This document is a lecture on diodes presented by Syed Usama Yunas. It discusses how diodes operate under forward and reverse bias. Under forward bias, a voltage applied in one direction allows current to flow as electrons move through the pn junction. Under reverse bias, a voltage applied in the opposite direction widens the depletion region and prevents current flow, with only a small reverse current possible. The document provides details on the movement of electrons and holes under each bias condition.

1. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
Lecture # 2
Course Title : Electronics 1(EEE231)
Course Instructor: Syed Usama Yunas
Office: Room – 24, Department of Electrical Engineering
Contact email: usamayunas@ciit-attock.edu.pk
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2. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
Electrons do not move through the pn junction of diode at
equilibrium and need external voltage to operate.
Forward bias is a condition that helps the electrons/current to
flow through the pn junction of diode.
A diode connected in forward bias in shown below,
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3. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
A dc voltage source 𝑉𝐵𝐼𝐴𝑆 is
connected across diode in order to
allow the current through the diode.
𝑉𝐵𝐼𝐴𝑆 is connected in such a way that
the positive terminal is connected to
the p region and negative terminal is connected to the n region.
 𝑉𝐵𝐼𝐴𝑆 must be greater than the barrier potential.
Resistor R is used to limit the current to a value that will not
damage the diode.
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4. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
How forward bias works:
 Negative side of voltage source pushes
free electrons which are the majority
carriers in the n region towards the pn junction. This flow of
free electrons is called electron current.
In p region, electrons loose their energy and immediately
combine with holes in the valence band.
Positive side of 𝑉𝐵𝐼𝐴𝑆 attracts electrons in the valence band
in the p region. The holes provide help to the electrons to
move through the p region.
The holes (not actually) but effectively move towards the
junction and the current caused by this effective flow is called
hole current.
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5. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
Flow of majority carriers and the voltage due to the barrier
potential across the depletion can be seen as,
More electrons flow through the depletion and more holes
move effectively so the overall number of electrons and holes
reduce in the depletion region. This overall situation cause the
depletion region to reduce or get narrow.
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6. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
The energy required in order to pass through the depletion
region is equal to the barrier potential. Barrier potential in case
of silicon is 0.7volts. So 0.7 volts is required for electrons in a
silicon diode to move through or cross the depletion region in
order to conduct electric current.
An additional small voltage drop occurs across the p and n
regions due to internal resistance of the doped semiconductor
material. This internal resistance is called dynamic resistance
which is very small and can be neglected.
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7. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
Reverse bias is a condition that essentially prevents current
through the diode.
A diode connected in reverse bias in shown below,
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8. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
A dc voltage source 𝑉𝐵𝐼𝐴𝑆 is
connected across diode.
𝑉𝐵𝐼𝐴𝑆 is connected in such a way that
the negative terminal is connected to
the p region and positive terminal is connected to the n region.
Resistor R is used to limit the current to a value that will not
damage the diode.
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9. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
How reverse bias works:
 Positive side of voltage source attracts
free electrons which are majority
carriers in the n region away from the pn junction.
When electrons are removed more holes are produced in the n
region.
 Electrons enter from the negative side of the voltage source
and move from hole to hole towards the depletion region.
This movement of electrons causes more number of
electrons in the depletion region.
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10. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
How reverse bias works:
State of diode immediately after reverse bias voltage
application can be seen as,
More holes in n region and more electrons in p region cause
the depletion region to wide.
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11. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
As the depletion region widens due to more number of holes
and electrons, electric field also increases in strength until
the potential across the depletion region equals bias voltage
𝑉𝐵𝐼𝐴𝑆
. At this point the flow of current stops and a very
small amount of current passes in reverse direction called
reverse current.
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12. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
Reverse current is caused due to small number of free
minority carrier electrons in p region. These electrons are
pushed towards the pn junction by the reverse bias voltage.
Electrons (after reaching the wide depletion region) cross the
depletion region, combine with the minority holes in the n
region and flow towards the positive bias voltage.
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13. Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department
COMSATS Institute of Information Technology, Attock Campus
Note: Conduction band in p region is at higher energy level
than the conduction band in n region. Therefore minority
carrier electrons in p region can travel through the pn
junction without any additional energy.
If reverse bias voltage is increased from a threshold voltage
(breakdown voltage), the reverse current will increase
drastically and could cause damage to diode.
For higher values of reverse bias voltage more minority
carrier electrons will speed through p region into n region.
These electrons are also high in energy and one electron
causes two more electrons to knock out in p region. The
process continues and multiplication of electrons keeps
happing, this process is called avalanche. Avalanche causes
high reverse current and could damage the diode.
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