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Topic: How does a transistor work? (Read 691 times) previous topic - next topic

Drew Davis

I'm going to control a motor according to the link below but I want to know why it works like that. I Understand why you need the base and the emitter but what function does the collector serve? Also what purpose does the capacitor serve?

Thanks!


http://www.instructables.com/file/F9LKDFGGU7FXUMH

James C4S


I Understand why you need the base and the emitter but what function does the collector serve?

The current allowed to flow from Collector to Emitter is proportional to the current flowing from base to emitter.  It's the basis of how a (NPN) transistor works.  https://www.google.com/search?q=how+does+a+transistor+work



Also what purpose does the capacitor serve?

Filtering the noise from the motor.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

AmbiLobe

A silicon transistor uses impurities like phosphorus and arsenic to produce diodes at two juctions. The base-emitter junction has a doped base and a heavily doped emitter so that the diode currents there have small numbers of charge carriers from the base and large number of charge carriers from the emitter crossing the junction. The amplification is because of that. A small base current produces a larger emitter current because that diode has holes and electrons flowing in unequal amounts. The collector is close to that diode, so diffusion lets that large emitter current go into the collector while the base has a small current. The collector is lightly doped so the diffusion of charge carriers proceeds from a volume with many carriers to a collector with few carriers.
I am going to get going.

Drew Davis

Thank you for all the explanations. I have learned so much from the Arduino Forum!

pwillard

#4
Jun 28, 2013, 05:16 am Last Edit: Jun 28, 2013, 05:40 am by pwillard Reason: 1
Gee... Why do people always want to place the inductive kick back diode in the wrong place.

Maybe it's not obvious on a physical layout (IE; Fritzing), but when you draw it as a schematic, something looks wrong.

(I'm not a fan of instructables... I pretty much only ever see bad advice... like "you don't need resistors on your LED's" and such.  This is just another example of someone almost understanding... but then *not* really...)

I put an example diode (D2) in the circuit to show where it *should* go.

Lets see shall we?


MarkT



The current allowed to flow from Collector to Emitter is proportional to the current flowing from base to emitter.  It's the basis of how a (NPN) transistor works.  https://www.google.com/search?q=how+does+a+transistor+work



When talking about semiconductor devices its usual to talk about the direction the mobile charge carriers flow,
not the conventional current direction, so don't think about current flowing from collector to emitter,
think about electrons flowing from emitter, via the base to the collector.   For a PNP transistor then
its holes flowing from emitter, via base, to collector.   Otherwise the names "emitter" and "collector"
make no sense.

When you look at a circuit as a whole, then use conventional current - its only an accident of history that we
consider electrons "negatively" charged, as the signs for electric charge were agreed upon before the
discovery of cathode rays and the electron.

Back to the transistor operation:  the base region is very thin - it needs to be to increase the
chances that a charge carrier diffuses far enough into the base region to wander into the reverse-biased
base-collector junction (thereafter the electric field in that junction can sweep the carriers over to the collector).
Note that in an NPN device the holes from the base are repelled by the collector voltage and all flow to the
emitter, the electrons from the emitter flow to the base and then get a chance to "see" the collector voltage and
be swept through - careful design allows this chance to be 99% or more,  This careful design is why the emitter
and collector are not equivalent (the details are important).
[ I won't respond to messages, use the forum please ]

AmbiLobe

How Do Bipolar Transistors Work?

Transistors amplify currents because of the diode physics. The current through a pn diode is made of the moving negative charges of electrons and the moving positive charges of holes. If the n-type semiconductor is heavily doped and the p-type part is lightly doped, then the diode current will be mostly electrons from the n-type semiconductor. The hole current will be smaller than the electron current that flows through that junction diode.

Transistors amplify currents because THE DIODE at the base-emitter junction has the emitter heavily doped with impurities and the base has a lower concentration of impurities. The ratio of impurity concentrations sets an upper limit on the amplification factor "hfe".

$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
NPN SILICON TRANSISTOR EXAMPLE

If the emitter has 1000 times as many phosphorus impurities per cubic cm, when compared to the concentration of boron impurities in the base region,  then hfe must be less than 1000.

If the emitter has the same number of phosphorus impurities per cubic cm, when compared to the concentration of boron impurities in the base region,  then hfe must be less than 1.

It is easy for a factory to make n-type silicon with 100,000 times the impurity concentration as an adjacent p-type impurity concentration. For specific numbers used in industry, expect n-type material with 10^21 phosphorus atoms per cubic cm and p-type material with 10^16 boron atoms per cubic cm. The ratio of impurity concentrations then is 100,000. So when that pn junction diode is forward biased, the current will have 100,000 times as many electrons crossing the pn junction than holes crossing the junction.

Transistors use that diode fact to provide an amplification of a small base current so it allows a large emitter current to diffuse to the collector.
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

Reference on diode physics

http://ecee.colorado.edu/~bart/book/book/chapter4/ch4_4.htm


see equations 4.4.18 and 4.4.19 to confirm
the diode current density ratio for electrons over holes Jn/Jp
is controlled by impurity concentrations Nd and Na, the number of donors and acceptor atoms per cc. The donors can be phosphorus for n-type silicon and the acceptors can be boron for p-type silicon.

I am going to get going.

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