CONTENTS
· HISTORY
· INTRODUCTION
· Applications OF GUNN DIODE
· Operation of the Gunn diode
· Gunn diode tuning
· Gunn diode oscillators
· Vi characteristic of gundiode
· Conclusion
· Bibliography
- certificate
GUNN DIODE
History
The Gunn diode is named for the physicist J.B. Gunn who, invented the gunn diode in the year1963, produced the first device based upon the theoretical calculations of Cyril Hilsum.
In some materials (III-V compounds such as GaAs and InP), after an electric field in the material reaches a threshold level, the mobility of electrons decrease as the electric field is increased, therebyproducing negative resistance. A two-terminal device made from such a material can produce microwave oscillations, the frequency of which is primarily determined by the characteristics of specimen of the material and not by any external circuit.
When the applied electrical field was about 2000 V/cm, he discovered oscillations of microwave frequencies.
In his own words:
... when I pushed the electric field up to the neighbourhood of 1000 to 2000 V/cm something entirely unexpected happened. Instead of a simple variation of current with voltage, all hell broke loose - the current started to jump up and down in a completely irregular way that very much resembled electrical noise mechanism I knew. The current variations were in the order of amperes rather than the nanoamperes you ordinarily see.
Introduction
gunn diode
A Gunn diode is also known as a transferred electron device (TED). It is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the aplied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity and reducing the gradient across it, preventing further conduction and current actually starts to fall down. In practice, this means a Gunn diode has a region of negative differential resistance.
The negative differential resistance, combined with the timing properties of the intermediate layer, allows construction of an RF relaxation oscillator simply by applying a suitable direct current through the device. In effect, the negative differational reisitance created by the diode will negate the real and positive resistance of an actual load and thus create a "zero" resistance circuit which will sustain oscillations indefinitely. Gunn diodes are therefore used to build oscillators in the 10 GHz and higher (THz) frequency range, where a resonator is usually added to control frequency.
These has a complex energy band structure. this is shown in the figure below:-
Empty Band
Forbidden Gaps
partly
filled band
 |
Filled band
The upper forbidden gap is very narrow. If voltage is applied across GaAs slice, electrons flow towards the positive end. But the applied field makes the electrons able to get transferred to the upper energy band, as the electrons require very less energy to jump to this level ( because of the complex energy band structure ). Thus instead of moving faster under the applied field, these electrons slows down. This is because, as they acquire more energy, they become less mobile.
Applications OF GUNN DIODE:
· Gunn diodes are reliable, relatively easy to install and the lower output power levels fall well belowthe safety exposure limits.
· They are ideally suited for use in low noise sources such as local oscillators, locking oscillators, low and medium power transmitter applications and motion detectionsystems.
· Higher power varieties can be used in phase-locked oscillators or as reflection amplifiers in point-to-point communication links and telemetry systems.
· Microwave sources have the advantagesover ultrasonic detectors of size and beamwidth, and over optical systems of working in dusty and adverse environments.
· The low voltage requirements of Gunn oscillators mean that battery or regulated mains supplies may be used, (battery drain can be reduced by using low current devices orby operation in a pulsed mode).
· The range of application of Gunn sensors for industrial and commercial use is extensive.
Operation of the Gunn diode
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.
It is found that 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.
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.
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 resonat 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.
Gunn diode oscillators
A Gunn device is not actually a diode" isn't true. They ARE diodes, because the word diode simply means it has two (active) electrodes. It doesn't have to have a P-N junction. In fact the term diode was coined for vacuum tube diodes (these actually may have three or four connections, as they require a heater supply, which may or may not form one of the active electrodes) which were made before commercially produced P-N diodes. Semiconductor diodes did exist before this but they were point contact diodes, but weren't called this. They were of course crystals and cat's whiskers, as used in crystal sets.
Gunn diodes have been around since John Gunn discovered that bulk N-type GaAs can be made to have a negative resistance effect. Gunn diodes have been a cheap source of microwaves ever since! They are used in many commercial applications for high frequency sources, including police radar, and even K-mart door openers. Ever wonder why your radar detector goes off when you pass a K-mart?
The I-V curves of a Gunn diode will help explain the effect. For low voltages (up to 1 volt perhaps), the Gunn diode behaves nearly as a linear resistor. Then at some point the current stops increasing with increasing voltage. This is known as the threshold voltage. Above this point the diode has negative resistance (curve slopes downward), which mean that it is just itching to oscillate! The operating point is usually about 4X the threshold voltage.
Below is a picture of a Gunn diode oscillator for W-band. Note the WR-10 waveguide, and the cheap heat sink. This bad boy must oscillate somewhere between 75 and 110 GHz, because that is the full extent of W-band. It is something we found in a lab drawer, for all we know it is a blown device. Nice use of a C-clamp to attach a heat sink!
GUNN OSCILLATOR
GRAPH
Vi characteristic of gundiode:
A rough approximation of the VI curve for a Gunn diode, showing the negative differential resistance region
A Gunn diode, also known as a transferred electron device (TED), is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the applied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity and reducing the gradient across it, preventing further conduction and current actually starts to fall down. In practice, this means a Gunn diode has a region of negative differential resistance.
The negative differential resistance, combined with the timing properties of the intermediate layer, allows construction of an RF relaxation oscillator simply by applying a suitable direct current through the device. In effect, the negative differential resistance created by the diode will negate the real and positive resistance of an actual load and thus create a "zero" resistance circuit which will sustain oscillations indefinitely. The oscillation frequency is determined partly by the properties of the thin middle layer, but can be tuned by external factors. Gunn diodes are therefore used to build oscillators in the 10 GHz and higher (THz) frequency range, where a resonator is usually added to control frequency. This resonator can be take the form of a waveguide, microwave cavity or YIG sphere. Tuning is done mechanically, by adjusting the parameters of the resonator, or in case of YIG spheres by changing the magnetic field.
Gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 terahertz.
The Gunn diode is named for the physicist J.B. Gunn who, in 1963, produced the first device based upon the theoretical calculations of Cyril Hilsum.
A negative resistance value means that 
is positive. This means the oscillation amplitude and energy grow exponentially with time. In practice, we can't ever obtain an oscillation whose energy grows larger without limit. Infinite powers and energies aren't accessible in the real world! Something always restricts the rate at which the system can ‘create’ oscillation power.
CONCLUSION
After going through this project I got more knowledge about “GUNN DIODE.”
This is my first experience to make a project on “GUNN DIODE”.
At last, I would like to thanks my economicsteacher“ER.POOJA SAHOTA” who gave this project to me in order to increase my knowledge about “GUNN DIODE”.
BIBILOGRAPHY
I have used following resources for making this assignment.
&
Resources are:-
· INTERNET
· NEWSPAPERS
· MAGAZINES
Links:-
Ø Bosch, B. Gunn Effect Electronics.
Ø Sze, S.M. Modern Semiconductor Device Physics.
Ø Sze, S.M. Physics of Semiconductor Devices.
Ø Moustakas, T. et al. “Growth and device applications of III-nitrides by MBE”, Journal of
Ø Foutz, B.E. et al., “Comparison of high field electron transport in GaN and GaAs”, Applied Physics Letters, 70 (21), 1997.
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