Tuesday, 8 January 2013

Gunn Diode or Transferred Electron Device


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Gunn Diode or Transferred Electron Device





This Gunn diode tutorial includes:
Gunn diodes are also known as transferred electron devices, TED, are widely used in microwave RF applications for frequencies between 1 and 100 GHz.
The Gunn diode is most commonly used for generating microwave RF signals - these circuits may also be called a transferred electron oscillator or TEO. The Gunn diode may also be used for an amplifier in what may be known as a transferred electron amplifier or TEA.
As Gunn diodes are easy to use, they form a relatively low cost method for generating microwave RF signals.

Gunn diode basics

The Gunn diode is a unique component - even though it is called a diode, it does not contain a PN diode junction. The Gunn diode or transferred electron device can be termed a diode because it does have two electrodes. It depends upon the bulk material properties rather than that of a PN junction. The Gunn diode operation depends on the fact that it has a voltage controlled negative resistance.
The mechanism behind the transferred electron effect was first published by Ridley and Watkins in a paper in 1961. Further work was published by Hilsum in 1962, and then in 1963 John Battiscombe (J. B.) Gunn independently observed the first transferred electron oscillation using Gallium Arsenide, GaAs semiconductor.

Gunn diode symbol for circuit diagrams

The Gunn diode symbol used in circuit diagrams varies. Often a standard diode is seen in the diagram, however this form of Gunn diode symbol does not indicate the fact that the Gunn diode is not a PN junction. Instead another symbol showing two filled in triangles with points touching is used as shown below.

Gunn diode symbol for circuit diagrams
Gunn diode symbol for circuit diagrams

Gunn diode construction

Gunn diodes are fabricated from a single piece of n-type semiconductor. The most common materials are gallium Arsenide, GaAs and Indium Phosphide, InP. However other materials including Ge, CdTe, InAs, InSb, ZnSe and others have been used. The device is simply an n-type bar with n+ contacts. It is necessary to use n-type material because the transferred electron effect is only applicable to electrons and not holes found in a p-type material.
Within the device there are three main areas, which can be roughly termed the top, middle and bottom areas.
Gunn diode construction
A discrete Gunn diode with the active layer mounted
onto a heatsink for efficient heat transfer
The most common method of manufacturing a Gunn diode is to grow and epitaxial layer on a degenerate n+ substrate. The active region is between a few microns and a few hundred micron thick. This active layer has a doping level between 1014cm-3 and 1016cm-3 - this is considerably less than that used for the top and bottom areas of the device. The thickness will vary according to the frequency required.
The top n+ layer can be deposited epitaxially or doped using ion implantation. Both top and bottom areas of the device are heavily doped to give n+ material. This provides the required high conductivity areas that are needed for the connections to the device.
Devices are normally mounted on a conducting base to which a wire connection is made. The base also acts as a heat sink which is critical for the removal of heat. The connection to the other terminal of the diode is made via a gold connection deposited onto the top surface. Gold is required because of its relative stability and high conductivity.
During manufacture there are a number of mandatory requirements for the devices to be successful - the material must be defect free and it must also have a very uniform level of doping.


Gunn diode operation basics

The operation of the Gunn diode can be explained in basic terms. When a voltage is placed across the device, most of the voltage appears across the inner active region. As this is particularly thin this means that the voltage gradient that exists in this region is exceedingly high.
The device exhibits a negative resistance region on its V/I curve as seen below. This negative resistance area enables the Gunn diode to amplify signals. This can be used both in amplifiers and oscillators. However Gunn diode oscillators are the most commonly found.
Gunn diode characteristic
Gunn diode characteristic
This negative resistance region means that the current flow in diode increases in the negative resistance region when the voltage falls - the inverse of the normal effect in any other positive resistance element. This phase reversal enables the Gunn diode to act as an amplifier and oscillator.

Gunn diode operation at microwave frequencies

At microwave frequencies, it is found that the dynamic action of the diode incorporates elements resulting from the thickness of the active region. When the voltage across the active region reaches a certain point a current is initiated and travels across the active region. During the time when the current pulse is moving across the active region the potential gradient falls preventing any further pulses from forming. Only when the pulse has reached the far side of the active region will the potential gradient rise, allowing the next pulse to be created.
It can be seen that the time taken for the current pulse to traverse the active region largely determines the rate at which current pulses are generated, and hence it determines the frequency of operation.
To see how this occurs, it is necessary to look at the electron concentration across the active region. Under normal conditions the concentration of free electrons would be the same regardless of the distance across the active diode region. However a small perturbation may occur resulting from noise from the current flow, or even external noise - this form of noise will always be present and acts as the seed for the oscillation. This grows as it passes across the active region of the Gunn diode.
Gunn diode operation
Gunn diode operation
The increase in free electrons in one area cause the free electrons in another area to decrease forming a form of wave. It also results in a higher field for the electrons in this region. This higher field slows down these electrons relative to the remainder. As a result the region of excess electrons will grow because the electrons in the trailing path arrive with a higher velocity. Similarly the area depleted of electrons will also grow because the electrons slightly ahead of the area with excess electrons can move faster. In this way, more electrons enter the region of excess making it larger, and more electrons leave the depleted region because they too can move faster. In this way the perturbation increases.
Gunn diode operation
Gunn diode operation - electrons in the peak move more slowly
The peak will traverse across the diode under the action of the potential across the diode, and growing as it traverses the diode as a result of the negative resistance.
A clue to the reason for this unusual action can be seen if the voltage and current curves are plotted for a normal diode and a Gunn diode. For a normal diode the current increases with voltage, although the relationship is not linear. On the other hand the current for a Gunn diode starts to increase, and once a certain voltage has been reached, it starts to fall before rising again. The region where it falls is known as a negative resistance region, and this is the reason why it oscillates.

A Gunn diode oscillator or transferred electron device oscillator generally consists of a diode with a DC bias applied and a tuned circuit.
The Gunn diode oscillator circuit or transferred electron oscillator uses the negative resistance over a portion of the V/I curve of the Gunn diode, combined with the timing properties within the device to allow the construction of an RF relaxation oscillator. When a suitable current is passed through the device it will start to oscillator.
The negative resistance created by the V/I characteristic will cancel out any real resistance in the circuit so that any oscillation will build up and will be maintained indefinitely while DC is applied. The amplitude will be limited by the limits of the negative resistance region of the Gunn diode.
Gunn diode characteristic
Gunn diode characteristic

Gunn diode tuning

The frequency of the signal generated by a Gunn diode is chiefly set by the thickness of the active region. However it is possible to alter it somewhat. Often Gunn diodes are mounted in a waveguide and the whole assembly forms a resonant circuit. As a result there are a number of ways in which the resonant frequency of the assembly can be altered. Mechanical adjustments can be made by placing an adjusting screw into the waveguide cavity and these are used to give a crude measure of tuning.
However some form of electrical tuning is normally required as well. It is possible to couple a varactor diode into the Gunn oscillator circuit, but changing the voltage on the varactor, and hence its capacitance, the frequency of the Gunn assembly can be trimmed.
A more effective tuning scheme can be implemented using what is termed a YIG. It gains its name from the fact that it contains a ferromagnetic material called Yttrium Iron Garnet. The Gunn diode is placed into the cavity along with the YIG which has the effect of reducing the effective size of the cavity. This is achieved by placing a coil outside the waveguide. When a current is passed through the coil it has the effect of increasing the magnetic volume of the YIG and hence reducing the electrical size of the cavity. In turn this increases the frequency of operation. This form of tuning, although more expensive, produces much lower levels of phase noise, and the frequency can be varied by a much greater degree.

Surface Mount Technology (SMT)

In recent years there has been a drammatic change from the use of leaded components to surface mount technology. These SMT components make the manufacturing process much easier and faster.

Passive components


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1 comment:

  1. Microwave diode (e.g. gunn diode ) – semiconductor or vacuum diode designed to operate in the range of microwave frequencies (very high frequencies). It is usually made of gallium arsenide GaAs (maximum operating frequency of 200 GHz). For several years work is being made on indium phosphide InP diodes, however their operation principles weren’t fully investigated yet. It is mostly met in generational, mixing and detection systems. Semiconductor microwave diodes are manufactured in special construction (from lead because of the sensitivity to electromagnetic pulses) with very low inductance and capacitance that enable placing them in the microwave circuit.

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