# Explain what is a transistor in plain English!

Can someone care to explain - in plain English - what a transistor is and how it works. I know computers are full of these but don't know much about them.

P.S. I know about this site from another one in the family of StackExchange (I have no engineering background) and hoped someone might help me with this. Please don't link to the wiki explanation. I've read that and I'm more confused.

I'm looking for a 7 year old explanation.

Thank you!

• See this answer to a similar question. – Martin May 19 '11 at 12:18
• possible duplicate of I don't care how a transistor works, how do I get one to work? – Kellenjb May 19 '11 at 12:34
• Eh, not sure if it is an exact duplicate. Slight difference between "How do I get one to work" and "How does it work". – Kellenjb May 19 '11 at 12:36
• The "Transistor man" cartoon is intended to show how a transistor works. A seven year old should be able to understand it. – Leon Heller May 19 '11 at 13:29
• If the Wikipedia explanation is confusing, then the solution is to fix it. Also, the link given is to the regular Wikipedia page. There is one in the Simple English Wikipedia: simple.wikipedia.org/wiki/Transistor – Kaz May 25 '14 at 13:20

For the moment, I'm going to lump together transistors, FET's (Field Effect Transistors), and relays. There are some other devices that could be lumped in there as well, but...

At the most basic level, all of these devices operate as a switch. Do something, and electricity will start or stop flowing. Stop doing something, and the electricity flow will change.

A relay is the easiest for beginners to understand. Current flowing through the coil causes an electromagnet to become magnetized. This makes a mechanical switch "flip". Turn off the current in the coil and the switch flips the other way.

In the case of a transistor, when current flows through one of the pins (called the "base") it causes more current to flow through the other two pins (the pins are called the collector and emitter). No current through the base means that there is no current flowing through the other two pins.

A FET is similar to a transistor, but instead of a current flowing through the base, it is a voltage on the base. And just to confuse matters the pins are not called Base, Emitter, and Collector. Instead they are called Gate, Source, and Drain. But the operation is very similar. Have the correct voltage on the Gate and electricity will flow through the other two pins. Don't have the correct voltage and the flow will stop.

Another important detail is that the transistor or FET doesn't have to be "fully on" or "fully off". If the Base or Gate is somewhere in between fully on & off, then the flow of electricity through the device will be "a little on" or "a little off". This doesn't work for relays.

And that's the beginner level explanation. Of course I glossed over a lot of details, but they are not important at this stage. What is important is that you can take Transistors and FET's and combine them in interesting ways to make all the cool electrical devices that we can't live without.

• +1 - Very difficult to answer this kind of question and make it accessible, interesting and correct. Nicely worded. – MikeJ-UK May 19 '11 at 15:56
• I hadn't seen this before, +1. – Olin Lathrop Mar 21 '12 at 13:11
• it's beautiful! – eGovind Mar 24 '13 at 13:45

At a 7 year old level, a transistor is like a controllable valve or tap (or faucet for our US friends)

A small turn of the tap (or small increase in voltage between the base and the emitter) creates a large flow (or large voltage between the collector and emitter)

So at a very basic level it acts as an amplifier.

However, in a computer it is generally used as a switch. Outside the small region where varying the base voltage drives a larger variation in the collector voltage it is effectively binary, so for allowed input voltages the output is always 0v or Vmax (this value depends on the power supply, type of transistor and circuit etc)

A bit more detail, while still not being too technical is on Wikipedia.

(and yes, I know I used flow and voltage in the same sentence:-)

• The controllable valve part is correct, but I don't like it when people say a transistor "acts as an amplifier", especially not to newbies. It's used to build amplifiers. An amplifier is the whole assembly, including the power source and a controllable valve. Transistors do not have a power source inside. The first amplifiers used carbon microphones as their controllable valve, but people don't call capsules of carbon "amplifiers". – endolith May 19 '11 at 14:02
• The output or current flowing through is related through multiplication of the input, so saying that it amplifies, is not incorrect. Fits with the dictionary definition of the word. – old_timer May 19 '11 at 16:41
• back to the faucet or valve or tap, turn it on a little you get a little water, but at some point you turn it on all the way and you dont get any more water, there is a limit, that is when the transistor becomes a switch, close the faucet, off, or open the faucet at least to the point that the flow is at a maxium and the switch is on. – old_timer May 19 '11 at 16:43
• I understand endolith's concern. Just because a transistor amplifies base current, doesn't make it an amplifier. An amplifier is a complete device. Logic gates can compute, but gates does not a computer make. – ajs410 May 20 '11 at 18:37
• A transistor definitely amplifies - it exactly fits the definition...within that small region. Outside that it approximates a switch well enough to be used as part of a logic gate - but that is due to the amplifier aspect of it being either cranked up against the limit of VCC or down at 0. – Rory Alsop May 20 '11 at 23:27

For a 7 year old:

Imagine you have a game controller stick that moves a couple of inches each way. It's connected to a robot arm that moves a couple of feet each way. The movements are just the same, but bigger.

Similarly, a transistor maps a lower-voltage current onto a higher-voltage current.

So, a small low-current circuit can use a transistor to control a motor or somesuch that requires a much higher current flow.

• That's a usage, not a description. – Andres Riofrio Mar 21 '12 at 2:53
• That's because I'm talking to a 7 year old. :) – Mark Harrison Apr 22 '14 at 17:45

If you imagine a sandwich made of two slices of copper bread separated by a slice of plastic cheese hooked up like this:

                     +-------------+
|             |
[COPPER]         |+
+-------[CHEESE]    [BATTERY2]
| +     [COPPER]         |
[BATTERY1]     |             |
|          |             |
+----------+-------------+


Then it's easy to see that no charge will leave either battery because the cheese is an insulator.

However, if the cheese is magical and becomes more and more copper-like as the voltage of battery1 is increased, more and more charge from battery2 will flow through the sandwich as the cheese becomes more and more conductive.

With the battery polarities shown and, in reality, the bread being "N" type silicon and the cheese being "P" type silicon, that's how transistors work.

• I don't understand the -2 vote, since from the POV of a 7 year old, using copper as an analog for "N" type material and cheese for "P" type material yields the classic bipolar transistor structure we all recognize, with the collector corresponding to the top copper conductor, the base corresponding to the cheese insulator, and the emitter corresponding to the bottom copper conductor. In reality, when the voltage across the base-emitter junction is increased, the base will become more and more "N" like, allowing more and more charge to flow through the now more "NNN" transistor-like sandwich. – EM Fields Jun 1 '14 at 8:50

I write this as another answer (instead of a comment) because of the following reason:

If I were a newbie/beginner (perhaps somewhat older than 7 years, for example: 15) I would be confused about a specific descreapancy to be observed in some of the answer. Here are some quotes

Similarly, a transistor maps a lower-voltage current onto a higher-voltage current.

In the case of a transistor, when current flows through one of the pins (called the "base") it causes more current to flow through the other two pins (the pins are called the collector and emitter).

A small turn of the tap (or small increase in voltage between the base and the emitter) creates a large flow (or large voltage between the collector and emitter)

As a beginner I ask myself (because I want to understand the working principle of the BJT): What is the controlling quantity: Current or voltage?