Gunn Diode working principles – Definition, Characteristics and Applications

Gunn Diode (e.g. 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.

Structure of gunn diode

Despite the fact that the Gunn diode is called a “diode”, it doesn’t have a p-n junction in its structure of gunn diode, 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.

Working principle of gunn diode

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 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.

Voltage-current characteristics

Characteristic of the Gunn diode is shown below (Fig. 6.).

Modes of operation of gunn diode

Copeland Suggested four basic modes of operation in doped majority diodes with low resistance connections, specifically,

• Gunn oscillation mode,
• Limited space charge accumulation mode,
• Stable amplification mode,
• Bias circuit oscillation mode.

Gunn oscillation mode

This mode is operated Together with the Area over the threshold Value, i.e. E Eth.

• The large field domain drifts across the hive until it Reaches anode or very low fieldvalue drops to under the sustaining field worth, i.e. E < Esus
• The frequency of oscillation is given by f= vdom/Leff, Where vdomis that the velocityofthe domain name and Leff is the effective length the domain name travels prior to a fresh domaingets nucleated.

The gunn oscillation mode has three sub-modes namely:

• Transit time domain mode,
• Delayed domain mod,
• Quenched domain mode.

Limited Space charge Accumulation LSA

• The coefficient of All doping divided by frequency (nlf) Ought to Maintain between 2 x 10^4 and 2 x 10^5.
• Here is the easiest Mode of functioning and it is composed of uniformly doped semiconductor with no inner space charge.
• Since the frequency is large that the domains don’t get enough time to form.
• Many of those domains find themselves at the negative Conduction state throughout a huge fraction of voltage cycle.
• A sizable portion of the System exhibits a uniform area Resulting in efficient power generation in the circuit controlled frequency.
• This mode is suitable to generate short pulses of large Peak power.LSA mode of operation may generated a number of watts of power with minimal efficiencies of 20%. The power output decreases by frequency, viz 1 W at 10 GHz and many mW at 100 GHz.
• its maximum working frequency Is a Lot lower than that Of the TT devices.
• Its limits are conducive to load requirements, Temperature and doping changes.

Stable amplification

In this mode the devices Display stable amplification in The transit period frequency. Negative conductance is used to avoid the creation of the domains. There’s three areas of amplification based on the product fL vary from 10^7 to 0.5 x 10^8.

Bias circuit oscillation

This mode occurs when there is either Gunn or Even LSA Oscillation and fL is smallor not about 10-12/cm2. The system displays amplification in the transit period frequency instead of spontaneous oscillation. This situation happens because the negative conductance is used without domain formation. Hence amplification of signals close to the transit time frequency can be achieved. After the diode is biased to threshold Gunn oscillation start resulting in abrupt reduction in the ordinary current of the circuit driving it to oscillations.

Gunn Diode Oscillator

Gunn diode oscillators or moved electron device oscillators would be the source of microwave power and also include Gunn’s diode or moved electron device (TED) as the primary component. In such devices, the Gunn diode is going to be found at the resonant cavity and is composed of 2 main elements i.e. DC bias along with tuning circuit.

DC offset

In the event of the Gunn diode, the DC bias grows, the current begins to increase within a first phase which lasts up into the voltage threshold. The current then proceeds to fall as the voltage rises until the breakdown voltage is attained. This area, which extends from the peak to the valley point, is known as the negative resistance region.

This property of the Gunn diode, together with its time properties, make it act like an oscillator supplied the best current flows through it. That is due to the negative resistance of the system nullifies the result of any real resistance present in the circuit. This causes the generation of continuous oscillations till a DC load is current, while preventing the oscillations away from growing. In addition, the amplitude of the resulting oscillations will likely be restricted by the bounds of this negative resistance area.

For gunn diode oscillator, the oscillation frequency Depends mostly on the centre active layer of this gunn diode. On the other hand, the resonant frequency could be tuned externally mechanically or by electric means. In the instance of an electronic tuning circuit, then the controller can be accomplished by means of a waveguide or even a microwave cavity or some varactor diode or even a YIG sphere. This diode is mounted in the cavity in this manner it disrupts the reduction resistance of the resonator, resulting in oscillations. On the other hand, in the event of mechanical tuning, the dimensions of the cavity or magnetic field (such as YIG spheres) is altered mechanically by way of, sayan adjustment , so as to tune the resonant frequency.

These Kinds of oscillators have been Utilized to generate microwave Frequencies from 10 GHz to many THz, as stated by the dimensions of the resonance chamber. Normally, coaxial and microstrip/planar established oscillators have low power factor and therefore are less temperature stable. On the flip side, dielectrically stabilized waveguide and resonator systems have greater power factor and may be readily thermally stable.

Gunn diode oscillator, can be utilized to generate frequencies Ranging from 5 to 65 GHz. Here because the used voltage Vb is diverse, the fluctuations Induced from the Gunn nozzle traveling along the cavity to receive a reflection from its Other finish and return to this first point after some time t determined by:

Where, l is the length of the cavity and c is the speed of light. From this, the equation for the resonance frequency of the Gunn Oscillator can be deduced as:

Where, n is the range of half-waves which could fit in the cavity for any particular frequency. This n ranges from 1 to 2 l/ctre in which tre is time for its gunna diode to respond to changes in the voltage.

This oscillations are initiated when the resonator load Will Be Slightly greater compared to the maximum negative immunity of this device. The oscillations then create amplitude intermia till the typical negative resistance of this gunn diode gets equivalent to the resonator resistance, and then continued oscillations can be obtained. Additionally, such relaxation oscillators have a huge capacitor connected via the gunn diode to prevent burning the device because of large amplitude signals.

Ultimately, it ought to be noticed that Gunn diode oscillators are commonly utilized as radio transmitters and receiversand rate detection sensors, parametric amplifiers, radar sources, motion monitoring detectors, motion sensors, remote vibration detectors, speed tachometers humidity screens and microwave transmitter-receivers (Gunnplexers), and also for automatic door openers, burglar alarmsand radar police, wireless LANsand collision avoidance methods, anti-lock brakes, pedestrian security systems, etc.

Advantages and disadvantages of gunn diode

Advantages of Gunn Diode:

• Higher basic frequency performance,
• Greater efficiency,
• Enhanced temperature stability,
• Enhanced turn-on characteristic,
• Reduced FM sideband noise.

Gunn diode disadvantages:

• Low Efficacy Under 10 GHz,
• Inadequate stability — Frequency changes with bias and temperature,
• FM noise high for Some applications,
• Small tuning range.

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.

Source