Transistors:

FET Transitor Symbols:

FET Junctions:

Transistors are 3-terminal semiconductor devices that have P-N junctions just like diodes, but have a more complex configuration. 
Transistors posses the magical property of "gain" and so are used to make amplifiers. 

There are many different types of transistors, but the most common type used for discrete (individual) transistors is the BJT (Bipolar Junction Transistor). 
BJT's are available in two configurations (polarities); NPN and PNP. The most common configuration is NPN. 

The 3 terminals of a BJT are designated base (B), emitter (E) and collector (C). 
A small current flowwing from the base to the emitter can control a large current flowing from the collector to the emitter. 
A BJT transistor will typically have a gain of 10 to 100 so the collector current will be 10 to 100 times bigger than the base current. 
The base and collector currents flow in the direction of the arrow in the symbol.

FET's are the most common type of transistor used in large scale integrated circuits. 
FET's can opperate at very low currents and so do not cause over-heating when millions of transitors are packed into a tight space. 

FET's are available in two configurations (polarities); N-channel and P-channel. 
The 3 terminals of a BJT are designated gate (G), source (S) and drain (D). 
A voltage across the Gate and source controls the current between the source and drain. 


NPN Junction Orientation:







Transistor Photos:








250px-NPN_BJT_-_Structure_&_circuit_svg:







300px-NPN_BJT_(Planar)_Cross-section_svg:













Transistor Experiment #1 (Gain Circuit):
Using an NPN transistor such as Jameco 38359 (2N3904), an LED such as Jameco 333973, 
a 10K ohm resistor such as Jameco 691104, a 470 ohm resistor such as jameco 690785, 
and a push button such as Jameco 149948, build the following circuit. 
Assemble the circuit on a solderless breadboard such as Jameco 20723. 
Use 22 AWG solid, insulated wire such as Jamco 36881 to make the connections. 
Use a 9 volt transistor radio battery with a battery clip such as Jamco 109154. 
When you push the button to the 10K resistor, the transistor turns on the LED. 
About 0.8 ma flows through the 10K resistor to the base of the transistor. 
The transistor amplifies this signal so that roughly 12 ma flows through the LED. 
When you break the connection, all currents go to zero and the LED goes out.  







Transistor Experiment #2 (Memory Circuit):
Build the memory circuit shown below with 2 NPN transistors, some resistors and an LED. 
When you put a negative-going pulse to point "A", point "C" goes low and the LED turns on. 
When you put a negative-going pulse to point "B", point "C" goes high and the LED turns off.
This circuit has "memory". It remembers the state of the LED until power is removed.