Why is silicon used to make microchips?
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.
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)