Transistors-Why can't a pn junction and np junction in series be a pnp transistor?

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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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  • #2
Vanadium 50
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Your junction has how many wires connected to it? How many does a transistor need?
 
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cheers so obvious!
 
  • #4
sophiecentaur
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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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