Transistor Confusion

[headinthemicrowave]

I don't know very much about electronics. I am definately a newbie. I am very confused about the operation of transistors. I've read articles, but none could explain it to me. I would like to know about how they operate and how they are used. Any and all explanations are welcome. Even the short ones with bad self-esteem.

 

[jin29_neci]

you must be able to forward biad the base & emitter to be able to switch the transistor. I hope this helps

 

[dknguyen]

Transistor

Are you trying to figure "the rules" to use transistors? Or are you actually trying to figure out what they do inside? Were you like me when you first started out? Trying to look at some simple circuits using transistors and tracing out how current was moving hoping to understand how to wire up a transistor? It is different for every kind of transistor and there are some rules you have to learn depending on the kind of transistor you are working with.

I will assume you are asking for "rules" first. Ask again if you want to get some insight onto the physics of a transistor (and give the type of transistor you want to know about). If you want to know what is happening inside the transistor, you have to know what happens in a diode first. So if that's the part you are interested in read up on it (stuff about majority and minority charge carriers inside a diode). A MOSFET's physical operation is conceptually much easier to understand than a BJT's physical operation.

Transistors have 3 terminals. Two terminals usually have a larger current going through them, and the third terminal controls "how much" the other two terminals open. The names of these terminals are different depending on the kind of transistor. Some "open and close" based on voltage, others based on current. The voltages and currents across specific terminals of the transistor also have to be at certain levels for the transistor to behave in certain ways. The two main types being MOSFET (voltage-controlled) and a BJT (voltage/current controlled). If you open/close them completely they can be used as digital switches. If you only do it "part-way" they can work as the basis for amplifiers and other analog circuits. Transistors have lots of little peculiarities depending on the type of transistor it is. A transistor essentially allows a small signal of voltage or current to control the flow of a much larger voltage or current. Similar to how you can't open a dam yourself with your barehands, but your tiny little finger can push a button that will open the dam for you.

It can get pretty complicated and about the only place you are going to learn about it is from an electronics textbook- not an article. I couldn't figure out how they worked myself until I took some courses in unversity. I suggest you get a textbook. Articles just won't cut it as far as I have seen. You also have to read the articles/books VERY carefully. They go a lot into graphs and equations and such because it's the only real detailed way to understand how a transistor behaves. Otherwise, it pretty much ends at what I just said above, unless you give me a specific situation to describe to you. It's much easier for me to clarify something for you if you refer me to a part of an article and ask something specific.

 

[headinthemicrowave]

I was just asking about "rules". I understand somewhat of how the depletion layers thin and thicken, and how it is used to "multiply" current, But I don't understand how to use them. Your explanation was very helpful, though. Thanks.

 

[dknguyen]

MOSFETs

For BJTs, it is a bit different and a few equations I don't want to go through. MOSFETs are more popular anyways.

With MOSFETs, there are two kinds NMOS (N-channel) and PMOS (and P-channel). This just describes where the N and P-type silicon goes in the setup. In NMOS and PMOS the same "parts" of the transistors have the opposite type of silicon. They are like inverted versionf of each other, as a result the rules for PMOS are the opposite. All MOSFETs have 3 terminals called the source, drain, and gate. The source and drain are "reversed" in PMOS and NMOS because they are inverted versions of each other.

The large current flows through the source/drain terminals. The voltage to control this flow is applied to the gate. BUT the voltage between the gate and source is what actually determines the "amount" of flow (in both NMOS and PMOS). I mention this because (for example) if you need at least 5V between gate-source to turn the NMOS completely on, you can just apply 5V to the gate if the source is connected to ground:

We want Vgs > 5V
Vgs > 5V = Vgate - Vsource(@ GND, 0V) -> Vgate = 5V

But if you have the source connected to other stuff which eventually connects to ground and this other stuff causes the source to be at 10V, then you actually have to apply 15V to the gate to the NMOS on.

We want Vgs > 5V
Vgs > 5V = Vgate - Vsource(@10V) -> Vgate = 15V

This above example was using the NMOS just as an on/off switch, turning it on and off completely. It also assumed that the voltage threshold across the source-drain required for it to operate this way was met or exceeded. Remember, that there are voltage thresholds that determine how the MOSFET operate. And voltage thresholds exist for the gate/source voltage AND source/drain voltage.

The voltage PMOS and NMOS thresholds are just opposites of each other. They are all across the same terminals on both types of transistors, but the difference is that they are in the opposite direction (ie. an operating mode voltage threshold for NMOS is voltage travelling from gate-to-source but the same threshold for PMOS (not necessarily the same number) would go from source-to-gate.

ie. If one threshold is measured across gate/drain for NMOS, the same threshold is also measured across gate/drain for PMOS (not gate/source which you might think is the "opposite" of gate/drain). The difference between these thresholds is that you are moving in opposite directions between two terminals when measuring the voltages, so for simplicity, the voltages are negatives of each other. (You understand about how voltage is measured right? The difference between the voltage from A to B vs Voltage from B to A?)

It's sort of confusing if you look at equation sheets because they tend to use use the NMOS setup as a standard. So when you read the PMOS equations they try to keep you moving in the same direction across terminals when measuring voltages as NMOS so everything is reversed and negative signs everywhere, etc.

The exact voltage-current characteristics of how the MOSFET conducts is dependent on certain threshold voltages across the source-drain and across the gate-source. It has different "modes" of operation. (ie. below a certain threshold, nothing conducts, between two specific thresholds, source-drain current increases almost linearily with applied gate-source voltage, and above a threshold source-drain current increases almost negligibly for an increase in gate-source voltage.

 

[Nigel Goodwin]

MOSFETs are more popular anyways.

Do you live on your own planet?, since when have MOSFET's been more popular than BJT's? - on planet Earth anyway!.

 

[dknguyen]

FETs

Pretty much all my courses use MOSFETs now whenever possible, digital and analog. It's like we learned about how BJTs work and then have been using FETs ever since.

Plus they seem to be more popular in ICs where most of them are used. Less silicon, smaller, cheaper, lower power, simpler control requirements. I just see them almost everywhere especially in digital. The only time I have seen them used in is certain analog amplifiers and select ADC/DAC configurations. Unless they are used in VLSI a lot more than I have been lead to believe. It seems to me that MOSFETs have replaced BJTs in a lot of low voltage applications, except for certain (high-frequency?) analog circuits and in very high voltage applications where things like IGBTs might serve better.

 

[Nigel Goodwin]

Pretty much all my courses use MOSFETs now whenever possible, digital and analog. It's like we learned about how BJTs work and then have been using FETs ever since.

I suggest you move out in the real world, where you will find BJT's out number MOSFET's by a massive margin!. In domestic electronics you might find just one MOSFET in a TV, but most have none in them.

Admittedly many IC's are MOS, but many aren't, but the question wasn't about IC's.

 

[dknguyen]

MOSFETs

Just curious, are BJTs used in more devices or does the actual number of them around outnumber MOSFETs (I'm thinking millions of MOSFETs on processors and other ICs)?

 

[Hero999]

It depends on the application. . .

Come to think of it he's probably right as when you think aof the billions and trillions of CMOS transistors etched onto ICs, yes there probably are move MOSFETs in the work than bjts.

However if you're talking about small signal discrete componants then I agree bjts are more popular.

 

[philba]

hmmm, civility seems to have abandoned us these days...

I think you have to consider cost and application. for power apps, mosfets are hands-down more popular. for smallish currents (low 100 mAs), a bjt is far and away the popular choice due to cost.

 

[Nigel Goodwin]

Just curious, are BJTs used in more devices or does the actual number of them around outnumber MOSFETs (I'm thinking millions of MOSFETs on processors and other ICs)?

There are also many millions of BJT's out there in IC's, although the really massive numbers on a single chip do tend to be MOS.

But the question still wasn't about chips, it was about transistors - and BJT's probably out number MOSFET's 1000's to one!. It's certainly short sighted of your course if they ignore BJT's to the extent you say?.

However, we were only taught one afternoon about valves, and that was only because the normal lecturer wasn't there!. At that time most TV's still used valves, particularly in the USA where they were slow to move to transistor sets.

 

[dknguyen]

???...vvv..alves?

 

[philba]

brittish for tubes.

 

[Nigel Goodwin]

brittish for tubes.

NO, tube is American for valve

 

[philba]

I think de Forest would argue with that...

 

[Nigel Goodwin]

Sorry, I thought the language we were using was called 'English'

We don't have 'trunks' on cars either, and don't wear 'pants' on the outside I'm getting bored with listing Americanisms now!.

BTW, what's he got to do with it anyway?

From Wikipedia:

In November 1904, he invented and patented the two-electrode vacuum-tube rectifier, which he called the oscillation valve. It was also called a thermionic valve, vacuum diode, kenotron, thermionic tube, or Fleming valve. This invention is often considered to have been the beginning of electronics, for this was the first vacuum tube. Fleming's diode was a vital unit in radio receivers and radars for many decades afterwards, until solid-state electronic technology took over.

In 1906, Lee De Forest of the USA added a control "grid" to the valve to create a vacuum tube RF detector called the Audion, leading Fleming to accuse him of copying his ideas. De Forest's device was shortly refined by him into an amplifier tube called the triode. The triode was vital in the creation of long-distance telephone and radio communications, radars, and early digital computers.

Notice the original name was 'valve'.

 

[philba]

notice I said "Brittish" not English. Also, from the wikipedia article a valve didn't have a grid. tubes have a grid. I'm not sure the point of arguing different cultures, though.

but perhaps the best way to leave it is that the US and Brittain are two countries divided by a common language.

 

[Nigel Goodwin]

notice I said "Brittish" not English. Also, from the wikipedia article a valve didn't have a grid. tubes have a grid. I'm not sure the point of arguing different cultures, though.

So do you call a diode a valve then?, and only triodes and above tubes?.

We're not 'arguing' cultures, we're 'discussing' them

but perhaps the best way to leave it is that the US and Brittain are two countries divided by a common language.

VERY old saying!.

But kindly notice, it's not 'British' we're using, it's 'English', just as you get 'Scottish', 'Welsh' and 'Irish'. This is becoming a hot topic in the UK, my daughter's even written a song called 'Patriot' about it - it's considered racist to fly an English flag in England, and they are banned from public buildings. Yet you can fly a Welsh flag in Wales, a Scottish flag in Scotland, or an Irish flag in Ireland - those countries even get to celebrate their national saints days, but we can't in England, apparently it's also 'racist'?.

One of the verses from Patriot:

We used to fly our flag
Proud of our country
But now we hide away
Where no one else can see

It's getting bad when a 15 year old girl writes like that!.

 

[philba]

why does it feel more like an argument, then?

Perhaps one thing I think we can agree upon is that political correctness is a pox upon civilization.