Electronic materials can be generally classified as Conductors, Insulators, or semiconductors.

Conductors allow the flow of electricity through them.
Insulators prevent and/or shield other materials from the flow of electricity.
Semiconductors allow the flow of electricity only under certain conditions.
Diodes are semiconductors.
Diodes are devices, which might be described as passing current in one direction only.
For this reason they are very useful as switches and especially important in binary functions where ON = 1, and OFF = 0.
Some schematic symbols for diodes are:
schematic symbols for diodes
Figure 1 - schematic symbols for diodes
The most common symbol is the first in Figure 1.
The straight bar end has the letter "k".
This denotes the "cathode" while the "a" denotes the anode.
The CATHODE is the POSITIVE (+) terminal.
The ANODE is the NEGATIVE (-) terminal.
Current can only flow from anode to cathode and not in the reverse direction.
This is one very important property of diodes.
The "arrow" indicates this flow of current.
In a real diode, a silver band indicates the KATHODE.
LAB EXERCISE: Testing a diode with a multimeter.
Obtain your kit and one diode from your teacher
Set the multimeter on the symbo
Place the positive (RED) terminal on one side of the diode and the negative terminal (BLACK) on the other side.
What is the reading on the meter?
Record it on a piece of paper
Reverse the polarity of the two terminals (switch them around and take a second reading)
Record it on a piece of paper
Can you tell which side is the anode and which side is the cathode? Explain.