The Physics of the Transistor
Jim Royston
Electronic Engineering
La Trobe University
• The B-E junction is forward biased and the C-B
junction is reverse biased.
• There is an exponential relationship between the
thickness of the B-E depletion region and the
current of electrons "injected" into the base as
minority carriers.
 VBE 

I C  I S exp 
 VT 
• The base region is very thin and has a lower
doping than the emitter or collector.
• The profile of concentration of minority carriers
(electrons in an NPN transistor) in the base is
linear.
• The amplifying action is produced by transferring
a current from a low-resistance circuit to a high
resistance circuit.
• transfer + resistor → transistor
• The C-B junction has a reverse bias leakage
current ICBO which is typically microamps or
nanoamps, and can be neglected unless
temperature effects are important.
• Approximately, ICEO ≈ β ICBO but both currents are
small.
• There is an injection of holes into the emitter,
where they are minority carriers, from the base –
this can be kept small by heavily doping the
emitter with donor impurities.
Gray, P. R. & Meyer, R. G., "Analysis & Design of Analog Integrated Circuits", 3rd edition 1993, New York, John Wiley & Sons.
References
• William Beaty "How Do Transistors Work?" 1995
http://amasci.com/amateur/transis.html
• Wikipedia "Bipolar Junction Transistors" accessed 2007
http://en.wikipedia.org/wiki/Bjt
• Gray, P. R. & Meyer, R. G., "Analysis & Design of Analog
Integrated Circuits", 3rd edition 1993, New York, John
Wiley & Sons.
• Bogart, T. F. et al, "Electronic Devices & Circuits", 3rd
edition 2001, Upper Saddle River, New Jersey, PrenticeHall (Pearson).
• Boylestad, R. L. & Nashelsky, L., "Electronic Devices
and Circuit Theory", 8th edition 2003, Upper Saddle
River, New Jersey, Prentice-Hall (Pearson).