How BJT works in terms of forward bias and reverse bias?

I knew how Bipolar Junction transistors work in terms of Forward bias and reverse bias.

Since im going to become IC design engineer , i need to know how the BJT works in terms of electrons, holes, carriers and flow of current. I have read some text books, but still i could not catch the important element.

I hope that someone can explain to me in a clear and step by step to show how BJT works?

bjt animation

In general, the emitter-base junction injects carriers both ways. Since the emitter is more heavily doped, the carriers from the emitter into the base are several orders of magnitude larger than the other way. The carriers make it across the base because of their kinetic energy and a built in field in the base due to the doping profile. They end up in the collector. Some of them do not make it across and recombine in the base. The base current from the external world supplies the charges to keep the base neutral.

how a bjt works

its nice to hear that u r going to become an IC design engineer. good luck

first of all stop thinking that the transistor is an amplifier and that Ib controls Ic.

i would recommend that u buy the book Microelectronic Circuits by Sedra Smith.

and go to the site http://amasci.com/amateur/transis.html

i hope that helps

bipolar transistor animation

Yes, thank u for the link and the explanations on how to think like IC design engineer. Now i underdstand the flow of charge in BJT.

bjt back injection

basically bjt is a active device that operates due to minority charge carriers.

the emitter of a base is highly doped followed by collector. the base is very thin & is very lightly doped.

when a forward bias voltage is applied to a emitter the majority charge carriers cross into the base. this junction can be viewed as a forward biased diode. the mojority charge carriers entering the base act like the minority carriers for base-collector junction. since this junction is generally reversed biased in the active region, the carriers fall through the potential barrier. some of this carriers make it through the base too.

as a result the emitter current is equal to the sum of the base & collector current ignoring the leakage current in the collecter junction. so regulating base current would result in the collecter current.


hpoe this topic get more clear.

animation of bjt

The mathematical representation is best. The base current is multiplied by a certain amount to give you collector and emitter current. This way you can ignore what the voltage does in terms of re. Anyways, the way I understand transistors has nothing to do with the way a diode works. I think the construction of the BJT was meant to mimick the mathematical model. The problem with the voltage representation is that you have re. But this is what design is all about. Because of re changes, you are left with a base current that does not multiply using the same factor. This is why you see the nonlinear transfer characteristic of the BJT. So stick with multiplying the current by a common factor and this is what it was designed for.

how bjt amplifier works

zahrein

Strictly speaking, IC design engineer is not required to know this. It's the device engineer who needs to know the solid-state device physics.

You may want to use a 3-ply NPN schematic to visualise, but technically it doesn't looks this way. In reality it is a N-type substrate, in it a P-well by diffusion or implantation, than a highly-doped N+ in the P-well. You can imagine 3 'U's or bowls such that the smallest bowl in the middle bowl and the middle bowl in the largest bowl.

Assuming an NPN Transistor,

1. In BJT, Emitter is heavily doped, i.e. N+. Base is p-type and narrow. Collector is moderately doped N.

2. With N+ so heavily-doped in Emitter, this prevents back-injection from Base and also provides a steep gradient (i.e. potential energy) for the electrons (majority carriers) in Emitter to become kinetically energetic to flow into Base, when Emitter-Base Junction is Forward Biased. Electrons in Base are now called minority carriers where the holes are the majority carriers.

3. The Base is delibrately made narrow to serve one purpose, i.e. to provide limited replacement holes when Emitter-Base Junction is forward-biased. Some holes must be present in the Base for Base current to enter (i.e. when electrons leave from the Base), in order to attract electrons from the Emitter. This principal concept comes from a PN junction diode called Drift when a diode is biased by applying an external source.

4. Since Collector is tied to the highest potential than Base, it creates a sizable electric field to attract most of the electrons from Base into Collector, and electrons leave from Collector, i.e. permitting the flow of Collector, which is β times than Base current.

BJT is called Bipolar instead of a Unipolar like MOSFET or JFET.
As you can see, both holes and electrons are active carriers. Electrons and Holes are opposite poles (i.e. having opposite polarities), thus calling any transistor with such distinction a Bipolar Junction Transistor.

bjt transistors animation

I thought you should refer to some solid state books .

Here is the list I recommand!

"Semiconductor Physics And Devices"written by Neamen
"Semiconductor Device Fundamentals"by Pierret

"Semiconductor Devices: Physics and Technology"
by Simon sze

Don't waste your time reading the electronics books such as sedra's,Milliman's......etc.
They just tell you the elementary things about BJT,FET.

But in my opinion,a IC designer doesn't need to
know the mechanism behind devices.

Good luck!

animated powerpoint on bjt biasing

Read millamn and halikies or sedra smith..

bjt working

you can download the files from following website....BJT working is explained with animation of electrons and holes.........all devices are there

http://www.malvino.com/ep/download1.html


can also check

http://www.faqs.org/docs/electric/Semi/SEMI_4.html

back injection bjt

I would like to explain this in "Ben G Streetman" style....

Consider a reverse biased photo diode.. There is a small current flowing in the diode owing to the injected charge carriers in the depletion region due to available light. Now as you increase the intensity of light, more and more electron-hole pairs are created which increases the current...

Now, imagine and replace the reverse biased photo-diode into the base and collector of a BJT. Now we need a charge injector... Here we need a source of charge carriers which will continuously inject charges into the depletion region of this reverse biased diode. (Remember that as base is very lightly doped when compared to collector, when this is reverse biased, most of the depletion region is formed in the base almost entirely covering it.)

Now the question is how to get this source of charge carriers - an Emitter of charges. The best thing is to have a forward biased diode! We now have a emitter-base diode which is forward biased to continuously provide the required charge carriers...

Thus we have a BJT working in the linear region when emitter-base junction is forward biased and base-collector junction is reverse biased..

Hope this gives an idea of working of BJT..

Re: bjt working

Excellent tutorial that i ever seen ........

Re: back injection bjt

excellent. BTW, what is Quantum Physics related to all of this?

srieda said:

I would like to explain this in "Ben G Streetman" style....

Consider a reverse biased photo diode.. There is a small current flowing in the diode owing to the injected charge carriers in the depletion region due to available light. Now as you increase the intensity of light, more and more electron-hole pairs are created which increases the current...

Now, imagine and replace the reverse biased photo-diode into the base and collector of a BJT. Now we need a charge injector... Here we need a source of charge carriers which will continuously inject charges into the depletion region of this reverse biased diode. (Remember that as base is very lightly doped when compared to collector, when this is reverse biased, most of the depletion region is formed in the base almost entirely covering it.)

Now the question is how to get this source of charge carriers - an Emitter of charges. The best thing is to have a forward biased diode! We now have a emitter-base diode which is forward biased to continuously provide the required charge carriers...

Thus we have a BJT working in the linear region when emitter-base junction is forward biased and base-collector junction is reverse biased..

Hope this gives an idea of working of BJT..

Click to expand...