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mph_Ragnarok
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Silicon is a insulator or semiconductor ?

Mon Jun 12, 2006 7:56 pm

My chem textbooks and my chem teacher tell me that silicon is a semiconductor. That means it partly conducts while it partly insulates? Whats that even mean ?? Thats like saying, " turn the light half on " . Its either on or off.......

Anyways, it gets even more confusing for me. I was reading about CPU fabrication and it said that pure pure silicon crystals are actually insulators ?? ( I thought they were " semi conductors ") and that they needed to make it dirty with germanium to actually let it conduct a little ?

And then they add copper wires into the silicon slices already dirty with the germanium, so that the copper wires can carry the electricity, but then if the surrounding silicon/germanium is already semi conductive, doesn't the whole situation turn into a mess ?
 
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Mon Jun 12, 2006 8:00 pm

By semi it means under certain configurations and environmental conditions, it will conduct electricity.

As usual, Google and Wikipedia are your friends.
 
daveagn
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Mon Jun 12, 2006 8:25 pm

Pure silicon is an insulator. They dope it with other elements to make it useful in electronics.
 
SNM
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Mon Jun 12, 2006 8:46 pm

Okay....
So, for an element to conduct, it needs to have free electrons that aren't strongly bonded that are available to flow through the material. Metals have lots of these free electrons; non-metals...don't.
Silicon is in a special place with regards to its electronic configuration. It doesn't have its own free electrons, but when you mix it with small amounts of nearby elements (like germanium, mixing it with these elements is called "doping"), then those elements take on silicon's electronic configuration (because the germanium forms electron bonds with the silicon) and there are now free electrons floating around from the germanium.
There aren't very many of these electrons, though, and I think they remain more tightly stuck to the germanium nuclei than electrons in metals. So the electrons can flow (and thus carry a signal current) only when subjected to the right kinds of electrical stress. The upshot is that by applying current (or maybe just voltage...) to one side of a transistor (forming transistors is the whole point, after all), you apply the right electric stress to the transistor to allow the freer electrons from germanium to flow through in directions perpendicular to the applied current.
Since the doped silicon can conduct only if it's also got another voltage source to its side, it's called a semi-conductor.


If this is unclear; I apologize, but I'm trying to dredge up some hated chemistry from last fall and electronics stuff from a bad computers class in high school. ;)
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FireGryphon
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Mon Jun 12, 2006 10:13 pm

mph_Ragnarok wrote:
My chem textbooks and my chem teacher tell me that silicon is a semiconductor. That means it partly conducts while it partly insulates? Whats that even mean ?? Thats like saying, " turn the light half on " . Its either on or off.......


Recall that there are different energy levels around a molecule, and a cerain number of electrons orbit the molecule in each energy level. The highest energy level that has electrons in it is called the 'valence' level, iirc. Electricity occurs when electrons from this valence layer are excited into a high enough energy state to flow with an electric current. How those electrons are 'lifted' is what separates a conductor, from an insulator, from a semiconductor.

Most conductors need a very small amount of voltage applied to them in order to excite their valence electrons. Because you need such a small amount of voltage, it is very easy to conduct electricity with these elements, so they are classified as conductors.

Insulators are elements that form very tight bonds with one another. Their valence electrons require very large amounts of voltage in order to excite them to an energy level where they can flow with electric current. Typically, applying too much voltage to an insulator such as wood will cause the insulator to burn way before its valence electrons are excited enough to conduct electricity. Since they effectively insulate current, they're called insulators.

Semiconductors are like a combination of both. A semiconductor will conduct electricity, but only if you apply a certain amount of voltage to it. Whereas a conductor will conduct electricity even with a very small voltage, a semiconductor will only conduct electricity if you apply, say, a medium amount of voltage; less than an insulator, but more than a conductor.

I don't feel like looking up exact numbers, but let's say that a typically doped silicon diode requires 1 volt to conduct electricity. You can apply .9V, and it'll just sit there as an insulator. Once you apply 1V or more, its valence electrons will be excited to a high enough energy level that they can join the current band and conduct electricity.

You can see how useful semiconductors are for computers. With a small amount of electricity, nothing happens (the 'off' state) but with a moderate increase in voltage, you have an electric current (the 'on' state). This is why binary is so important: 0 == off (low voltage), 1 == on (high voltage).

Silicon in its natural form is more or less an insulator (it conducts some electricity, but not much), so it is typically doped with other elements to make it a more usable semiconductor. Germanium is also a semiconductor.


Anyways, it gets even more confusing for me. I was reading about CPU fabrication and it said that pure pure silicon crystals are actually insulators ?? ( I thought they were " semi conductors ") and that they needed to make it dirty with germanium to actually let it conduct a little ?


Germanium is a semiconductor just like Silicon. Doping elements for silicon typically include, but are not limited to, things like phosphorus and boron. This is because phosphorus and boron have five valence electrons (silicon and germanium each have four). In a pure silicon or germanium lattice, all four valence electrons will be used to bond to other molecules. After dopoing, however, when the bonds are all formed, there's an extra electron: four valence electrons from the dopant bond to the semiconductor's four electrons, and the dopants last valence electron is free to be picked off.

Ninja edit: Rewritten for clarity.
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just brew it!
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Mon Jun 12, 2006 10:51 pm

Semiconductors like silicon and germanium exist in a sort of limbo between conductors and insulators.

Pure silicon does not conduct electricity well (though still somewhat better than insulators). But elements from silicon's column of the periodic table have an interesting property: when doped with small amounts of impurities from the columns to the left or right, the crystal structure ends up with a surplus (N type) or deficit (P type) of electrons. In an N type doped semiconductor, current can flow in the conventional way, i.e. by electrons flowing through the material. In a P type doped semiconductor, current can flow by having the absence of an electron -- i.e. a "hole" -- move through the crystal structure, in the opposite direction.

When P and N type doped semiconductors are juxtaposed, a diode is formed. A diode is a simple one-way electrical gate which allows current to flow only in one direction. When the junction is "forward biased" (positive voltage applied to the P end and negative voltage to the N end), holes are pushed towards the boundary from the P end and electrons are pushed towards the boundary from the N end; the holes and electrons annihilate each other (sort of like matter and anti-matter) at the boundary, allowing current to flow. OTOH when the junction is "reverse biased" (negative on the P end and positive on the N end), the holes and electrons are pulled away from the boundary, and very little current flows.

A 3-layer sandwich (N-P-N or P-N-P) forms what is known as a bipolar transistor; this was the most common type of transistor up until the early 1970s. Bipolar transistors allow a small current applied to the middle layer to control a much larger current flowing between the two outer layers.

Nearly all contemporary chips use MOS (metal oxide semiconductor) transistors, in what is referred to as CMOS (complementary MOS) configuration. In CMOS, the transistors have a channel composed of P or N type semiconductor, and a 'gate' electrode which is insulated from the channel by a layer of silicon dioxide only a few atoms thick. A voltage applied to the electrode can attract or repel the electrons or holes in the channel, switching the channel on or off (i.e. switching it from low resistance to high resistance, or vice-versa). Since CMOS circuits are operated by voltage levels instead of current flow like the older bipolar designs, they use much less power; this is what made the whole consumer electronics and personal computing revolution possible.

SNM: Minor nit... germanium is from the same column of the periodic table as silicon. To produce charge carriers, you need to dope with elements from the columns to the left or right. Germanium was actually used as the raw material for transistors before silicon was; it isn't a dopant per se (though IIRC it is sometimes used to alter the crystal structure in other ways... e.g. in stressed silicon designs).
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Action Jim
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Mon Jun 12, 2006 11:07 pm

Some background from an earlier post of mine:

A very brief sketch of the band theory of conduction: There are really four types of standard electronic conductors: insulator, semiconductor, semimetal, or conductor. The difference between these types of materials is a result of their different band gaps.

So what is a band gap? Quantum mechanics tells us that electrons around an atom exist in discrete energy states. When many atoms interact, these states are pertubed slightly, resulting in a range of energies that atoms can exist in a material. For some materials due to their atomic characterisitics and crystalline structure, there is a range of energies that electrons cannot access. This range of energies is called a band gap. The band gap separates the valence band, which is composed of the outer shell electrons of the atoms, and the conduction band, which are the electrons in a material that are available for electronic conduction. At absolute zero, all of the electrons in a material are in the valence band - this is called the ground state of the material. With increasing temperature, some electrons can be promoted to the conduction band (this number is based off of Fermi-Dirac statictics).

In a conductor, the energies of the valence band and the conduction band overlap, ensuring that some electrons are always available for conduction. In a semimetal, the bandgap os a material is effectively zero, making is very easy for the material to have a high number of conduction electrons at room temp. A semiconductor has a low band gap (for example, the band gap of Si is 1.1 eV), making it an insulator who's conductivity increases with increasing temperature (as more and more electrons possess enough energy to overcome the band gap). An insulator is a semiconductor with a very large band gap, making it very unlikely that electrons can contribute to conduction.


We can see by this that the main defining characteristic of a semiconductor is that it's conductivity increases as the temperature of the material increases, because more electrons are made available for condution. This is indepenedent of applied voltage.

And to elaborate on conductivity the role of dopants in a semiconductor( from here):

For electronics, a big issue is the doping levels in both diamond and Si. Intrinsic Si would be useless for devices. In order to make a device out of silicon, regions of the silicon are implanted with impurities (usually boron or phosphorus), or doped. These doping levels result in the appearance of states within the band gap near either the valence band (B) or conduction band (P). These states in Si are separated from the main bands of the material by a very small energy, making it very likely that most of the doping electrons are promoted into their respective bands, and the creation of conductive areas on a Si wafer. This allows for the creation of transistors, which are composed of different regions of doped material where applied voltages can control the passage of electrons, resulting in switching.


So, Si is not an insulator when it is undoped, it is still a semiconductor. By doping, we can adjust the conductivity of the Si.
Last edited by Action Jim on Mon Jun 12, 2006 11:09 pm, edited 1 time in total.
Good bye everyone.
 
SNM
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Mon Jun 12, 2006 11:08 pm

Wow. My post just got pwned. So much for the low-chem approach. :o

jbi wrote:
SNM: Minor nit... germanium is from the same column of the periodic table as silicon.

Oh, my bad. I just went with what Ragnarok gave me. ;)
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just brew it!
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Mon Jun 12, 2006 11:10 pm

Whoa there, AJ... you're gonna scare the poor guy off! :lol:
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Action Jim
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Mon Jun 12, 2006 11:13 pm

just brew it! wrote:
Whoa there, AJ... you're gonna scare the poor guy off! :lol:


Sorry, I just get excited when I can post something technical here.

I'll go back to talking about poop now...
Good bye everyone.
 
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Mon Jun 12, 2006 11:20 pm

Action Jim wrote:
just brew it! wrote:
Whoa there, AJ... you're gonna scare the poor guy off! :lol:

Sorry, I just get excited when I can post something technical here.

I'll go back to talking about poop now...

Hehe... :lol:

But given all the followups people have posted, I think we've given a pretty decent broad-brush overview of "Semiconductors 101". :D

I leave you all with this link, for your edification and/or amusement. :wink:
Last edited by just brew it! on Mon Jun 12, 2006 11:20 pm, edited 1 time in total.
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FireGryphon
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Mon Jun 12, 2006 11:20 pm

just brew it! wrote:
Whoa there, AJ... you're gonna scare the poor guy off! :lol:


You know, the explanations in this thread start out simple and get more complex. Reading the thread in order is a pretty good approach. You can stop whenever your head starts to hurt. :)

just brew it! wrote:
I leave you all with this link, for your edification and/or amusement. :wink:


I came across that when I took quantum physics in college. It was pretty useful. :wink:
Last edited by FireGryphon on Mon Jun 12, 2006 11:28 pm, edited 2 times in total.
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just brew it!
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Mon Jun 12, 2006 11:28 pm

FireGryphon wrote:
You can stop whenever your head starts to hurt. :)

This rule could be applied to many other aspects of life as well. Software development... listening to loud music... drinking.

I think you may gave stumbled onto a universal truth here! :D
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FroBozz_Inc
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Tue Jun 13, 2006 8:03 am

Action Jim wrote:
Sorry, I just get excited when I can post something technical here.

I'll go back to talking about poop now...


8) :lol: Heh! But would p00p be an insulator or a semiconductor?

:D
 
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Tue Jun 13, 2006 9:05 am

FroBozz_Inc wrote:
8) :lol: Heh! But would p00p be an insulator or a semiconductor?

:D

That probably depends on whether it's still wet, or dried out. And on what the producer was consuming. :D
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FireGryphon
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Tue Jun 13, 2006 9:12 am

just brew it! wrote:
That probably depends on whether it's still wet, or dried out. And on what the producer was consuming. :D


I don't think corn conducts electricity too well.
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Buub
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Tue Jun 13, 2006 2:06 pm

What a bunch of BS. That stuff doesn't make any sense.

You know it's all done with Magic. Why do you think people who do "magic" tricks these days are all actually illusionists? Because Intel and AMD are using up the Magic as fast as they can make it!
 
FroBozz_Inc
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Tue Jun 13, 2006 2:19 pm

IC's all run on magic smoke anyway.

When a part blows up? The magic smoke is let out and that's why it's dead.

We all strive to not let out the magic smoke! :D
 
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Tue Jun 13, 2006 2:40 pm

FroBozz_Inc wrote:
IC's all run on magic smoke anyway.

Yup. I have an exclusive deal with the companies for delivering the smoke.

But honestly, have anyone of you watched a chip die emanting the blue flame? I've never screwed up anything that bad that it had to go in flames :P
 
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Tue Jun 13, 2006 3:51 pm

The original Apple II floppy drive had two interesting properties:

1. Power was supplied over the same ribbon cable which carried the data signals.

2. The cable wasn't keyed.

The consequence of plugging the cable in backwards was that flames would shoot out of two ICs on the floppy drive's logic board, leaving holes (in the approximate shape of the silicon die) in the tops of the IC packages.

A lot of Apple II floppy drives suffered this fate back in the day.
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