3. 3
When an electron of energy E faces a wall of potential V(x),
a change in the probability function of the electron can be
observed.
This is due to the tunneling nature of those particles.
In tunnel diode, due to heavy doping, the barrier is very
thinner than the usual.
The Schrödinger equation shows that there exists a large
probability of electrons to propagate though the barrier.
The Schrödinger equation is given by
4. 4
The time independent Schrödinger
equation is given by:
V(x) – potential of the surface
E – energy of the electron
After all derivation,
we arrive at,
The negative exponential indicates that the
probability wave function of the electron
decreases exponentially during tunneling.
5. 5
Tunnel diode is a highly doped silicon or germanium
diode. It is highly doped such that I part of impurity for
every 1000 atoms is doped.
It is not the usual case in which 1 part of impurity for
every 10^8 atoms is doped.
Thus, the width of the depletion layer fairly decreases,
facilitating the tunneling of electrons to occur.
6. Symbol of the tunnel diode is:
6
Tunnel diode is fabricated by using either Germanium or Gallium
Arsenide.
Anode is p-type region as it attracts electrons and cathode is n-
type region as it attracts the holes.
It is doped heavily using pentavalent and trivalent impurities in
order to decrease the width of the depletion layer.
8. 8
Unbiased Tunnel
Diode
When no voltage is applied the n-type CB
partly covers the p-type VB.
This occurs due to heavy doping.
The holes and electrons energy levels
within the P-type & N-type remain the
same correspondingly.
9. 9
When temperature increases, electrons tunnel
from the n-region of the CB to the p-region of the
VB.
In the same way, the holes tunnel from the p-
regions VB to the n-region CB.
In this condition, the diode will be unbiased which
means there is no flow of current through the
diode.
10. 10
When a small voltage is applied to the
tunnel diode which is less than the built-
in voltage no forward current flow
through the junction.
However a small number of electrons in
CB of n-region will tunnel to the empty
states of the VB in p-region.
This will create a small forward bias
tunnel current
Thus tunnel current starts flowing with a
small application of voltage
When small voltage
applied
11. 11
When the voltage applied to the tunnel
diode is slightly increased, a large number
of free electrons at n-side and holes at p-
side are generated.
Because of the increase in voltage, the
overlapping of the CB and VB is
increased.
The energy level of an n- side CB
becomes exactly equal to the energy level
of a p-side VB. As a result, maximum
tunnel current flows.
Applied voltage is slightly
increased
12. 12
If the applied voltage is further
increased, a slight misalign of the
conduction band and valence band
takes place.
Small amount electrons tunnel from
the conduction band of n-region to
the valence band of p-region and
cause a small current flow.
Thus, the tunneling current starts
decreasing.
When applied voltage is further
increased
13. 13
When applied voltage is largely increased
If the applied voltage is largely
increased, the tunneling current
drops to zero.
At this point, the CB and VB no
longer overlap and the tunnel diode
operates in the same manner as a
normal p-n junction diode.
14. ADD A FOOTER 14
It is an excellent conductor in reverse direction.
For small voltages, the resistance remains small
At forward current conduction, dI/dV is low.
At peak voltage, the current is at its peak current and the
resistance is 0.
The current up to this point is called Tunneling current.
The further increase in voltage after the peak voltage
leads to decrease in the current whose resistance
(slope) is negative. This region from the peak current to
valley point is called the negative resistance region.
Once valley current point is reached, the diode acts as
the normal p-n junction diode where the current
continues to increase with the increase in voltage. The
current from this valley potential is called conventional
diode current.
15. 15
• Tunnel diodes are used as logic memory storage devices.
• They are used in relaxation oscillator circuits.
• They is used as an ultra high-speed switch.
• They are used in FM receivers.
16. 16
ADVANTAGES
• Low cost,
• Low noise,
• Simplicity,
• High speed,
• Eco friendly, and
• Low power.
DISADVANTAGES
• Low output-voltage swing and
the fact that it is a two-
terminal device.
• There is no isolation between
input and output, and this
leads to serious circuit-design
difficulties.
