5. HISTORY
• Tunnel Diode was invented in August 1957 by Leo Esaki.
• In 1973 he received the Nobel prize in physics for discovering
the electron tunneling effect used .
• well into the microwave frequency region, made possible by
the use of the quantum mechanical effect called tunneling.
6. Definition
• An extremely stable semiconductor diode, having a
very narrow highly doped p-n junction, in which
electrons travel across the junction by means of the
tunnel effect.
7. Construction
• There are several steps for this which are given below
1. Firstly, it reduces the width of the depletion layer to an
extremely small value (about 0.00001 mm).
2. Secondly, it reduces the reverse breakdown voltage to a very small
value (approaching zero) with the result that the diode appears to be
broken down for any reverse voltage.
8. construction
3. Thirdly, it produces a negative resistance section on the V/I
characteristic of the diode.
It is called a tunnel diode because due to its extremely thin
depletion layer, electrons are able to tunnel through the potential
barrier at relatively low forward bias voltage (less than 0.05V).
Such diodes are usually fabricated from germanium, gallium-
arsenide(GA As) and gallium antimonites (Gabs).
10. V/I Characteristic
• As seen, forward bias produces immediate conduction
i.e. as soon as forward bias is applied, significant current
is produced.
• The current quickly rises touts peak value Ip when the
applied forward voltage reaches a valueVp (point A ).
11. V/I Characteristic
• When forward voltage is increased further, diode current starts
decreasing till it achieves its minimum value called valley current
Iv corresponding to valley voltageVv (point B).
• For voltages greater thanVv, current starts increasing again as in
any ordinary junction diode.
13. V/I Characteristic
• As seen from Fig between the peak point A and valley point B, current decreases
with increase in the applied voltage.
• In other words, tunnel diode possesses negative resistance (— R N) in this region.
• In fact, this constitutes the most useful property of the diode. Instead of
absorbing power, a negative resistance produces power.
• By offsetting losses in L and C components of a tank circuit, such a negative
resistance permits oscillations.
14. V/I Characteristic
• Hence, a tunnel diode is used as a very high frequency
oscillator.
• Another point worth noting is that this resistance increases as
we go from point A to B because as voltage is increased, current
keeps decreasing which means that diode negative resistance
keeps increasing.
15. Negative Resistance (– RN)
• It is the resistance offered by the diode within the negative
resistance section of its characteristic. It equals the reciprocal of
the slope of the characteristic in this region.
It may also be found from the following relation
R N = − dV/dI.
• Its value depends on the semiconductor material used
(varying from − 10 Ω to − 200 Ω).
17. Tunneling diode theory
•
At zero forward bias, the energy levels of conduction electrons
in N-region of the junction are slightly out of alignment with
the energy levels of holes in the P-region.
• As the forward voltage is slightly increased, electron levels
start getting aligned with the hole levels on the other side of
junction thus permitting some electrons to cross over.This kind
of junction crossing is called tunneling.
18. Tunneling diode theory
• As voltage is increased to peak voltage (VP), all conduction
band electrons in the N-region are able to cross over to the
valence band in the P-region because the two bands are
exactly aligned. Hence, maximum current (called peak
current IP) flows in the circuit.
•After VP as the applied voltage is increased, current
starts decreasing because the two bands start
gradually getting out of alignment.
19. • It reaches minimum value (called valley current) when the
two are totally out of alignment at a forward bias of VV (valley
voltage).
• For voltages greater than VV, current starts increasing again
exactly as it does in the case of an ordinary P-N junction diode.
• Tunneling is much faster than normal crossing which
enables a tunnel diode to switch ON and OFF much faster than
an ordinary diode.That is why a tunnel diode is extensively
used in special applications requiring very fast switching
speeds like high-speed computer memories and high
frequency oscillators .
20. Backward diode applications
The characteristics of the backward diode make it suitable for a limited
number of applications where other diodes may not perform as well.
• Detector :
The backward diode provides a linear detection characteristic for small
signals. Additionally the fact that there is no charge storage in its mode of operation
means that it can be used for signals with frequencies extending to 50 GHz and more
21. Backward diode applications
• Rectifier:
The diode is suitable for rectifying signals with peak voltages between
about 0.1 and 0.6 volts.
• Switch:
In view of its speed of operation, the diode is sometimes used
for very high speed switching applications.
22. Advatages and disadvantages
• The advantages of a tunnel diode are :
• 1. low noise
• 2. ease of operation
• 3. high speed
• 4. low power
• 5. Insensitivity to nuclear radiations
23. Advatages and disadvantages
1. The voltage range over which it can be operated properly is 1V or less.
2. Being a two-terminal device, it provides no isolation between the input
and output circuits
