The tunnel diode was invented by leo esaki in 1958 and named after the inventor which is also called esaki diode. It's use for oscillation and generating high frequency current. A tunnel diode is from junction of heavily doped N-type and P-type semiconductors making it conductive in a different way. Just like other diodes, it consists of a cathode and an anode. Electrons flows our of the cathode and the anode recieves the electrons thus making current to flow from the anode to cathode when connected in forward bias. Tunnel diode are usually fabricated from germanium and can also be manufactured from gallium antimonide, silicon and gallium arsenide.
Circuit symbol of a tunnel diode.
Between the junction of the N-type and P-type semiconductor is a depletion layer that offers resistance to the flow of electrons across the barrier, free electrons and holes are absent in this region. The width of the depletion layer depends on the amount of impurities added. Large number of doped impurities results to little or no depletion layer while little of few number of doped impurities results to large or longer depletion layer which offers resistance to current flow, and must be overcome with much voltage.
Working principle of a tunnel diode.
Tunnel diodes work on a quantum mechanics principle called "tunneling". Tunneling means the direct flow of electrons from the N-type semiconductor to the P-type semiconductor across the small depletion region. In normal P-N junction diodes, the depletion width is large as compared to the tunnel diode. This width depletion layer or region in need diodes opposes the flow of current, therefore the depletion layer acts as a barrier. To overcome this barrier, an enough supply of voltage is needed, which then makes current to start flowing through the junction.
Tunnel diodes mode of operation.
Tunnel diodes operates in different modes depending on the amount of current applied to it and the direction of applied voltage.
•ON mode.
Like other diodes, it behaves like an on switch when current flows from the anode to cathode. It's connected in circuits with the cathode attached to a negative power terminal and the anode connected to the positive power terminal.
•OFF mode.
The diode blocks current and acts like an off switch when connected in reverse bias. In circuits it can be intentional used as a permanent switch in off mode to block current from flowing by connecting the anode to a ground and the cathode to the +ve terminal of the circuit.
•Tunnel mode.
1. When a small voltage is applied.
When a little voltage is applied to the tunnel diode which is lesser than the built-in voltage of the depletion layer, no forward current will flows through the junction. However, a small number of electrons in the conduction band of the n-region will tunnel the empty states of the valence band in P-region. This will create a small forward bias tunnel current. Thus, tunnel current starts flowing with a small application of voltage.
2. Volatage applied is increased slightly.
When the voltage applied to the tunnel diode is slightly increased, a large number of free electrons at the n- region and holes at p-side are generated. Due to the increase in voltage, the overlapping of the conduction band and valence band is increased.
3. Volatage applied is greatly increased.
If the applied voltage is largely increased, the tunneling current drops to zero. At this point the conduction band and valence band is longer overlapping and it operates in the same way as p-n junction diodes. If the applied voltage is greater than the built-in potential of the depletion layer, the regular forward current starts flowing through it.
Circuit appllications of tunnel diodes.
•Tunnel diodes are use in FM receiver's circuit.
• They are use in high speed oscilator circuit.
•They are use as an high speed switch.
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Power Electronics