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Why are semi metals (conductors) used in making photo electric cells ,transistors and microchips in computers?
Question Date: 2016-09-26
Answer 1:

An excellent question! In short, the electronic properties (particularly the electrical conductivity) of semiconductors can be engineered and controlled. See below for the more materials science explanation.

You've probably learned that metals conduct electricity easily, semiconductors less easily, and insulators not at all. This is partially true, but it really depends on the temperature and even the material you are talking about. Most of the time we talk about room temperature because that's the temperature we and our computers operate in. However, it's possible to make a semiconductor conduct electricity or a metal turn into an insulator depending on the temperature.

The most basic level a computer processes information is in binary (long strings of 0's and 1's). This system of 0's and 1's is encoded in the hardware of the computer as no current (== 0) or some current (==1). This also works with voltages, polarized light, magnetism, or any signal/material property that can have two distinct states.

Semiconductors are the basis of many electronic devices because we can control the electronic conductivity (specifically around room temperature). Silicon has been the material of choice, mostly because it is cheap, abundant, and has good enough electronic properties, though there is active research in looking for alternate semiconducting materials. We control the conductivity via doping, or intentionally incorporating impurities.

Depending on the impurity incorporated, the resulting semiconductor is called either p-type or n-type. The naming is based on if the carriers responsible for the conductivity are electrons or holes (which can be thought of as the lack of an electron).

In materials science, we have a more stringent definition for whether a material is a metal, semiconductor, or insulator based on whether or not it has a band gap. When we talk about material properties, we are mostly concerned with electrons and the energy levels they can live in. In metals, there is overlap between the energy levels with electrons and those without. This is basically the physics for why electrons are able to conduct so easily in metals.

In semiconductors and insulators, there is a range of energies for which no electrons can exist, aka a band gap. Semiconductors have smaller band gaps compared to insulators. Electrons can jump over the gap with sufficient amount of energy. This can be thermal (i.e., heat) or electromagnetic (i.e., light). For an illustration see here . For any practical purposes, you can't really "turn off" the conductivity in a metal, and the amount of energy to make insulators conductive is too high.

For silicon, the p-type or n-type doping introduces carriers that can conduct at room temperature. The main unit for most electronic devices is the p-n junction , which consists of putting p-type silicon together with n-type silicon into once device. This configuration lets you control current depending on the bias voltage you put on the device. Other electronic elements like the transistor, LED, solar cells are simply more complicated designs, but all operate using the same principles as the pn-junction. More info here on the pn-junction , if you're curious.

Hope this helps!

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