Semiconductors Defined

From: Subject: Semiconductors Defined Date: Mon, 17 Mar 2008 12:22:21 -0700 MIME-Version: 1.0 Content-Type: multipart/related; type="text/html"; boundary="----=_NextPart_000_00C5_01C88829.8CC88F90" X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3198 This is a multi-part message in MIME format. ------=_NextPart_000_00C5_01C88829.8CC88F90 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: mhtml:http://googlemail.com/attachment?ui=2&ik=c3000552be&realattid=f_fdw0tvia1&attid=0.2&disp=safe&view=att&th=118b923feb996151&saduie=1egname5ggcxpt9w0hz1a4jhb1xr3mi&sadet=1205781749368&sads=53ba4a51a4d1bd0acbba337b2c19b1fd Semiconductors Defined Semiconductors Defined = -=20 35

The band structure = described on=20 the previous page seems to have no room for semiconductors. =20 Conductors have half-filled valence bands, so electrons move = freely. =20 Insulators have full valence bands, making it difficult for = electrons to=20 move. But if an electron in an insulator can gain enough = energy to=20 jump to the (empty) conduction=20 band, available states abound. Materials with = properties we=20 now associate with semiconductors were first identified in the = early=20 1800s, but they remained little more than a scientific curiosity = until the=20 1900s. Over time, scientists discovered that they could = control the=20 conductivity of certain materials, turning a good insulator into a = decent=20 conductor by changing certain attributes, such as the temperature = of the=20 substance or the amount of impurities found in it. These = materials=20 that could conduct upon demand were called semiconductors. =20 Semiconductors made of one material (such as silicon) with no = impurities=20 are called pure,=20 or intrinsic,=20 semiconductors.=20

Click on the Semiconductor image to the = right to=20 open up an animation of conduction in = semiconductors.

Band diagram of a=20 = Semiconductor

<= /CENTER>Another = Word of=20 Warning: As was emphasized before, the motion of = electrons in=20 solids is complex. Not only do the diagrams and animations = included in=20 these materials display an infintesimal percentage of the number of = electrons=20 actually present in a physical device and show motion only in one = dimension,=20 they also exaggerate the number of conduction electrons. Pure = Silicon at=20 room temperature has perhaps one conduction electron for every=20 10¹³ (that's ten trillion) atoms.¹ The=20 author begs your forgiveness for not drawing all = 10¹³ atoms for each conduction electron but feels the important = physics is=20 better demonstrated by the simpler, albeit somewhat inaccurate, diagrams = and=20 animations.=20

The difference between conduction in semiconductors = and=20 conduction in conductors is evident in the effect of temperature on=20 resistance. You may have learned in a physics class that resistance=20 increases (and conductivity=20 decreases) as a resistor gets hot. Heating any device results in = more=20 atomic vibrations. If the atomic cores=20 are vibrating more, electrons will have decreased mobility. Voila = -=20 increased resistance. This is the end of the story for conductors, = but the=20 resistance of semiconductors depends upon temperature in an additional=20 manner. Increasing the temperature of intrinsic semiconductors = provides=20 more thermal energy for electrons to absorb, and thus will increase the = number=20 of conduction electrons. Voila - decreased resistance. This second = effect in=20 semiconductors is much greater than the effect of atomic vibrations, so=20 increasing the temperature of a semiconductor ends up decreasing = its=20 resistance.=20

¹ Numbers taken from The Quantum = Dot, by=20 Richard Turton, a very good book that has unfortunately gone out of=20 print.
=20

What exactly is = the=20 relationship between the number of conduction electrons and the = familiar=20 concepts of resistance and resistivity? =
Go to the next page to find=20 out!

Copyright =A9 2003=20 Doris Jeanne Wagner and Rensselaer Polytechnic Institute. All = Rights=20 Reserved.

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