Electronics

Copyrights © Syed Usama Yunas , Faculty Member Electrical Engineering Department

COMSATS Institute of Information Technology, Attock Campus

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Lecture # 2

Course Title : Electronics 1(EEE231)Course Instructor: Syed Usama Yunas

Office: Room – 24, Department of Electrical EngineeringContact email: [email protected]



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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,

Forward Bias



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A dc voltage source is connected across diode in order to allow the current through the diode. 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.must be greater than the barrier potential.Resistor R is used to limit the current to a value that will not

damage the diode.

Forward Bias (continued…)



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How forward bias works: Negative side of voltage source pushesfree 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 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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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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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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Reverse bias is a condition that essentially prevents current through the diode.

A diode connected in reverse bias in shown below,

Reverse Bias



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A dc voltage source is connected across diode.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.

Reverse Bias (continued…)



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How reverse bias works: Positive side of voltage source attractsfree electrons which are majoritycarriers 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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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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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 . 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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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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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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Electronics