18-12-2012, 06:20 PM
SPECIAL PURPOSE ELECTRONIC DEVICES
[attachment=44296]
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. Impurity concentration in normal diode is 1 part in 8 10 in Tunnel diode 1 part in
3 10 .Normally a electron or hole must have energy greater than or equal to potential
energy barrier, to move to other side of the barrier. For very thin barrier there is a large
probability of electron penetrating through the barrier which is called as “Tunneling:.
Under normal Forward bias operation
As voltage begins to increase, electrons at first tunnel through the very narrow p–n junction
barrier because filled electron states in the conduction band on the n-side become aligned with
empty valence band hole states on the p-side of the p-n junction. As voltage increases further
these states become more misaligned and the current drops – this is called negative resistance
because current decreases with increasing voltage. As voltage increases yet further, the diode
begins to operate as a normal diode, where electrons travel by conduction across the p–n
junction, and no longer by tunneling through the p–n junction barrier. Thus the most important
operating region for a tunnel diode is the negative resistance region.
Reverse bias operation
When used in the reverse direction they are called back diodes and can act as fast rectifiers with
zero offset voltage and extreme linearity for power signals.Under reverse bias filled states on the
p-side become increasingly aligned with empty states on the n-side and electrons now tunnel
through the pn junction barrier in reverse direction.
Operation of a variable capacitor
They key to understanding how a varactor or varicap diode works is to look at what a
capacitor is and what can change the capacitance. As can be seen from the diagram
below, a capacitor consists of two plates with an insulating dielectric between them.
The capacitance of the capacitor is dependent upon the area of the plates - the larger the
area the greater the capacitance, and also the distance between them - the greater the
distance the smaller the level of capacitance.
A reverse biased diode has no current flowing between the P-type area and the N-type
area. The N-type region and the P-type regions can conduct electricity, and can be
considered to be the two plates, and the region between them - the depletion region is the
insulating dielectric. This is exactly the same as the capacitor above.
As with any diode, if the reverse bias is changed so does the size of the depletion region.
If the reverse voltage on the varactor or varicap diode is increased, the depletion region of
the diode increases and if the reverse voltage on varactor diode is decreased the depletion
region narrows. Therefore by changing the reverse bias on the diode it is possible to
change the capacitance.
[attachment=44296]
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. Impurity concentration in normal diode is 1 part in 8 10 in Tunnel diode 1 part in
3 10 .Normally a electron or hole must have energy greater than or equal to potential
energy barrier, to move to other side of the barrier. For very thin barrier there is a large
probability of electron penetrating through the barrier which is called as “Tunneling:.
Under normal Forward bias operation
As voltage begins to increase, electrons at first tunnel through the very narrow p–n junction
barrier because filled electron states in the conduction band on the n-side become aligned with
empty valence band hole states on the p-side of the p-n junction. As voltage increases further
these states become more misaligned and the current drops – this is called negative resistance
because current decreases with increasing voltage. As voltage increases yet further, the diode
begins to operate as a normal diode, where electrons travel by conduction across the p–n
junction, and no longer by tunneling through the p–n junction barrier. Thus the most important
operating region for a tunnel diode is the negative resistance region.
Reverse bias operation
When used in the reverse direction they are called back diodes and can act as fast rectifiers with
zero offset voltage and extreme linearity for power signals.Under reverse bias filled states on the
p-side become increasingly aligned with empty states on the n-side and electrons now tunnel
through the pn junction barrier in reverse direction.
Operation of a variable capacitor
They key to understanding how a varactor or varicap diode works is to look at what a
capacitor is and what can change the capacitance. As can be seen from the diagram
below, a capacitor consists of two plates with an insulating dielectric between them.
The capacitance of the capacitor is dependent upon the area of the plates - the larger the
area the greater the capacitance, and also the distance between them - the greater the
distance the smaller the level of capacitance.
A reverse biased diode has no current flowing between the P-type area and the N-type
area. The N-type region and the P-type regions can conduct electricity, and can be
considered to be the two plates, and the region between them - the depletion region is the
insulating dielectric. This is exactly the same as the capacitor above.
As with any diode, if the reverse bias is changed so does the size of the depletion region.
If the reverse voltage on the varactor or varicap diode is increased, the depletion region of
the diode increases and if the reverse voltage on varactor diode is decreased the depletion
region narrows. Therefore by changing the reverse bias on the diode it is possible to
change the capacitance.