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The p-n junction

Differences between p- and n-type semi-conductors

We have seen that the addition of specific elements to semiconductor materials turns them into p-type semiconductors or n-type semiconductors. The differences between n- and p-type semiconductors are summarised in [link] .

The p-n junction

When p-type and n-type semiconductors are placed in contact with each other, a p-n junction is formed. Near the junction, electrons and holes combine to create a depletion region.

The p-n junction forms between p- and n-type semiconductors. The free electrons from the n-type material combine with the holes in the p-type material near the junction. There is a small potential difference across the junction. The area near the junction is called the depletion region because there are few holes and few free electrons in this region.

Electric current flows more easily across a p-n junction in one direction than in the other. If the positive pole of a battery is connected to the p-side of the junction, and the negative pole to the n-side, charge flows across the junction. If the battery is connected in the opposite direction, very little charge can flow.

This might not sound very useful at first but the p-n junction forms the basis for computer chips, solar cells, and other electronic devices.

The following simulation allows you to see what happens in semi-conductors and to make semi-conductors.

run demo

Unbiased

In a p-n junction, without an external applied voltage (no bias), an equilibrium condition is reached in which a potential difference is formed across the junction.

P-type is where you have more "holes"; n-type is where you have more electrons in the material. Initially, when you put them together to form a junction, holes near the junction tends to "move" across to the n-region, while the electrons in the n-region drift across to the p-region to "fill" some holes. This current will quickly stop as the potential barrier is built up by the migrated charges. So in steady state no current flows.

Now when you put a potential different across the terminals you have two cases: forward biased and reverse biased.

Forward biased

Forward-bias occurs when the p-type semiconductor material is connected to the positive terminal of a battery and the n-type semiconductor material is connected to the negative terminal.

The electric field from the external potential different can easily overcome the small internal field (in the so-called depletion region, created by the initial drifting of charges): usually anything bigger than 0.6V would be enough. The external field then attracts more e- to flow from n-region to p-region and more holes from p-region to n-region and you have a forward biased situation. the diode is forward biased and so current will flow.

Reverse biased

In this case the external field pushes e- back to the n-region while more holes into the p-region, as a result you get no current flow. Only the small number of thermally released minority carriers (holes in the n-type region and e- in the p-type region) will be able to cross the junction and form a very small current, but for all practical purposes, this can be ignored.

Of course if the reverse biased potential is large enough you get what is called avalanche break down and current flow in the opposite direction. In many cases, except for Zener diodes, you most likely will destroy the diode.

Real-world applications of semiconductors

Semiconductors form the basis of modern electronics. Every electrical appliance usually has some semiconductor-based technology inside it. The fundamental uses of semiconductors are in microchips (also known as integrated circuits) and microprocessors.

Integrated circuits are miniaturised circuits. The use of integrated circuits makes it possible for electronic devices (like a cellular telephone or a hi-fi) to get smaller.

Microprocessors are a special type of integrated circuit.

Research project : semiconductors

Assess the impact on society of the invention of transistors, with particular reference to their use in microchips (integrated circuits) and microprocessors.

The p-n junction

  1. Compare p- and n-type semi-conductors.
  2. Explain how a p-n junction works using a diagram.
  3. Give everyday examples of the application.

End of chapter exercises

  1. What is a conductor?
  2. What is an insulator?
  3. What is a semiconductor?

Questions & Answers

explain why HCL is a Bronstead lowry acid . use an equation of HCL in water to explain
Mihlali Reply
what is electron. ( you should not allowed to tell it properties in it defination.)
vigyan Reply
a charged particle.
Siyabonga
investigative question
Obakeng Reply
how do you create a bomb
Nasty Reply
how can I create a bomb
Nasty
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Nasty
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Nasty
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Nasty
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Nasty
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Nasty
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Nasty
how can I create a bomb
Nasty
how can I create a bomb
Nasty
if you want to create bomb use this chemical substance oxidised, sulphuric acid,tobacco,gas and flammable substance , mix all this substance together and put in strong mental bottle like gas bottle put in . you must close the bottle very parked and carry that bottle carefully and shake in 5 minutes
Aaron
after shake put this bottle were the temperature is 100 degrees Celsius after moment that bottle will be high explosion like bomb
Aaron
what is mass
asmerom Reply
is object amount matter contain by object
Aaron
what is hypothesis
Aaron
hypothesis is a tentative explanation that does not have facts and it can be proven to be wrong or right
MPHO
wath is physical science
Siphelo Reply
what is physics
Dineo Reply
the branch of science concerned with the nature and properties of matter and energy
Thabisto
use a vector scale diagram and represent the two displacements(5m east and 7m west)
itumeleng Reply
what os covalent?
Shellah Reply
can i ask something?
Uapfa Reply
Ask
Precious
how to calculate enthalpy on a reaction?
Uapfa
how do you learn this chapter festet
Lisakhanya Reply
it's not all about learning it fast but understanding the basics, the concepts. formulas .. if you juss understand the basics you know the chapter .understand it
Precious
you are right precious
MPHO
can i ask smething.?
Uapfa
electric field around a point
Sameer Reply
The force per positive unit charge
Mpho
what is Kevlar made of ?
Crazy Reply
How do I determine the number of valence electrons for an atom
Kamohelo Reply
look at its group number on periodic table. group 2 elements will have 2 valence electrons and so on for all the groups. (correct me if I'm wrong but this is how I remeber it )
Siyabonga
how to find a point between to charges that is zero?
Thyrin Reply

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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