# An explanation of a transistor

1. Nov 2, 2013

### res3210

Hey guys,

So I built a circuit a little while back using a flyback transformer and a 2n3055 transistor. Basically it just gave me a massive hi voltage output an I saw little inch long plasma arcs. If you guys need a circuit diagram, I can send you the link or just post it here. The main point of this, however, is that it got me to thinking that the transistor was the main reason why the FBT was working (since it uses a 12v DC source for current). I am just wondering if anyone can provide a good explanation for how exactly a transistor works? Or could you provide me with some recommendations for reading materials? I'm a second year physics major and I have a fairly good understanding of electrical systems, so I'm not too worried about the rigor of the reading materials, just the clarity. Any and all help would be greatly appreciated!

2. Nov 2, 2013

### cabraham

A 2nd year student must be careful w/ high voltage. This arc you created has to do with operation of inductors as well as transistors. Please refrain from any circuit with shock hazard. When you study inductors you will gain insight.

3. Nov 2, 2013

### res3210

I'm fairly familiar with inductors, but I just don't understand how the AC current is generated in order to cause induction.

4. Nov 2, 2013

5. Nov 2, 2013

### meBigGuy

If a switch turns on and off repeatedly, it essentially creates AC at the frequency at which it switches.

When the switch closes there is current in the inductor creating a field, and when the switch opens the field collapses and generates whatever voltage is required as the inductor tries to maintain the current flow. The transistor operates as a switch. When a small current is driven into the base (to emitter), a large current can flow from collector to emitter. The flyback design exploits that.

With that as a high level view it is worthwhile to understand more about exactly how the transistor is doing what it does, especially with your physics background. The issues will come up again and again so are worth learning now.

6. Nov 4, 2013

### sophiecentaur

Personally, I found that most explanations of the operation of a transistor were too complicated for a beginner. The initial (only) appreciation of what's going on doesn't really involve solid state Physics. You can look upon an amplifying device as a variable resistor and the input signal corresponds to varying the angle of the knob. Some transistors are used as simple 'switches',being either ON or OFF, according to the state of the input. The amount by which the transistor is switched on depends upon the value of a small amount of current that is fed into the base connection.
Thereafter, things get much harder. Imo, the old thermionic valve was much easier to explain because it just involved electrons flowing through a vacuum and the effect of the volts on a grid of wires on the amount of current that flowed from the Anode. My Dad explained it to me when I was very young and it made good sense. I don't think the transistor thing would have (but he didn't know about them in the 1950s).

7. Nov 4, 2013

### res3210

So does the current start by flowing into the base of the transistor? Either that or the emitter? It looks like there is tunneling involved with the electrons in the emitter, as they have some probability to be absorbed into the base, or diffuse across the base into the collector. What I am confused about mainly still is how this process causes amplification.

8. Nov 4, 2013

### Okefenokee

No, it's not about tunneling. It's about the quantum statistics inside semiconductor crystals.

Think of a diode. A diode blocks current in one direction because it has an empty region inside (no carriers there). If you apply a forward voltage across a diode the empty region will shrink until the diode can conduct.

A solid state trasistor is a double ended diode. It blocks current in both directions unless you do something to the base. In the case of a BIPOLAR JUNCTION TRANSISTOR (BJT) you must inject current into the base. The base current gets swept across the deplected regions by built-in voltages and causes conduction. Its a diode that you can turn on and off.

A FIELD EFFECT TRANSISTOR (FET) is a little different. An FET has a base that connects to a conductive plate adjacent to a region with few charge carriers. If you apply the correct voltage to the base then charge carriers will be attracted to the plate which create a channel for conduction.

That was a very lean description of how transistors work. There's much more to the physics of how they work. You could make a whole career out of studying it if you wanted to. Not that I did, but I at least took a 4 credit hour course on it.

From a circuit perspective, the most important thing to know is that a transistor is an amplifier.

Think of it like a water spigot. A small power input (turning the spigot) controls a large power output (the flow of water).

I can recommend Semiconductor device Fundamentals by Pierret. It's an undergrad textbook for electrical engineers. It's a dense book though.

9. Nov 4, 2013

### meBigGuy

1. How does the circuit work
2. How does a transistor work.

Two different subjects.

In the circuit you are describing (a 1 transistor self exciting flyback), The base-emitter current causes the transistor to conduct from collector-to-emitter (Beta*Ib). When the transformer field builds up, a reverse current induced in the base winding causes the transistor to shut off. The Field collapses, causing a spark. The field collapsing allows the transistor to conduct again and the cycle repeats.

The physics of how current into the base-emitter junction of a transistor causes a larger current to flow from collector to emitter is basic transistor theory. It can be as shallow or as deep a subject as you want. A good place to start is what Okeefenokee said. Don't listen to anyone tell you how hard it is. Try this for an overview of a switch http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html then this for a little more switch detail http://www.electronics-tutorials.ws/transistor/tran_2.html and then go here to start to learn about NPN junctions. http://en.wikipedia.org/wiki/Bipolar_junction_transistor Or, just start at the last article and then go to the other two..

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook