NPN transistor

N-P-N transistor:

• NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped semiconductor (the "base") between two N-doped layers.
• A small current entering the base is amplified to produce a large collector and emitter current.                                                                                                                                                         
  

PNP transistor

P-N-P transistor:

• The other type of BJT is the PNP, consisting of a P-doped semiconductor between two layers of P-doped material.
• A small current leaving the base is amplified in the collector output.
• A PNP transistor is ON when its based is pulled low relative to emitter.