Transistor Action: Understanding Vbe, Vc & Ic

[neorich]

"Transistor Action"

Dear Forum,

I have a question about what is often referred to as "transistor action".

I understand how a transistor operates within a circuit, and the characteristics of such a circuit, but I would like to learn physically how the bipolar transistor is able to control a collector current through base current or base voltage?

Lets assume the transistor is properly biased and the base emitter junction is passing current i.e. Vbe>0.6V. When a base voltage is applied and hence a base current is flowing there then appears a collector current of hfe*Ib. OK. This collector current is supplied by the source (normally written as Vcc).

Can you please answer the following questions.

1.) Does a change in base voltage (or current) cause the collector voltage (voltage at the bottom of a collector resistor) to change, thereby causing a change in current where the current is given by (Vcc-Vc)/Rc.?
2.) If this is the case, how does the change in Vb get "translated" into a change in Vc?
3.) Does a change in base current (or voltage) cause a change collector current to be drawn which then passes through the resistor and drops a voltage thereby changing the collector voltage i.e. Vc=Vcc-IcR?
4.) I guess this question is does the collector voltage change first, leading to a change in collector current, OR does the collector current change first leading to a change in collector voltage?

5.) Now for the interesting question. Whether 1 or 3 is correct (or both, or neither) how does the change in Vb (or Ib) pass through the transistor and lead to a change in Vc (or Ic). What physical processes are happening in the transistor materials. Mathematical or qualitative explanations welcomed.

Thank you very much.

Kind Regards

neorich

 

[neorich]

Dear Forum, It has been several days with no reply. Can you tell me if this is a difficult question? Is it a poorly understood area of physics? Or is it in the wrong area of the forum?

Regards

neorich

 

[marcusl]

The web is full of sites that explain transistor operation. Start with wikipedia, then search on semiconductor and bipolar transistor operation, transistor tutorials, etc. Here are a few representative sites:

http://www.spectrum.ieee.org/may04/3992/s101"

http://www.uoguelph.ca/~antoon/tutorial/xtor/xtor.html"

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html"

You'll see that transistors are current-driven devices rather than voltage-driven. The bias voltage permits current to flow across the B-E junction, and charge carriers diffuse out of the base towards the collector. If you still have questions, please come back and post them.

 

[neorich]

Thanks,

The first link there was very helpful.

Regards

neorich