automatic door openers.
Gunn Diode – Principle of operation
Gunn diode’s principle of operation is based on the 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, while electric field increases 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, a cyclic formation of the area of low electron mobility and high electric field start to re-create. Due to cyclical phenomenon, oscillations are generated, which frequency can reach up to 100 GHz. After exceeding that frequency border, oscillations start to fade.
Gunn Diode – Gunn Effect
The Gunn Effect can occur only in selected types of semiconductors from groups as A3B5 and A2B6. Their main feature is the specific arrangement of energy bands. In the case of the Gunn diode, the Gunn effect will be explained in the form of GaAs material.
The energy in the semiconductor band model is deposited on the vertical axis, while the horizontal axis represents the geometric coordinate as position. The horizontal and main horizontal banding bands appear in the form of a homogeneous solid. This effect occurs without the occurrence of external polarization sources. The position of energy bands depends on the electron and its momentum relative to the location of crystallographic axes. The horizontal band takes the form of valleys, if on his axis is deposited momentum, not the standard x coordinate. The material from which Gunn diode is manufactured is GaAs. It has two main features that influence the determination of the Gunn effect:
- It is composed of two sub-bands, which are located in the conduction band, which occur in the form of two valleys shifted on the axis of the momentum. The process of passing the baseband to the sub-band, the bottom of which is 1.36 eV value from the baseband peak, occurs without the loss of the momentum of the electron. There is also a second passage called an oblique passage. During the transition, the momentum value is changed. The transition is the second sub-band with the bottom positioned 0.36 eV higher than the first sub-band.
- High activity and mobility of electrons in the second, higher sub-band are smaller than in the previous one sub-band. If the mobility of energy carriers into a semiconductor is greater, then also its conductivity increases.
The electrons carried with the velocity proportional to the field strength and mobility of the loads are caused by applying a homogeneous electric field. The high mobility of electrons occurs in the lower sub-bands. This occurs after applying a small field strength. As the value of the electric field increases, the electrons have the energy to get into the second sub-band. This causes an increase in the resistivity of the entire semiconductor because electrons are stopped under conditions of reduced mobility. GaAs material achieves the phenomenon of reduced resistivity after exceeding the electric field value of 3.5 x 105 V/m. The negative differential conductance occurring as a function of I(V) characteristic, in such conditions has the possibility of creating a semiconductor in the non-junction area.
Interesting presentation worth checking out as file Gunn diode oscillator ppt can be found here.
Gunn Diode – Voltage-current characteristics
Characteristic of the Gunn diode is shown below (Fig. 6.).
Gunn Diode Oscillator
Gunn diode oscillator – After biasing the diode with DC voltage into its negative resistance region (as shown on the Fig. 6.), 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 before. However, this parameter can be further adjusted by other, external factors. Gunn diodes are capable of being used in oscillator constructions in the frequency range of GHz – THz.
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 capturing 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,
- The antenna made of two wires 28.6 mm long each.
Step 1: Mounting the system
The 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 the 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, the 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 the 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 may answer some of your questions:
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.