Silicon Semiconductor properties

March 21, 2020
Intel now makes processors
Why only silicon chip is used in computers? Does it have any special property?
Answer 1:

Great question! Silicon is an interesting example where material properties are not the only deciding factor for what makes it into consumer products.

Why is Si used in chips for computers?
Silicon is actually not the most optimal electronic material, but it is used instead of other materials with better electronic properties because it is cheap and extremely abundant. In fact, Si is the eighth most abundant element on earth (albeit, you need to do processing beforehand). It's part of what makes all of your electronics affordable, and is often the barrier for other materials to substitute it. But there's a lot of research across the world to discover materials that are comparable or better to silicon. It's an exciting time in electronic materials research!

In general, materials for electronic devices are semiconductors. This is because you want to be able to have a device that can switch between an on and off state at room temperature (otherwise you would need to use your computer in somewhere like a freezer or sauna!). Metals conduct electrons so it would be hard to switch them off, and similarly insulators do not conduct electrons, so turning them on would be equally difficult. Semiconductor materials have properties in that sweet middle spot that's just right. Silicon is an example of such a semiconductor. It is not the best electronic material, but it is good enough.

What makes Si a less than ideal electronic material?
There are most definitely many of reasons for this, such as lower carrier mobilities (e.g. electrons don't move as fast; recall that current is C/s, so slower carriers means you get less current out per second). You can compare it to germanium and gallium arsenide, which have better electronic material properties.

And some extra interesting stuff:
What distinguishes between a metal, semiconductor, and insulator?
The key feature that is often used to differentiate among metals, semiconductors, and insulators is the energy gap. In solid state physics, you find out that electrons in solids live at particular energies called bands. Metals have no gap between bands, which means that electrons can flow between bands and conduct. Semiconductors and insulators have a gap between bands, with the upper chunk of bands called "conduction bands" and lower chunk of bands called "valence bands"; semiconductors have a smaller gaps. This is better understood with a schematic. It turns out that these bands vary in energy as you move across these materials in different directions, so that there are energy minima and maxima between the valence and conduction bands. The band gap is often defined as the energy difference between the conduction band minimum (CBM) and valence band maximum (VBM) (also better understood with a picture; the momentum axis is essentially a particular spot and direction in the material). In this schematic, the CBM and VBM are directly between each other; this is called a "direct band gap". But this need not be the case, the CBM and VBM could be located in different spots, in which case the gap is an "indirect band gap."

This is important for Si in solar cells for instance. Silicon has an indirect band gap, which is less efficient than materials with direct band gaps. But because Si is so cheap and abundant, it is still the dominant material used in making solar cells.

This is quite a bit to absorb (and is in fact a condensed version of many physics and materials science classes), but if this peaks your interest, I'd recommend looking into materials science for future study!

Hope this helps!
Answer 2:

A computer chip needs to be built out of a semiconducting material, and most of these chips are indeed built with silicon even though there are other possible materials.

The most important reason for why silicon is the most popular material for computer chips is that billions of dollars and incredible amounts of time have been invested over the past several decades perfecting the process of making very reliable, ever smaller transistors with millions and sometimes billions of working transistors per chip. So even if another material might be better than silicon, a huge amount of time (many years!) and money (billions of dollars!) would still be needed to perfect the designs and manufacturing process.

The first transistor was created in 1947 at Bell Labs and was made with Germanium, another semionducting material. The first silicon transistor was created in the 1950's. Transistors can also be made from many other materials.

One reason for why silicon was chosen over germanium is that silicon operates better at high temperatures because the bonds with the electrons are stronger in silicon than in germanium. Germanium has weaker bonds to its electrons and at high temperatures these bonds may be broken and lead to worse performance.

Another reason why silicon became dominant is that it is very easy to create a high quality thin insulator on the surface of a silicon chip, because you can just put silicon in a hot furnace with oxygen and it will form a thin film of silicon dioxide which performs as an excellent insulator which gave it a huge advantage when MOSFETs were first created. Modern computer chips are formed entirely with insulated gate MOSFET devices...

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