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Saturday, 23 February 2019

Gunn Diode and Tunnel Diode

Gunn Diode

Gunn diodes are most commonly used microwave oscillators. It was discovered in 1963 by Gunn; he discovered the bulk transferred effect in Ga As and InS.


Gunn Effect in Microwave Engineering: 

If a voltage of about 50V is applied across a thin slice of Ga As, then negative resistance will be encountered under certain conditions. Basically, these consist merely in applying a voltage gradient across the slice in excess of about 3700 V/cm. Oscillations occur across the slice if connected to a suitably tuned circuit. Thus we see that the voltage gradient across the slice of Ga As is very high, giving rise to high electron velocity and so oscillations that occur are at microwave frequencies, with a cavity normally being used as a tuned circuit. This effect is called 'Gunn Effect'.



Gunn Effect is a bulk property of semiconductors and so it does not depend, like other semiconductor effects on either junction or contact properties. It is independent of total voltage or current, and is not affected by magnetic fields or different types of contacts. This effect occurs only in N-type materials as it is associated with electrons and not with holes. The electric field in V/cm is the factor resulting in the presence or absence of oscillations. A threshold value of 3.7kV/cm is the minimum to excite oscillations. The frequency of oscillations produced is related closely to the time that electrons take to traverse such a slice of N-type material due to the voltage applied. This proposes that a group of electrons is formed by cycle (called domain) and it reaches at the positive end of the slice to stimulate oscillations in the related tuned circuit.

It is quite possible to use small slices of Ga As mounted directly in a cavity without encapsulation.

Now-a-days, commercial Gunn diodes, encapsulated in packages are available. A typical Gunn diode is shown in Figure. The device is grown epitaxially onto a copper or gold plated molybdenum electrode, out of Ga As doped with silicon, tellurium or selenium. The substrate is very highly doped Ga As with the top layer highly doped, whereas, the active layer is less heavily doped.
Gunn Diode Construction
A typical commercial diode uses a 10V supply and has a typical power dissipation of 1W and a D.C. current of 10mA. Its power output is 20mW with an efficiency of 2% and frequency of oscillation lying between 8 and 12 GHz. The Gunn diodes oscillate anywhere in the X-band with 20% mechanical tuning arrangement and about 400 MHz of electrical tuning.

Applications:

Gunn diodes can replace reflex klystron in all fields such as receiver local oscillators, parametric amplifier pumps, signal generators, frequency modulated power oscillators. It is also used in short range communication links, RADAR for small boats, etc.

Tunnel Diode

Tunnel diodes are fabricated by doping the semiconductor materials at a very high level, one in one thousand or one in one hundred. Germanium and gallium arsenide (Ga As) are used to fabricate tunnel diodes. Construction details are shown in Figure. It is also called Esaki diode. Ge and Ga As are usually used to fabricate tunnel diodes because of high electron mobility and reasonable gap energy. It is possible for the carriers to tunnel through the potential barrier if it narrow enough (typically 10-6 cm) and if available energy level exists on the other side.

Tunnel Diode Construction
The V-I characteristics of a tunnel diode are shown in Figure.
Tunnel Diode VI Characteristics
The right hand rising portion is the normal forward biased diode region of the device. The current variation in the vincinity of the origin is due to quantum mechanical tunneling of electrons through narrow space charge region of the junction. As the applied voltage is increased from zero, tunneling current first increases and then decreases to zero. This decreases in current with increasing voltage results in a negative resistance region. When the forward voltage is further increased, the tunneling effect cease, and current increases as in the case of an ordinary PN diode.
In reverse bias, tunnel diodes behave like a conductor.

Applications: Tunnel diodes are useful in high frequency circuits, microwave oscillators, parametric amplifiers, etc. 


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