Why Can't P-N Junction Connected Series Act as a P-N-P Transistor?

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Discussion Overview

The discussion revolves around the question of why a p-n junction connected in series with an n-p junction cannot function as a p-n-p transistor. The scope includes conceptual understanding of semiconductor devices and their operational principles.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that the base element of a p-n-p transistor must be thin, approximately 10 wavelengths of light, and implies that in the given setup, the base would be too thick.
  • Another participant questions the number of wires connected to the junction and compares it to the number needed for a transistor, hinting at a fundamental difference in structure.
  • A later reply elaborates that two diodes connected in series have three possible terminals, but emphasizes that they do not interact in the same way as a transistor, where the base is integral to both diodes, allowing for control of current flow.

Areas of Agreement / Disagreement

Participants express differing views on the operational principles of diodes versus transistors, indicating that there is no consensus on a definitive answer to the original question.

Contextual Notes

Limitations in understanding may arise from assumptions about the thickness of the base and the interaction between the diodes in series, which are not fully explored in the discussion.

phjw
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Hi I'm practising past papers however have no answers.
one of the questions says:

Why can a p-n junction connected in series with an n-p junction not act as an p-n-p transistor.

My only thoughts were that the base element must be thin (approx 10 wavelengths of light) and in thus in this set up the base would be to thick.

However I'm sure there must be a better answer any help would be amazing cheers ozzie
 
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Your junction has how many wires connected to it? How many does a transistor need?
 


cheers so obvious!
 


Vanadium 50 said:
Your junction has how many wires connected to it? How many does a transistor need?
Actually, two diodes connected in series HAVE three possible terminals. The point is that two separate diodes have no effect on each other. A transistor is different in that the base is part of both of the two diodes that 'appear' to be in it. So current into the (thin) base and from the emitter can affect/control the current that flows from collector to emitter (through the collector-base diode which is reverse biased or the wrong way round). A piece of wire between two diodes wouldn't achieve this at all.
 

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