Intro to Transistors
Wednesday, 13 May 2026
9:34 PM
Akin to diodes, transistors are solid state devices created from semiconductor material. They are
used as Amplifiers, Oscillators and Electronic Switches. The purpose of a transistor is to control
currents. It controls a large current with a small current.
The transistor transfers its internal resistance from the low resistance between the emitter and base
terminals, over to a much higher resistance between the collector and base terminals.
High R Reverse
Biased
Collector
Base
Low R Forward
Biased
Emitter
A transistors internal makeup is akin to a pair of diodes being placed back to back. The key difference is
that while a diode is made up on one side of N type material and the other is P type material a transistor
has 3 segments resulting in either an NPN or PNP configuration. Because of this, on end of the formation
will act as a forward biased diode while the other end will act as a reversed biased diode thus the low
and high resistance values
High E Reverse
Bias
Low R Forward
Bias
Because transistors have three segments they also have two PN junctions as opposed to the single
found in diodes. In each case the transistor has 3 main components.
The Emitter is a heavily doped material meaning it can transmit a large number of current carriers inside
the semiconductor material. This would mean there are many free electrons in an N-type material or
that there are many holes for electrons to fit into for a P-type material.
The Base is a very thin and lightly doped material, opposite in nature to the Emitter and Collector.
Because it is so thin and lightly doped it can't support as much current flow as the Emitter or Collector
and instead allows current to flow from the Emitter to the Collector.
The Collector is moderately doped allowing it to carry more current than the Base but not as much as
the Emitter. The Collector does however have the largest are of the three segments because it has to
dissipate most of the heat generated during current flow.
NPN transistors and PNP transistors
are represented by a very similar
symbol, a circle with three lines that
meet inside at a fourth line. The
shortest of these lines represents the
base, the line with the arrow is the
emitter and the remaining line is the
collector. The only difference between
the two symbols is that the NPN type
will have the arrow pointing out of the
circle while the PNP type will have the
arrow pointing into the circle.
The phrase "Never Points iN" can be
The Collector is moderately doped allowing it to carry more current than the Base but not as much as
the Emitter. The Collector does however have the largest are of the three segments because it has to
dissipate most of the heat generated during current flow.
circle while the PNP type will have the
arrow pointing into the circle.
The phrase "Never Points iN" can be
used to remember the NPN type as the
arrow points out, by elimination, the
PNP type has the arrow pointing in.
Transistors have identification
numbers. An example would be
2N3304 where the 2 indicates
transistor (2 PN Junctions), the N
indicates it is solid state and the 3304
identifies the transistor type.