<< Chapter < Page Chapter >> Page >

Summing up, in an NPN transistor, a small flow of electrons from emitter (E) to base (B) allows a much larger flow of electrons from emitter (E) to collector (C). Given that conventional current (flowing from + to - ) is in the opposite direction to electron flow, we say that a small conventional current from base to emitter allows a large current to flow from collector to emitter.

A PNP transistor works the other way. A small conventional current from emitter to base allows a much larger conventional current to flow from emitter to collector. The operation is more complicated to explain since the principal charge carrier in a PNP transistor is not the electron but the `hole'.

The operation of NPN and PNP transistors (in terms of conventional currents) is summarized in [link] .

An overview of bipolar transistors as current amplifiers. (Left) An NPN transistor. (Right) A PNP transistor.

Interesting fact

The transistor is considered by many to be one of the greatest discoveries or inventions in modern history, ranking with banking and the printing press. Key to the importance of the transistor in modern society is its ability to be produced in huge numbers using simple techniques, resulting in vanishingly small prices. Computer “chips” consist of millions of transistors and sell for Rands, with per-transistor costs in the thousandths-of-cents. The low cost has meant that the transistor has become an almost universal tool for non-mechanical tasks. Whereas a common device, say a refrigerator, would have used a mechanical device for control, today it is often less expensive to simply use a few million transistors and the appropriate computer program to carry out the same task through "brute force". Today transistors have replaced almost all electromechanical devices, most simple feedback systems, and appear in huge numbers in everything from computers to cars.

Interesting fact

The transistor was invented at Bell Laboratories in December 1947 (first demonstrated on December 23) by John Bardeen, Walter Houser Brattain, and William Bradford Shockley, who were awarded the Nobel Prize in physics in 1956.

The field effect transistor (fet)

To control a bipolar transistor, you control the current flowing into or out of its base. The other type of transistor is the field effect transistor (FET). FETs work using control voltages instead. Accordingly they can be controlled with much smaller currents and are much more economic to use.

Interesting fact

No-one would build a computer with billions of bipolar transistors — the current in each transistor's base might be small, but when you add up all of the base currents in the millions of transistors, the computer as a whole would be consuming a great deal of electricity and making a great deal of heat. Not only is this wasteful, it would prevent manufacturers making a computer of convenient size. If the transistors were too close together, they would overheat.

A field effect transistor (FET). The diagram on the top shows the semiconductor structure. The diagram underneath shows its circuit symbol.

Questions & Answers

how do you see a product if it is exothermic
Entle Reply
When ∆H<0
does these circles shown show the observer ?
Amahle Reply
can you mix kc nd pressure
Khanyisile Reply
equation for dissociation
Amanda Reply
under which topic
acids and bases
Newton's law
it the bases that dissociate
How does a catalyst affect the rate of chemical reaction
Rebone Reply
it speeds up the reaction without being by the reaction. in other words it increases the Reaction rate by lowering the activation energy of a reaction
it only speed up the reaction not to affect the reaction
what is a compound?
Sithembiso Reply
define Doppler effect
Emihle Reply
It is the change in frequency of a wave in relation to an observer who is moving relative to the wave source.
Doppler effect is the change in frequency or pitch detected by a listener because the velocity of the sound source is different from that of a listener relative to the medium of sound propagation
explain how color is an unreliable clue to.the identify minerals
Camille Reply
Hi I want to know where the topic for electric circuits are?
Zahra Reply
hello.. How to deal with Reactions rate?. i dont Understand them
Simphiwe Reply
Firstly you must know how energy changes during chemical reactions...secondly the collision theory.... Rate and extent of reactions.... And factors that affect the rate of chemical reactions... If u capture all the topic then this chapter will be easy for you the is no way you can fail it...
thank you
Heii.. I'm struggling with chemical equilibrium.
how do I attempt chemical equilibrium?
doppler effect question
Mabikinyane Reply
How do I differentiate from the formula of a listener who is moving away and who is moving towards the source
if the its away you will subtract on the numerator and add in the denominator but if its towards its the opposite
how cn I calculate sound of a source if it's unkown
Simphiwe Reply
Use what you were given. What were you given?
where can I find acids and bases
muravha Reply
how to calculate avarage power?
Bongani Reply
good result for bursary
Soluiswayi Reply
Good day Sir
Good day Mam
good day
How are you

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now

Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 12 physical science' conversation and receive update notifications?