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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 is frequency directly proportional to kinetic energy max. and what is the relationship between the wavelength and the energy of a photon. eg if a photon with a greater wavelength and a photon with less frequency. what would be the energies. as in which one would be greater and why
Pearl Reply
relationship wave depend n enrgy greadt reflect of energy n the form of tex shape
thulare
what is Cell capacity?
liphelo Reply
what are hydrocarbons
Nyiko Reply
compounds that are made up of hydrogen and carbon only
Patience
They are compounds that consist only with hydrogen &carbon elements.
Duncan
how do i calculate the work done on an object by a frictional force
Emmelda Reply
@ Emmelda it's W = FxCos180 Where 180° is the angle between Frictional force and where the object is going, it's always 180° because it opposes the motion. Then F is the magnitude of the force and x is the displacement
Vukona
Ohm's law state that provide the temperature remains constant, that potential difference is directly proportional to the current flowing in the circuit
Ezile Reply
state ohms law
Booi
what does ohm's law state ?
Gugu Reply
ohm's law state that the potential difference across a conductor is directly proportional to the current in the conductor at constant temperature
Nqobile
ohm'law in metalic conductor is directly proportional to the potential difference provided the temparature
Nandise
what is the principle on which generator is based
Nqobile Reply
what is spectrum
Mpeh Reply
It is a different combinations of frequency of light or any types of waves.
Duncan
Heii guys what's the topic? Am neww
Vannessa Reply
In nuclear reactions, is it necessary that there must only be one element on the product side and a radioactive particle or you can have a product that are both radioactive particles?
Jobena Reply
conservative force is a force when the work done in moving the object between the two points that does not depend on the path way
Bongiwe Reply
what is conservative force?
Mh Reply
It is a force , that does not have any external features affecting it
Duncan
what does the alternating current do
Kwanga Reply
what is definition of hydrocarbon
jessha Reply
it is the atoms that Contain of C atoms and H atoms only
Xolani
You mean molecules
Jc
yea 💯
Xolani
what's the definition of le chatelier?
Karabo
what is electromagnetic
Daniel
differentiate between DC motor and Ac generator
Kwanga
dc motor converts electrical energy into mechanical energy while ac generator converts mechanical into electical
Ophola
DC has a Split-ring but then Ac has a slip-rings.
Joyous
what's the other name of galvanic cell?
Cwenga
galvanic cell is also called Voltanic cell
Bongiwe
is the chain whereby there is only Hydrogen and Carbon atoms only.
Hlonny
what is the difference between AC and DC moto
Mihle
please check the answer from Ophola and Joyous
Nomsa
AC motor consists of 2 slip rings and a DC motor has a single split ring commutator
Vukona
ac have slip rings n DC has split rings
mudodzwa
want to ask about Rays,,,,i want to know about angle symbols
Freddy Reply

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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