http://en.wikipedia.org/wiki/Tunnel_diode
Tunnel diode
A tunnel diode or Esaki diode is a type of semiconductor diode that is capable of very fast operation, well into the microwave frequency region, by using the quantum mechanical effect called tunneling.
It was invented in August 1957 by Leo Esaki when he was with Tokyo Tsushin Kogyo, now known as Sony. In 1973 he received the Nobel Prize in Physics, jointly with Brian Josephson, for discovering the electron tunneling effect used in these diodes. Robert Noyce independently came up with the idea of a tunnel diode while working for William Shockley, but was discouraged from pursuing it.[1]
These diodes have a heavily doped p–n junction only some 10 nm (100 Å) wide. The heavy doping results in a broken bandgap, where conduction band electron states on the n-side are more or less aligned with valence band hole states on the p-side.
Tunnel diodes were first manufactured by Sony in 1957[2] followed by General Electricand other companies from about 1960, and are still made in low volume today.[3] Tunnel diodes are usually made from germanium, but can also be made in gallium arsenide andsilicon materials. They are used in frequency converters and detectors.[4] They havenegative differential resistance in part of their operating range, and therefore are also used as oscillators, amplifiers, and in switching circuits using hysteresis.
In 1977, the Intelsat V satellite receiver used a microstrip tunnel diode amplifier (TDA) front-end in the 14 to 15.5 GHz band. Such amplifiers are considered state-of-the-art, with better performance at high frequencies than any transistor-based front end.[5]
The highest frequency room-temperature solid-state oscillators are based on resonant-tunneling diode (RTD).[6]
There is another type of tunnel diode called a metal–insulator–metal (MIM) diode, but present application appears restricted to research environments due to inherent sensitivities.[7]
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[edit]Forward bias operation
Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow p–n junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states on the p-side of the p-n junction. As voltage increases further these states become more misaligned and the current drops – this is called negative resistance because current decreases with increasing voltage. As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the p–n junction, and no longer by tunneling through the p–n junction barrier. The most important operating region for a tunnel diode is the negative resistance region.
[edit]Reverse bias operation
Main article: Backward diode
When used in the reverse direction they are called back diodes (or backward diodes) and can act as fast rectifiers with zero offset voltage and extreme linearity for power signals (they have an accurate square law characteristic in the reverse direction). Under reverse bias filled states on the p-side become increasingly aligned with empty states on the n-side and electrons now tunnel through the pn junction barrier in reverse direction.
[edit]Technical comparisons
In a conventional semiconductor diode, conduction takes place while the p–n junction is forward biased and blocks current flow when the junction is reverse biased. This occurs up to a point known as the “reverse breakdown voltage” when conduction begins (often accompanied by destruction of the device). In the tunnel diode, the dopant concentration in the p and n layers are increased to the point where the reverse breakdown voltagebecomes zero and the diode conducts in the reverse direction. However, when forward-biased, an odd effect occurs called quantum mechanical tunnelling which gives rise to a region where an increase in forward voltage is accompanied by a decrease in forward current. This negative resistance region can be exploited in a solid state version of the dynatron oscillator which normally uses a tetrode thermionic valve (or tube).
The tunnel diode showed great promise as an oscillator and high-frequency threshold (trigger) device since it would operate at frequencies far greater than the tetrode would, well into the microwave bands. Applications for tunnel diodes included local oscillators for UHF television tuners, trigger circuits in oscilloscopes, high speed counter circuits, and very fast-rise time pulse generator circuits. The tunnel diode can also be used as low-noise microwave amplifier.[8] However, since its discovery, more conventional semiconductor devices have surpassed its performance using conventional oscillator techniques. For many purposes, a three-terminal device, such as a field-effect transistor, is more flexible than a device with only two terminals. Practical tunnel diodes operate at a few milliamperes and a few tenths of a volt, making them low-power devices.[9] The Gunn diode has similar high frequency capability and can handle more power.
Tunnel diodes are also relatively resistant to nuclear radiation, as compared to other diodes. This makes them well suited to higher radiation environments, such as those found in space applications.
[edit]Longevity
Esaki diodes are notable for their longevity; devices made in the 1960s still function. Writing in Nature, Esaki and coauthors state that semiconductor devices in general are extremely stable, and suggest that their shelf life should be "infinite" if kept at room temperature. They go on to report that a small-scale test of 50-year-old devices revealed a "gratifying confirmation of the diode's longevity". As noticed on some samples of Esaki diodes, the gold plated iron pins can in fact corrode and short out to the case. This can usually be diagnosed, and the diode inside normally still works.[
http://www.radio-electronics.com/info/data/semicond/tunneldiode/tunneldiode.php
Tunnel diode
The tunnel diode was found many microwave applications because semiconductor devices of the day could not reach these frequencies. Although not widely used today, it is still sometimes mentioned and it is a fascinating device.
The tunnel diode was discovered by a Ph.D. research student named Esaki in 1958 while he was investigating the properties of heavily doped germanium junctions for use in high speed bipolar transistors. In the course of his research he produced some heavily doped junctions and as a result found that they produced an oscillation at microwave frequencies as a result of the tunnelling effect. It was subsequently found that other materials including gallium arsenide also produced the same effect.
Tunnel diode structure
The tunnel diode is similar to a standard p-n junction in many respects except that the doping levels are very high. Also the depletion region, the area between the p-type and n-type areas, where there are no carriers is very narrow. Typically it is in the region of between five to ten nano-metres - only a few atom widths.
As the depletion region is so narrow this means that if it is to be used for high frequency operation the diode itself must be made very small to reduce the high level of capacitance resulting from the very narrow depletion region.
Mode of operation
The characteristic curve for a tunnel diode shows an area of negative resistance. When forward biased the current in the diode rises at first, but later it can be seen to fall with increasing voltage, before finally rising again. The reason for this is that there are a number of different components to forming the overall curve. The main two are the normal diode current across the junction, and the current arising from the tunnelling effect. It is this last component that is of interest in a tunnel diode.
Tunnelling is an effect that is caused by quantum mechanical effects when electrons pass through a potential barrier. It can be visualised in very basic terms by them "tunnelling" through the barrier.
The tunnelling only occurs under certain conditions. This means that it peaks when a certain voltage is placed across the junction. This results in the current increasing to a point beyond that which would be expected for a standard pn junction. As the voltage across the diode is increased the effect reduces and the current through the device falls. This results in a negative resistance region on the curve of te diode that can be used to provide gain.
Advantages and disadvantages
One of the main reasons for the early success of the tunnel diode was its high speed of operation and the high frequencies it could handle. This resulted from the fact that while many other devices are slowed down by the presence of minority carriers, the tunnel diode only uses majority carriers, i.e. holes in an n-type material and electrons in a p-type material. The minority carriers slow down the operation of a device and as a result their speed is slower. Also the tunnelling effect is inherently very fast.
The tunnel diode is rarely used these days and this results from its disadvantages. Firstly they only have a low tunnelling current and this means that they are low power devices. While this may be acceptable for low noise amplifiers, it is a significant drawback when they are sued in oscillators as further amplification is needed and this can only be undertaken by devices that have a higher power capability, i.e. not tunnel diodes. The third disadvantage is that they are problems with the reproducibility of the devices resulting in low yields and therefore higher production costs.
Applications
Although the tunnel diode appeared promising some years ago, it was soon replaced by other semiconductor devices like IMPATT diodes for oscillator applications and FETs when used as an amplifier. Nevertheless the tunnel diode is a useful device for certain applications.
Try to add more relations to gunn diode applications
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