Monday, 9 March 2015

Quantum Stuff #4

 Quantum Stuff  #4


This week we will be looking at particles on a quantum level in the form of the component that makes the modern day computer work. The transistor.

Transistors can be either on or off. It controls the flow of electricity by letting it through and cutting it off. It works because of semi-conductors. So what is a semiconductor?

A semiconductor conducts electricity better than an insulator but nearly as well as a metal. Pure silicon is an example of this. Because of the 4 electrons each atom has it can easily form a tetrahedral crystal but bonding to its 4 nearest neighbours. And few particles gain the energy to escape their bonds. These mobile charges make silicon a semi-conductor. 

But to make these semi-conductors useful in transistor a process called 'doping' has to happen. Doping is adding another substance to enhance performance the same way an athlete would but on an atomic level.

There are two types of doping. The n-type and the p-type.
The n-type is adding a 5 electron substance such as Phosphorus into pure silicon which adds an extra electron which makes it a better conductor as there are more mobile charges. It has a similar structure to silicon and can therefore fit into the structure.
The p-type is adding a 3 electron substance which leaves a positive hole (where there is no electron) where electrons move towards which creates this never ending flow of electrons which again makes it a better conductor. It has a similar structure to silicon and can therefore fit into the structure.

But what has this got to do with transistors? The n-type and p-type are placed together in layers with the semiconductors being insulated by an oxide layer (grey layer).

But the n-type and p-type do not stay to themselves and the n-type fills in the holes in the p-type silicon and cause what sic called the 'depletion layer'. So what's been depleted? The moving charges. With all of the spaces filled here is no longer any moving electrons. Because the p-type has no longer got any positive holes it becomes negative and repels any electrons trying to come across from the n-type silicon. So it acts like an barrier. This is the transistor off.

To turn it on you have to apply a small positive voltage to the gate. This thins the depletion layer and lets electrons get through which creates a conducting channel and therefore turns the transistor on. 

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