introduction :
A tunnel diode is a highly doped semiconductor device that conducts current by quantum mechanical tunneling. it is also called as esaki diode named after leo esaki who got Nobel prize in physics for discovering electron tunneling in 1973.
Tunneling Phenomenon:
According to classical laws of physics, a particle must have an energy at least equal to the height of a potential-energy barrier if it has to move from one side of the barrier to the other. In other words, energy has to be supplied from some external source so that the electrons on one side of junction climb over the junction barrier to reach the other side. However if the barrier is thin such as in tunnel diode, the Schrodinger equation(Quantum Mechanics) indicates that there is a large probability that an electron will penetrate through the barrier. This will happen without any loss of energy on the part of electron. This quantum mechanical behavior is referred to as tunneling and the high-impurity P-N junction devices are called tunnel-diodes. The tunneling phenomenon is a majority carrier effect.
Final statement: if electron energy is grater than or equal to height of barrier then electron crosses that barrier without any loss of energy.
Construction:
Depletion region in tunnel diode is very small due to very heavy doping.Tunnel diodes are usually fabricated from germanium, gallium or gallium arsenide. These all have small forbidden energy gaps and high ion motilities. Silicon is not used in the fabrication of tunnel diodes due to low (Ip,I v)value.
A small tin dot is soldered or alloyed to a heavily doped pellet of n-type Ge, GaSb or GaAs. The pellet is then soldered to anode which is also used for heat dissipation. The cathode contact is connected to the tin dot via a mesh screen used to reduce inductance. The diode has a ceramic body and a hermetically sealing lid on top.
Working and characteristics :
For small forward voltages owing to high carrier concentrations in tunnel diode and due to tunneling effect the forward resistance will be very small. As voltage increase she current also increases till the current reaches Peak current. If the voltage applied to tunnel diode is increased beyond the peak voltage the current will start decreasing. This is negative resistance region. It prevails till valley point. At valley point the current through the diode will be minimum. Beyond valley point the tunnel diode acts as normal diode. In reverse biased condition also Tunnel diode is an excellent conductor due to its high doping concentrations.
Tunnel diodes are made from Germanium or gallium arsenide due to their highest peak voltage to valley point swing. The ratio of high peak current to valley current quantifies the maximum voltage swing allowed in negative resistance region.
applications:
- Tunnel diode can make a very stable oscillator circuit.
- It is also used as an amplifiers and mixers.
- Due to the tunneling mechanism, it is used as an ultra high speed switch.
- Due to its negative resistance, it is used as a relaxation oscillator circuit.
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