2. Diode
o A Semiconductor diode consists of an n-material region and a p-material
region separated by a pn junction.
o In simple words a diode conducts in one direction and doesn’t conduct in
other direction.
o P-n junctions consist of two semiconductor regions of opposite type.
• the region on the right (n-region) has free electrons
• the region on the left (p-region) has many holes.
o Term diode means two electrode i.e. Anode (+) and Cathode (-).
o Arrow indicates direction of conventional current through it.
3.
4. Formation of Diode
o As a result of recombination, a large number of positive (in the n-region) and
negative (in the p region) ions builds up near the p-n junction, essentially
depleting the region of any conduction electrons or holes -termed the
depletion region.
o The regions nearby the p–n junction gets charged, generate an electric field.
o A point is reached where the total negative charge in the depletion region
repels any further diffusion of electrons (negatively charged particles) into
the p region (like charges repel) and the diffusion stops.
o In other words, the depletion region acts as a barrier to the further
movement of electrons across the junction. This potential difference is called
the barrier potential.
5. o The barrier potential, VB, is the amount of voltage required to move
electrons through the electric field the electric field
o –At 25°C, it is approximately 0.7 V for silicon and 0.3 V for germanium
o –As the junction temperature increases, the barrier potential decreases,
and vice versa.
Figure: Formation of depletion region
6. Biasing of Diode
o With no external voltage applied to diode, the depletion region available at
junction which prevents the current to flow through it.
o Bias means that a voltage (positive or negative) is applied to a “node" in the
circuit to offset or alter the node's voltage or the output of that circuit.
o Thus required to be externally biased to make current flow.
o There are two types of diodes on the basis of biasing
o Forward Biased Diode
o Reversed Biased Diode
7. Forward Biased Diode
o When +ve terminal of battery is connected to the P-type material and - ve terminal
to the N-type material.
o The negative terminal of the bias-voltage source pushes the conduction-band
electrons in the n region toward the p-n junction, while the positive terminal pushes
the holes in the p-region toward terminal pushes the holes in the p-region toward
the p-n junction.
o When it overcomes the barrier potential (VB), the external voltage source provides
the n-region electrons with enough energy to penetrate the depletion region and
move through the junction
o In Forward bias is the condition that permits current due to majority current carriers
through a diode. As Vd increases, the depletion width decrease until a flood of
majority carriers start passing through
9. Reversed Biased Diode
o Reverse bias is the condition that prevents current through the diode.
o The negative terminal of the V BIAS source is connected to the p-region, and
the positive terminal is connected to the p-region, and the positive terminal
is connected to the n-region.
o If the external reverse-bias voltage is increased to a large enough value,
reverse breakdown occurs.
o Minority conduction-band electrons acquire enough energy from the
external source to accelerate toward the positive end of the diode, colliding
with atoms and knocking valence electrons into the conduction band.
11. V/I Characteristics of Forward Biased Diodes
o If the external voltage applied on the silicon diode is less than 0.7 volts, the
silicon diode allows only a small electric current. However, this small electric
current is considered as negligible.
o If Forward Biased diode then V/I then
R = VF / IF
12. o When the external voltage applied on the Silicon diode reaches 0.7 volts,
the p-n junction diode starts allowing large electric current through it.
o At this point, a small increase in voltage increases the electric current
rapidly. The forward voltage at which the silicon diode starts allowing large
electric current is called cut-in voltage.
o The cut-in voltage for silicon diode is approximately 0.7 volts.
o If the external voltage applied on the Germanium diode is less than 0.3
volts, the germanium diode allows only a small electric current. However,
this small electric current is considered as negligible.
o When the external voltage applied on the germanium diode reaches 0.3
volts, the germanium diode starts allowing large electric current through it.
At this point, a small increase in voltage increases the electric current
rapidly. The forward voltage at which the germanium diode starts allowing
large electric current is called cut-in voltage. The cut-in voltage for
germanium diode is approximately 0.3 volts.
13. V/I Characteristics of Forward Biased Diodes
o The wide depletion region of reverse biased p-n junction diode completely blocks
the majority charge carrier current. However, it allows the minority charge carrier
current.
o The free electrons (minority carriers) in the p-type semiconductor and the holes
(minority carriers) in the n-type semiconductor carry the electric current.
o The electric current, which is carried by the minority charge carriers in the p-n
junction diode, is called reverse current.
o In n-type and p-type semiconductors, very small number of minority charge carriers
is present. Hence, a small voltage applied on the diode pushes all the minority
carriers towards the junction. Thus, further increase in the external voltage does not
increase the electric current. This electric current is called reverse saturation current.
14. o In other words, the voltage or point at which the electric current reaches its
maximum level and further increase in voltage does not increase the electric
current is called reverse saturation current.
o Hence, the reverse saturation current remains constant with the increase in
voltage. However, if the voltage applied on the diode is increased
continuously, the p-n junction diode reaches to a state where junction
breakdown occurs and reverse current increases rapidly.
o The reverse saturation current is depends on the temperature. If
temperature increases the generation of minority charge carriers increases.
Hence, the reverse current increases with the increase in temperature
15. o The number of minority charge carriers
generated in the germanium diodes is
greater than the silicon diodes. Hence, the
reverse saturation current in the germanium
diodes is greater than the silicon diodes.
o The voltage or point at which junction
breakdown occurs is called breakdown
voltage. The breakdown voltage of a p-n
junction diode is depends on the width of
depletion region. The p-n junction diodes
with wide depletion region have high
breakdown voltage whereas the p-n junction
diodes with narrow depletion region have
low breakdown voltage.
