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

In summary, the question asks why a p-n junction connected in series with an n-p junction cannot function as a p-n-p transistor. The reason is that the base element must be thin, and in this setup, it would be too thick. However, the real answer is that a transistor requires three terminals, while two diodes connected in series only have two. Additionally, the base in a transistor is part of both diodes, allowing for control of the current flow. A piece of wire between two diodes would not achieve this.
  • #1
phjw
7
0
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


Your junction has how many wires connected to it? How many does a transistor need?
 
  • #3


cheers so obvious!
 
  • #4


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.
 

1. Why can't a P-N junction connected in series act as a P-N-P transistor?

P-N junctions connected in series do not have the necessary components to function as a P-N-P transistor. A P-N-P transistor requires three doped regions, with two P-type regions sandwiching a N-type region. In a series connection, only one P-N junction is present, making it impossible to have the necessary three regions.

2. What components are necessary for a P-N-P transistor to function?

A P-N-P transistor requires three doped regions, with two P-type regions sandwiching a N-type region. This creates two P-N junctions, allowing for the transistor to function as an amplifier or switch.

3. Can a P-N junction connected in series still act as a single diode?

Yes, a P-N junction connected in series can still act as a single diode. However, it will not have the characteristics of a P-N-P transistor and will not function in the same way.

4. Are there any alternative setups that can be used to create a P-N-P transistor?

Yes, a P-N-P transistor can also be created by connecting two P-N junctions in parallel, with the N-type regions connected together. This creates the necessary three doped regions for the transistor to function.

5. Why is a P-N-P transistor preferred over a P-N junction in series?

A P-N-P transistor has the ability to amplify and switch signals, making it more versatile for use in electronic circuits. A P-N junction in series can only act as a single diode, limiting its functionality.

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