Tunnel diode definition, construction, workings are explained in detailed here. it is a highly Doped as
compared to PN-junction diode. It has negative resistance region .it works on
the principle of Tunneling effect which is obtained by creating an extremely
thin depletion layer. Tunnel diode is also known as ESAKI diode. Because Leo Esaki
invented tunnel diode in august 1957.
What is a Tunnel Diode?
Tunnel
Diode is the PN-Junction
Diode device that exhibits negative resistance. When the voltage is
increased then the current flowing through it decreases. It works on the
principle of Tunneling effect.
The tunnel diode is a
two terminal device with n-type semiconductor as cathode and p-typesemiconductor as anode. The tunnel diode Circuit symbol is
as shown below.
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| Equivalent Circuit Diagram of Tunnel Diode |
Tunnel Diode & PN-junction Diode
Differences
In normal PN-junction diode
doping concentration of impurity adding is 1:108 impurity atoms. This
causes the depletion region of normal diodes is of one micron. This depletion
region prevents the movement of electrons from N region to P region and holes
from P region to N region.
In Tunnel Diode the impurity
concentration is very high as compared to conventional diodes .that is in order
of 1:103 impurity atoms. This corresponds to an impurity
concentration exceeding 109 per cm3. Then the depletion
region width of Tunnel diode is gets
reduced due to tunneling effect.
The conventional diode offers
large resistance when it is connected to reverse bias condition. And it offers
low resistance when it is connected to forward bias. When increase the forward
voltage it will causes to increase of current.
In Tunnel
diode when we connected to reverse bias voltage it produces considerable
current. Under forward bias condition it gives a curve it is triple valued. It
offers three currents at three different forward different voltages.
Construction of Tunnel Diode
Germanium and Gallium
arsenide are used in the manufacturing of Tunnel diodes. Silicon tunnel diodes
can be made but their peak to valley current ratio is very low. the
construction of tunnel diode is shown in below.
 |
| Construction of Tunnel Diode |
Tunnel Diode Working
We can see the working
of Tunnel diode is with the help of
energy band diagrams.
No-Bias Condition:
Under no-bias
conditions, there are no filled states on one side of junction which are at the
same energy as the empty allowed states on other side. Hence, there can be no
flow of charge in either direction across the junction and current is zero. So,
it is simply stated as the Tunnel diode is in open-circuit condition.
 |
Un-Biased Tunnel Diode Junction
|
Forward Bias Condition
In forward bias
condition filled levels on N-side are raised to higher energy level than the
empty levels of P-side. So, electronics tunnel takes place from N-side to
P-side (similar to reverse bias condition) and current will increase in forward
characteristics. When forward bias is increased more current will flow. Finally
maximum level of tunneling is reached at this place the energy levels are
directly oppositive
 |
| Forward Biased Tunnel Diode Junction |
If further
we increase forward bias current flowing of tunnel diode is reduces at one
stage the current reaches minimum value
If
further we increase forward bias current would be increases because the forward
voltage is again greater than the barrier potential.
 |
| Tunnel Diode Increased Forward biased |
Reverse Bias Condition
In
reverse bias of Tunnel diode means the P-side is connected to negative terminal
of battery and N-side is connected to the positive terminal of battery.
The filled energy
levels on the P-side are oppositive to the empty energy levels on N-side
Tunneling takes place. The electronics tunnels through the barrier from the
P-side higher energy levels to N-side lower energy levels. This will cause considerable
current is flowing in this condition.
 |
| Reverse Biased Junction Of Tunnel Diode |
If increases the
reverse bias, electrons further tunnel from P-side to N-side .this will cause
increases the reverse current.
V-I Characteristics of Tunnel Diode
Advantages of Tunnel
Diode
·
Low
cost
·
Low
noise
·
Easy of operation
·
High
speed
·
Low
power
Disadvantages of Tunnel
Diode
·
It
is a two terminal device so, it cannot be worked as a amplifier
· Low output voltage swing.
Applications
of Tunnel diode
·
It
is used as switches. Switching time for this is tunnel diode is around 50 ps.
·
These
are also used as oscillators.
·
Used
as mixers