How Transistors Work: A Simple Explanation for Beginners

In summary, a transistor is fundamentally different from a two-diode model and has specific regions designed to increase its effectiveness. The physics involved include thermal vibrations and the behavior of electrons or holes in the different regions of the transistor. The base current is made up of both injection and transport components.
  • #1
PainterGuy
940
69
Hello everyone, :smile:

I am trying to understand how a transistor works. Let's start with a NPN transistor. I read somewhere that NPN is just two diodes joined by sandwiching their 'P' regions between the 'N' regions. In other words by joining their anodes together.

Please have a see on this diagram:-
http://img577.imageshack.us/img577/8670/functioningoftransistor.jpg

I hope you understand the diagram. Actually both circuits are the same. In the second circuit I have used 'diode version' of a transistor. I have never problem accepting that D2 will be forward biased. "y" point will be more +ve than "x" point. But I do not understand how D1 will be forward biased because the point "w" (the cathode of D1) will be more +ve than the point "y" (the anode of the D1). In order for the D1 to be forward biased its anode should be more +ve than its cathode. Tell me please in simple way how this works. Much grateful for any help I can get.

Cheers
 
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  • #2
The two-diode model only works when you have no biasing, and are not using the transistor as a transistor. Just as two diodes tied together don't give you a transistor, a transistor (when used as such) does not act as two diodes tied together.

Nevertheless, this can be a handy thing to remember when you run out of diodes.
 
  • #3
MATLABdude said:
Nevertheless, this can be a handy thing to remember when you run out of diodes.

You owe me a new keyboard! :rofl:
 
  • #4
berkeman said:
You owe me a new keyboard! :rofl:

And after you run out of diodes, Zeners, and appropriate resistors, you can also cascade 10 of them together when you fry your last 7805 and need to drop from 12 V to 5 V!

Okay, okay, not so much. But I have seen a transistor kludged to work as a simple diode (and where large voltages weren't being encountered).
 
  • #5
The workings of a BJT transistor are fundamentally different from anything you could deduce from a two-diode model.

A transistor can act as a pair of common anode diodes (or common cathode for PNPs), but that's a sub-optimal use for them. Whereas diodes would be made with two approximately equivalent p- and n- type silicons, transistors have their regions specifically sized and doped in order to increase the size of the effect.

As for the physics, I'll do my best to explain it.

The understand the physics of an NPN transistor, you need to think about what the electrons are doing. For PNPs, the concept is the same, just replace "electrons" with "holes".

In normal operation, the BE (Base-Emitter) junction is forward biased, an the BC junction reverse biased. The emitter is very heavily doped, and teeming with free electrons, so when it is forward biased, thermal vibrations cause a huge amount of electrons to get pulled into the base. ("The cup runneth over", so to speak.) This is where is gets the name "emitter" from. The base is lightly doped, and very small compared to the emitter, so only a small fraction of the elections can combine with the holes, forming the base current. The electrons that don't recombine have nowhere else to go except to the collector.

The Wikipedia article has an interesting little paragragh:
To minimize the percentage of carriers that recombine before reaching the collector–base junction, the transistor's base region must be thin enough that carriers can diffuse across it in much less time than the semiconductor's minority carrier lifetime. In particular, the thickness of the base must be much less than the diffusion length of the electrons. The collector–base junction is reverse-biased, and so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector–base junction. The thin shared base and asymmetric collector–emitter doping is what differentiates a bipolar transistor from two separate and oppositely biased diodes connected in series.
 
  • #6
berkeman said:
You owe me a new keyboard! :rofl:
Was there something funny about that? I thought everyone did that at one point in time, especially during school. I did it once when I didn't have enough diodes in my kit to make a Class AB push-pull amplifier. Though I used a pair of JFETs instead of BJTs. And it didn't work.

Of course, it wouldn't be the first time I missed something because of my naivety.

Or did I use varactors? I can't remember...
 
  • #7
Jiggy-Ninja said:
The workings of a BJT transistor are fundamentally different from anything you could deduce from a two-diode model.

A transistor can act as a pair of common anode diodes (or common cathode for PNPs), but that's a sub-optimal use for them. Whereas diodes would be made with two approximately equivalent p- and n- type silicons, transistors have their regions specifically sized and doped in order to increase the size of the effect.

As for the physics, I'll do my best to explain it.

The understand the physics of an NPN transistor, you need to think about what the electrons are doing. For PNPs, the concept is the same, just replace "electrons" with "holes".

In normal operation, the BE (Base-Emitter) junction is forward biased, an the BC junction reverse biased. The emitter is very heavily doped, and teeming with free electrons, so when it is forward biased, thermal vibrations cause a huge amount of electrons to get pulled into the base. ("The cup runneth over", so to speak.) This is where is gets the name "emitter" from. The base is lightly doped, and very small compared to the emitter, so only a small fraction of the elections can combine with the holes, forming the base current. The electrons that don't recombine have nowhere else to go except to the collector.

The Wikipedia article has an interesting little paragragh:

What you've described as "base current" is just a small portion of the base current. When the b-e jcn is fwd biased, holes from the base transit into the emitter. This is the injection component of base current. The electrons which recombine in the base constitute the transport component of base current.

There is a 3rd, & equally important component of base current which is the "displacement" or "charging" component. In order to fwd bias the b-e jcn, it must be energized by charging said jcn. As frequency increases, the current needed to rapidly charge & discharge the b-e jcn increases. Hence current gain, beta, decreases w/ increasing signal frequency. The freq where beta equals unity is "ft", the transition frequency.

The light doping in the base region does indeed minimize the number of recombinations in said base region, but it is quite small in comparison to the injection component. A lightly doped base results in fewer holes injected from base to emitter. For a silicon device, the injection component greatly exceeds the transport (recombination in base region) component of base current. At very high frequencies the charging component prevails as ft is approached.

I just thought that this deserves mention. Search through my postings & you will find many threads where this is discussed in great detail.

Claude
 
  • #8
Would that "displacement" current be a one time thing, though? Like a capacitor in parallel with the junction?

And you're talking about AC being applied to the BE with no DC bias, right? With a DC bias current and a small AC signal, you'd just have to deal with the r'e equivalent resistance, right?
 
  • #9
No it's a steady thing. If the ac signal inputted to the bjt amp stage is 1.0 Hz, a very small but continuous displacement current exists. At a signal frequency of 10 Hz, 10 times more exists.

I'm including dc bias w/ the ac signal. The re' equivalent b-e jcn resistance also features a diffusion capacitance in parallel, C_pi. Any reference text detaila the full hybrid pi bjt model.

At low freq, the injection component of base current is the largest. At high freq, the displacement component becomes larger. The transport component, OTOH, is the smallest of the 3 at high freq. At very low freq, the displacement is the smallest.

Although the transport component of base current is quite small, it is absolutely important. Without it, transistor action would not be possible. When electrons emitted from emitter into the base region recombine, ionization occurs, resulting in a net builup of charge. For every electron acquired in the base via recom, an electron exits the base terminal to maintain charge neutrality. Otherwise, the local E field due to accumulated charges via recom, would negate the E field in b-e jcn, resulting in no more electrons being emitted from the emitter.

Again, the transport component of base current is very small but necessary for transistor action to sustain. Typically for silicon, for every 5000 electrons emitted from the emitter, 4,999 reach the collector, & 1 recombines in the base. Then 1 electron exits the base resulting in charge neutrality.

Meanwhile, for a beta value of 124 typical, 40 holes are injected from base to emitter (for every 5000 electrons from e to b). These holes recombine with electrons in base region. To maintain charge neutrality, 40 electrons enter the emitter.

These are ballpark numbers, & they vary with temp, device type, bias level, etc. I'm just conveying the order of magnitudes involved w/ the various quantities. The displacement current adds to these values. At a freq of "ft" the amount of displacement current needed is so great that the overall beta value is unity. For a bjt to be effective as an amplifying device, the operating frequency must be well below ft.

Does this help?

Claude
 
  • #10
You might also like to google 'Miller Effect' in relation to displacement current, as this version is more common in electronics circles. I note there are lots of hits.
 

1. How does a transistor work?

A transistor works by controlling the flow of electrons between two terminals, known as the collector and emitter, by using a third terminal called the base. The base terminal acts as a switch, allowing a small current to control a larger current flow between the collector and emitter.

2. What is the purpose of a transistor?

A transistor is used as a basic building block in electronic circuits to amplify or switch electronic signals. It can also be used as a voltage or current regulator, oscillator, or in other applications.

3. What are the different types of transistors?

There are two main types of transistors: bipolar junction transistors (BJTs) and field-effect transistors (FETs). Within these categories, there are further subtypes such as NPN and PNP BJTs, and MOSFETs and JFETs.

4. How is a transistor different from a vacuum tube?

A transistor is a solid-state device made of semiconductor materials, whereas a vacuum tube is a glass tube containing electrodes and a vacuum. Transistors are smaller, more efficient, and have a longer lifespan compared to vacuum tubes.

5. What are some common uses of transistors?

Transistors are used in a wide range of electronic devices, including computers, radios, televisions, and smartphones. They are also used in power supplies, motor control systems, and many other applications.

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