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Bipolar Junction Transistor (BJT) theory
28 questions By Tony R. Kuphaldt
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Question 19 of 28
Are the collector and emitter terminals of a transistor interchangeable? If not, what is the physical difference between the emitter and collector?
Reveal answerThe emitter is smaller and more heavily “doped” than the collector.
Notes:Ask your students if there is any way to distinguish the emitter and collector terminals on a transistor, from external meter measurements. There is!
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Question 20 of 28
A beginning electronics student is just learning about transistors, and reads in the textbook that a bipolar transistor (either NPN or PNP) can be thought of as two diodes connected back-to-back as such:
Acting on this idea, the student proceeds to connect two 1N4001 rectifying diodes back-to-back and try to use it as a transistor. This idea does not work: although the diode pair reads the same patterns of continuity as a transistor would, it does not amplify. Explain why.
Note: this is a fairly deep question, and may not be answered without an understanding of charge carrier energy levels and semiconductor junction behavior.
Reveal answerThis makeshift transistor will not work because the metal connection between the two “P”-type materials (the diode anodes) precludes the injection of minority carrier (conduction band level) electrons into the “P” material of the collector-side diode.
Follow-up question: what do you suppose is really meant by the textbook’s statement of bipolar transistors being equivalent to back-to-back diodes, if two diodes connected back-to-back do not exhibit amplifying behavior? Is this a completely wrong statement, or is there some truth to it?
Notes:The idea for this question came from personal experience. I actually tried to build my own transistor out of two back-to-back diodes, and failed miserably. It took many years before I finally understood enough about semiconductor physics to realize why it would not work!
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Question 21 of 28
How would you explain the necessary conditions for conduction of an electric current through a BJT? Describe must be done to a BJT in order for it to conduct a current.
Reveal answerThe base-emitter PN junction must be forward-biased, and the polarity of voltage between collector and emitter must be such that the collector current adds with the base current to equal the emitter current.
Notes:This is perhaps the most important question your students could learn to answer when first studying BJTs. What, exactly, is necessary to turn one on? Have your students draw diagrams to illustrate their answers as they present in front of the class.
What surprises me is the following: I did not see any mentioning of the most important BJT parameter: Transconductance gm.
(Instead, you are stating that the beta-value would be a very important parameter - which is NOT the case!).
Like some other low-level electronic books and papers you treat the BJT as a cittent-controlled device - this is simply wrong.
There is not a single proof that the BJT would be a CCCS !
However, there are many theoretical explanations, measurements and observable facts which clearly show that the BJT is - of course - voltage-controlled. Who can deny it ? With which arguments? Ic=beta x Ib is just a formula - a misinterpretation of Ib=Ic/beta.