Bipolar Junction Transistor

Q1: What is a bipolar junction transistor (BJT)?
A1: A bipolar junction transistor is a two-junction, three-terminal semiconductor device. "Bipolar" refers to the presence of oppositely-charged charge carriers (electrons and holes). The device is divided into three regions:

1. Emitter
2. Base
3. Collector

A lead is connected to each region.

Notice the junctions separating the base from the emitter and collector

Q2: What are some types of BJTs?
A2:

1. NPN
   
2. PNP

Q3: Why doesn't the NPN transistor act like two PN junction diodes connected in series?
A3: Although the models above suggest that a symmetry shared by the emitter, base, and collector regions, the actual devices are designed to avoid this symmetry.

Q4: How do I label the circuit variables associated with the NPN and PNP ?

A4: A single letter subscript indicates the region to which the current is associated with. A two letter subscript indicates the voltage at the first letter with respect to the second letter.

NPN Circuit Variables
Even though there's are 6 circuit variables,
2 can be written in terms of the others
using KCL and KVL. Same goes for the PNP.

PNP Circuit Variables
Notice that current directions and
letter order in the subscripts for voltage
 are reversed with respect to the NPN

Q5: How might I see the NPN and PNP in a schematic?

A5: 

NPN Symbol
Notice the arrow pointing away
from the base and towards the emitter

PNP Symbol
Notice the arrow pointing towards
 the base and away from the emiter