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This module is adapted from the Connexions module entitled Introduction to Bipolar Transistors by Bill Wilson.

Let's leave the world of two terminal devices (which are all called diodes by the way; diode just means two-terminals) andventure into the much more interesting world of three terminals. The first device we will look at is called the bipolar transistor . Consider the structure shown in [link] :

Structure of a npn bipolar transistor.

The device consists of three layers of silicon, a heavily doped n-type layer called the emitter, a moderately doped p-type layercalled the base, and third, more lightly doped layer called the collector. In a biasing (applied DC potential) configurationcalled forward active biasing , the emitter-base junction is forward biased, and the base-collector junction isreverse biased. [link] shows the biasing conventions we will use. Both bias voltages are referenced tothe base terminal. Since the base-emitter junction is forward biased, and since the base is made of p-type material, V EB must be negative. On the other hand, in order to reverse bias the base-collector junction V CB will be a positive voltage.

Forward active biasing of a npn bipolar transistor.

Now, let's draw the band-diagram for this device. At first this might seem hard to do, but we know what forward and reversebiased band diagrams look like, so we'll just stick one of each together. We show this in [link] , which is a very busy figure, but it is also very important, because it shows all of the important features in theoperation the transistor. Since the base-emitter junction is forward biased, electrons will go from the (n-type) emitter intothe base. Likewise, some holes from the base will be injected into the emitter.

Band diagram and carrier fluxes in a bipolar transistor.

In [link] , we have two different kinds of arrows. The open arrows which are attached to the carriers, show uswhich way the carrier is moving. The solid arrows which are labeled with some kind of subscripted I , represent current flow. We need to do this because for holes, motion and current flow are in the same direction,while for electrons, carrier motion and current flow are in opposite directions.

Just as we saw in the last chapter, the electrons which are injected into the base diffuse away from the emitter-basejunction towards the (reverse biased) base-collector junction. As they move through the base, some of the electrons encounterholes and recombine with them. Those electrons which do get to the base-collector junction runinto a large electric field which sweeps them out of the base and into the collector. They "fall" down the large potentialdrop at the junction.

These effects are all seen in [link] , with arrows representing the various currents which are associated with eachof the carriers fluxes. I Ee represents the current associated with the electron injection into the base, i.e., it points in the opposite direction from themotion of the electrons, since electrons have a negative charge. I Eh represents the current associated with holes injection into the emitter from the base. I Br represents recombination current in the base, while I Ce represents the electron current going into the collector. It should be easy for you to see that:

I E I Ee I Eh
I B I Eh I Br
I C I Ce

In a "good" transistor, almost all of the current across the base-emitter junction consists of electrons being injected intothe base. The transistor engineer works hard to design the device so that very little emitter current is made up of holescoming from the base into the emitter. The transistor is also designed so that almost all of those electrons which areinjected into the base make it across to the base-collector reverse-biased junction. Some recombination is unavoidable, butthings are arranged so as to minimize this effect.

Questions & Answers

what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Chemistry of electronic materials. OpenStax CNX. Aug 09, 2011 Download for free at http://cnx.org/content/col10719/1.9
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