Monthly Archives: November 2015

Transistors; The Brain Cells of Computers

Like the neurons in human brains, billions of transistors allow for your computer to function. Despite having been invented 50 years ago, little is known about transistors by the general public; here’s a quick rundown to get you up to date.

Transistors are electronic components made of silicon. They can work either as amplifiers or switchers. Let’s start with their work as amplifiers:

Transistors are capable of receiving a small electric current at their input and releasing a much larger electric current at their output. This was first utilized for hearing aids; the transistors allowed for the electric currents generated by tiny microphones worn in the ear to then be converted into much louder sounds played out of a tiny loudspeaker.

Transistors can also function as switches in that the small current entering at the input then switches on a larger current that exits out the output. This is fundamental to how computer circuit boards function; whether or not a transistor is switched on (a current is run through it) can be read as a 0 or a 1, allowing for binary code to be stored via the presence or absence of an electric current. Computer chips contain billions of transistors.

transistor under microscopeBecause there are so many transistors, they have to be extremely tiny and correspond to extremely tiny electrical currents to all fit inside your smartphone. The most advances (and tiny) transistors work by controlling the movements of individual electrons, so they end up being so small you could fit 500 million to two billion on the surface of your fingernail.

You may be wondering how you even make something so small. Here’s how:

As mentioned before, transistors are made out of silicon, which is normally an insulator but in this case has been transformed into a semiconductor by introducing impurities (a process called chemical doping) into the material. The process works by either removing electrons from the silicon and making it more prone to being positively charged (called p-type) or adding electrons to make it more prone to being negatively charged (called n-type).

Once you’ve set up your two types of silicon, you can stick them together and fashion for yourself a kind of silicon sandwich; the junction between the two materials will become normal silicon because the lack and abundance of electrons will start to even out. Now you’ve got a diode (also called a rectifier). Diodes only allow for currents to flow through them in one direction because of the properties of the junction between the two slabs of different-typed silicon.

transistor under micro2To make a junction transistor, you have to actually use three layers of typed silicon, meaning your options are p-n-p or n-p-n. “Once electrical contacts are attached to all three layers, the component will either amplify a current or switch it on or off.

Check out a n-p-n transistor: let’s call the two contacts connected to the n-type silicon the emitter and the collector, and let’s call the contact connected to the p-type the base. Given that a small positive voltage is applied to the base, while the emitter is made negative and the collector is made positive, electrons are going to be pulled from the emitter into the base and then from the base to the collector. That means the transistor is turned on.

Remember that the base current can switch the amplified current on or off. It needs to be outfitted with a positive charge for the current to be amplified on its journey from the emitter to the collector.