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A semiconductor is a substance whose resistivity lies between that of a conductor and an insulator. The resistance of a semiconductor decreases as the temperature increases. Semiconductors are usually made from Silicon or Germanium

Conduction in semiconductors:

Conduction in semiconductors A Si atom has 4 electrons in its outermost shell. In a piece of Si at low temp. (near 0 K) each of these electrons forms covalent bonds with 4 other Si atoms

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As the temp. of the semiconductor is increased, these electrons gain more energy. Some gain enough energy to break free of their bonds, and wander through the piece of material. Once an electron moves out of a bond, it leaves behind a ‘hole’ in that bond

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This hole is positive, and so can attract nearby electrons which then move out of their bond etc. Thus, as electrons move in one direction, holes effectively move in the other direction Electron moves to fill hole As electron moves in one direction hole effectively moves in other

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NB. This is a difference between conduction in metals and semiconductors, in metals conduction is due solely to movement of electrons, in semiconductors it is due to movement of negative electrons and positive holes.

Intrinsic semiconductors:

Intrinsic semiconductors When conduction in a semiconductor is due to electrons moving from - to + and an equal no. of holes moving from + to -, it is called intrinsic conduction The resistance of an intrinsic semiconductor, even at room temp. is quite high

To increase conduction in a semiconductor:

To increase conduction in a semiconductor 3 ways (a) Increase the temperature As the temp. increases the no. of electrons which have enough energy to break free of their bonds increases, and so the conduction increases. A thermistor is a device whose resistance changes rapidly as the temperature changes

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(b) Shine light on surface. When light shines on some semiconductors, (e.g. cadmium sulphide) it gives sufficient energy to the valence electrons for them to break free of their bonds, and so increases conduction. Such a semiconductor is called an LDR Its resistance varies from several M Ω in darkness, to 100 Ω in daylight-used in automatic streetlight circuits

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(c ) Adding small amounts of impurities to the semiconductor can also effect its conduction. This process is known as doping


Doping Si has 4 valence electrons. B has 3 valence electrons. If a small amount of B is introduced into the Si lattice, at each location that a B atom occurs in the bond, there will be one electron missing, in other words, a positive hole exists.

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This means that even before you raise the temperature there are already holes in existence, which, if a pd is applied across the material will cause conduction to occur.

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Thus, in a semiconductor doped with B, there are more holes than electrons. Positive holes are thus the majority charge carriers, and the system is called a p-type semiconductor

N-type semiconductors:

N-type semiconductors If, instead, the semiconductor has small quantities of phosphorus (which has 5 valence electrons) added to it, then at each location where P occurs, there is an extra electron.

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Thus the semiconductor has more electrons than holes. The electrons are the majority charge carriers and the system is called an n-type semiconductor

Extrinsic conduction:

Extrinsic conduction -increased conduction due to the presence of impurity elements is called extrinsic conduction

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NB -it is important to note that even though there are extra FREE electrons or holes, the piece of material is still electrically neutral. Overall there are still the same TOTAL no. of electrons as protons

P-N Junction:

P-N Junction If a piece of p-type an n-type semiconductors are placed next to each other, at the boundary some of the electrons from the n-type jump the boundary to fill some of the holes in the p-type. This has several effects

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Firstly, the p-type has now gained extra electrons, and so is negatively charged. Similarly the n-type is now positively charged. Thus there is a pd across the material – this is known as the junction voltage. For Si the junction voltage =0.6V, for germanium = 0.2V

Depletion layer:

Depletion layer Also, at the boundary region (junction) there are no free electrons and no holes (as they have cancelled each other) Thus, in this region there are no free charge carriers, so no conduction can occur. This insulating layer is called the depletion layer

PN junction (diode):

PN junction (diode) Symbol Depletion layer forms an insulator between the 2 sides P type N type

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In order to get current to flow through the diode it is necessary to break down the depletion layer (i.e. drive free electrons and free holes into the layer)

Reverse biased pn junction:

Reverse biased pn junction If the + of the battery is connected to the n-type and the – terminal to the p-type, the free electrons and free holes are attracted back towards the battery, hence back from the depletion layer, hence the depletion layer grows. Thus a reverse biased pn junction does not conduct current

Forward biased pn junction:

Forward biased pn junction In this case the + of the battery is connected to the p-type and the – of the battery to the n-type, the free electrons and the free holes are repelled by the battery and driven into the depletion layer. Once the applied pd of the battery is greater than the junction voltage, the depletion layer breaks down and the diode conducts


exp To investigate the variation of current with voltage for a semiconductor diode Very NB experiment See book, p289 NOTE in the reverse biased case there is a vv small leakage current ( μ A). –this remains constant over a wide range of current, if current gets bigger the insulating properties of the diode break down and a large current flows which destroys the diode-happens at the breakdown voltage

Rectification of AC:

Rectification of AC A diode only allows current to flow in one direction. Thus a diode can be used to convert ac to dc See p 291 fig 25.17

Integrated circuits:

Integrated circuits Contains diodes, transistors, capacitors and resistors all fabricated on a single piece of silicon

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