TUNNEL DIODE
PRINCIPLE OF
OPERATION AND CHARACTERISTICS OF TUNNEL DIODE:
A tunnel diode
or Esaki diode is a type of semiconductor
diode which is capable of very fast operation, well into the microwave
frequency region, by using quantum
mechanical effects.
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.
Fig:Tunnel
diode schematic symbol
These diodes have a heavily dopedp–n junction only some 10 nm (100 Å) wide. The heavy doping results in a broken bandgap, where conduction bandelectron
states on the n-side
are more or less aligned with valence bandhole states on the p-side.
Tunnel diodes were manufactured by Sony for the first time in 1957 followed by General Electric and other companies from about 1960,
and are still made in low volume today. Tunnel diodes are usually made from germanium, but can also be made in gallium arsenide and silicon materials. They can be used as oscillators, amplifiers, frequency
converters and detectors.
Tunnelling Phenomenon:
In electronics, Tunneling is known as
a direct flow of electrons across the small depletion region from n-side
conduction band into the p-side valence band. In a p-n junction diode, both
positive and negative ions form the depletion region. Due to these ions,
in-built electric potential or electric field is present in the depletion
region. This electric field gives an electric force to the opposite direction
of externally applied voltage.
As the width of the depletion
layer reduces, charge carriers can easily cross the junction. Charge carriers
do not need any form of kinetic energy to move across the junction. Instead,
carriers punch through junction. This effect is called Tunneling and hence the
diode is called Tunnel Diode.
Due to
Tunneling, when the value of forward voltage is low value of forward current generated
will be high. It can operate in forward biased as well as in reverse biased.
Due to high doping, it can operate in reverse biased. Due to the reduction in
barrier potential, the value of reverse breakdown voltage also reduces. It
reaches a value of zero. Due to this small reverse voltage leads to diode
breakdown. Hence, this creates negative resistance region.
Tunnel
Diode Working Phenomenon
Unbiased Tunnel Diode
In an unbiased tunnel diode, no
voltage will be applied to the tunnel diode. Here, due to heavy doping
conduction band of n – type semiconductor overlaps with valence band of p –
type material. Electrons from n side and holes from p side overlap with each
other and they will be at same energy level.
Some electrons tunnel from the
conduction band of n-region to the valence band of p-region when temperature
increases. Similarly, holes will move from valence band of p-region to the
conduction band of n-region. Finally, the net current will be zero since equal
numbers of electrons are holes flow in opposite direction.
Small Voltage Applied to the Tunnel
Diode
When a small
voltage, that has lesser value than the built-in voltage of the depletion
layer, is applied to the tunnel diode, there is no flow of forward current
through the junction. Nevertheless, a minimal number of electrons from the
conduction band of n region will start tunneling to valence band in p region.
Increased Voltage Applied to the
Tunnel Diode
When the
amount of voltage applied is increased, the number of free electrons generated
at n side and holes at p side is also increased. Due to voltage increase,
overlapping between the bands are also increased.
Further Increased Voltage Applied to
the Tunnel Diode
A further
increase in the applied voltage will cause a slight misalignment of the
conduction band and valence band. Still there will be an overlap between
conduction band and valence band. The electrons move from conduction band to
valence band of p region. Therefore, this causes small current to flow. Hence,
tunnel current starts decreasing.
Largely Increased Voltage Applied to
the Tunnel Diode
The tunneling
current will be zero when applied voltage is increased more to the maximum. At
this voltage levels, the valence band and the conduction band does not overlap.
This makes tunnel diode to operate same as a PN junction diode.
When applied
voltage is more than the built-in potential of the depletion layer the forward
current starts flowing through the tunnel diode. In this condition, current portion
in the curve decreases when the voltage increases and this is the negative
resistance of tunnel diode. Such diodes operating in negative resistance region
is used as amplifier or oscillator.
V-I
Characteristics of Tunnel Diode
Due to
forward biasing, because of heavy doping conduction happens in the diode. The
maximum current that a diode reaches is Ip and voltage applied is Vp. The
current value decreases, when more amount of voltage is applied. Current keeps
decreasing until it reaches a minimal value.
Advantages of tunnel
diodes:
·
Environmental
immunity i.e peak point is not a function of temperature.
·
low
cost.
·
low
noise.
·
low
power consumption.
·
High
speed i.e tunneling takes place very fast at the speed of light in the order of
nanoseconds
·
simplicity
i.e a tunnel diode can be used along with a d.c supply and a few passive
elements to obtain various application circuits.
Applications for tunnel
diodes:
·
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.
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