16
\$\begingroup\$

On my quest to better understand how computers work at a deep level I have come to the question of why, exactly, silicon is used in microchips. I always assumed, naively, that silicon had a very high electrical resistance and so it made a good material to sandwich other materials with low electrical resistance (i.e. gold) in. And that this was the way that microchips were made.

After actually doing some research I see that I was wrong and that silicon is a 'semiconductor'. To keep this short I'll just skip forward and just say that I don't understand what a semiconductor is and why it's good for making microchips. I've seen several explanations and they either confused me, or the explanations completely contradicted each other, but the basic gist is that a semiconductor is somewhere in-between a conductor and an insulator. Why is that useful for making integrated circuits?

\$\endgroup\$
  • 8
    \$\begingroup\$ A semiconductor is a material whose electrical behavior can be modified by impurities added to it (doping), and can be used to make circuit elements with varying resistances, voltage threshold behaviors, and so on, depending on influencing factors such as applied electrical and magnetic fields. Silicon is cheap, ubiquitous (common sand) and convenient, but Germanium and other semiconductor materials are also used where called for. \$\endgroup\$ – Anindo Ghosh Mar 5 '13 at 5:55
  • \$\begingroup\$ So, essentially, the manufacturer uses these impurities to create pathways where the electrical current can travel? Is that correct? \$\endgroup\$ – Cole Rowland Mar 5 '13 at 6:22
  • \$\begingroup\$ Not pathways: That would be more like a PCB, with copper as pathways. Actual materials behavior is modified, e.g. increase current through a base-emitter junction and the current through the collector-emitter junction increases, in a bipolar junction transistor, for instance, if the silicon at either side of the junctions is suitably "doped". \$\endgroup\$ – Anindo Ghosh Mar 5 '13 at 6:44
  • \$\begingroup\$ Sorry, 'pathways' may have been the wrong word. I meant that by adding different impurities you can adjust the chemical properties of the silicon thereby controlling how the electrical current 'flows' through the chip. Is that correct? \$\endgroup\$ – Cole Rowland Mar 5 '13 at 7:14
  • 5
    \$\begingroup\$ Why downvote this question? I have upvoted to restore it to 0. If you are going to downvote please provide feedback so that the question can be improved. \$\endgroup\$ – bhillam Mar 5 '13 at 7:34
18
\$\begingroup\$

Any of a number of semiconductor materials can be and are used, indeed the first transistor was actually a Germanium (Ge) transistor. the real reason why Si is so dominant comes down to 4 principal reasons ( but #1 is the primary reason):

1) It forms an oxide that is of very high high quality, seals the surface with very few pin holes or gaps. - this allows gap MOSFET to be more easily made as the SiO2 forms the insulating layer for the Gate, - SiO2 has been called the chip designers friend.

2) It forms a very tough Nitride, Si3N4 Silicon Nitride forms a very high bandgap insulator which is impermeable. - this is used to passivate (seal) the die. - this also used to make hard masks and in other process steps

3) Si has a very nice bandgap of ~ 1.12 eV, not too high so that room temperature can't ionize it, and not so low that it has to high leakage current.

4) it forms a very nice gate material. Most modern FET's used in VLSI (up until the latest generations) have been called MOSFET but in actual fact have used Si as the gate material. It turns out that it is very easy to deposited non-crystalline Si on surfaces and it is easily etched to great precision.

Basically the success of Si is the success of MOSFET, which with scaling and extreme integration has driven the industry. Mosfet's are not so easily manufactured in other material systems, and you can't drive the same level of integration in other semicondcutors.

GeO2 - is partially soluble

GaAs - does not form a oxide

CO2 - is a gas

Semiconductors are used because with selective contamination (called dopants) you can control the properties of the material and tailor it's operation and operational mechanisms.

\$\endgroup\$
  • \$\begingroup\$ en.wikipedia.org/wiki/Semiconductor_device_fabrication \$\endgroup\$ – sheetansh Mar 5 '13 at 11:17
  • \$\begingroup\$ This is a great answer. \$\endgroup\$ – Rocketmagnet Mar 5 '13 at 11:20
  • 1
    \$\begingroup\$ +1 but I would also think the high availability and low cost of the material is another good reason. \$\endgroup\$ – kenny Mar 5 '13 at 11:27
  • 1
    \$\begingroup\$ Silicon was already the dominant semiconductor material before FETs were commonly used. \$\endgroup\$ – Olin Lathrop Mar 5 '13 at 12:55
  • 1
    \$\begingroup\$ One more benefit of silicon compared to alternatives (like GaAs, which was the "semiconductor of the future" for many years) is physical robustness. From what I've been told, if you made a 200 mm GaAs wafer it would be likely to shatter if you just looked at it funny, This is one of the reasons GaAs fabs have stuck with much smaller (3" and 4"?) wafers, making GaAs that much more uneconomical compared to Si. \$\endgroup\$ – The Photon Mar 8 '13 at 16:44
6
\$\begingroup\$

To sketch why a semiconductor is good for creating circuits, start with your understanding that it is in between a conductor and an insulator, and add the fact that impurities (dopants) and other processing (oxide layers) can modify its behaviour to make parts of it conduct better, and other parts conduct worse. Add in the fact that electrical charges attract or repel each other (opposites attract, like charges repel).

Now imagine a channel where electrons can flow, insulated from a conducting layer nearby, which you control the voltage on. Make that layer negative, and its electric field repels the electrons in the channel - even through the insulator - stopping them entering the channel. Make it positive and it attracts electrons into the channel from the -ve terminal, where they can flow through it to the +ve terminal. So you can control the flow of current with the voltage on the insulated layer.

This is a Field Effect Transistor or FET. - the insulated layer is called the gate; the -ve terminal is called the source, and the +ve terminal is the drain.

As electrons flow in the channel, it is called a N-channel FET (N for Negative)

There are other devices you can build on a semiconductor, with more depth of understanding, but hopefully this is enough to show the basic principle.

As to why silicon? Of perhaps a dozen possible semiconductor materials, it is particularly convenient and reliable, as well as being almost as cheap as sand (which is mostly silicon dioxide)

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.