# Help w/ Bipolar Junction Transistor

Discussion in 'Homework Help' started by TurboDsm4g63, Jan 25, 2009.

1. ### TurboDsm4g63 Thread Starter New Member

Dec 10, 2008
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Can anyone help understand the BJT theory a little bit better, The way my Instructor explains it and the book are two very different ways.

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Can you refine your question to provide details on what you want to know?

hgmjr

3. ### Tesla00010010 Member

Jun 20, 2008
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Basically,the transistor have charge carriers in its N and P parts.When current comes from the emitter it tries to go through the base because of the battery polarity (forward biased to the emitter-base junction) but the electric field in the collector collects almost all the electrons from the base.This is because the base is made very thin and it has very few charge carriers,also the collector is more heavily doped and its made of the same material from the emitter,P or N.So those electrons collected from the emitter will go back to the positive side of the battery,since the base-collector junction is reverse biased.It is kind of easily understood when you figure emitter current coming and splitting between base and collector.This means,both base and collector connected to the same battery or source.Also,emitter current is always the sum of base and collector current.For the reasons explained before,changes in base current affect collector current,and thus there cannot be collector current if there is no base current.This is all you need to know.

However,it is more difficult to visualize transistor operation when you have two different sources feeding emitter-base and base-collector circuits.For example,if you connect a small solar cell in the base and a bigger voltage source between emitter and collector,how does the collector "knows" that it has to amplify that small current through the base and drain current from the battery?In the textbooks they explain you how transistor works but not exactly why it amplifies,at least not in perfection.Transistor depend not only on electricity laws but also on quantum mechanics laws (which sometimes even PhD physicists cant understand at all)and the process of adding impurities to the crystal has to do with chemistry also.So,i think you should not worry so much about those details and focus more on the transistor in a component level.

Last edited: Jan 27, 2009