Diodes are small semi conductors that have a P-junction and an N-junction.
Diodes are made through a process called Doping.
The P and N junctions are made on a "loaf" or "canvas" of silicon. Phosphorous, boron, germanium, and other ingredients are added to give the silicon specific electrical properties.
P-Type silicon is silicon with "holes" in its atoms. Since electrons are a negative charge, the absence of electrons form these "holes" and gives the silicon it's positive charge.
N-Type silicon is phosphorous doped which gives silicon an extra electron. The phosphorous electron can move through the silicon loaf crystal with relative ease... which means N-type can carry electrons.
Both P-Type and N-Type silicon conducts electricity. The resistance of both types is determined by the proportion of holes to extra electrons. Thus both kinds act like resistors, they will conduct in both directions.
Inside a Diode however, this is not the case. P-Type and N-Type silicon is combined forming a PN Junction where electrons can only flow in one direction.
How the Diode Works:
The diode conducts electricity in one direction only, normally regular silicon rectifier diodes work in Forward Bias mode; the N-Junction connected to negative, the P-Junction side connected to positive.
Diodes will not conduct in reverse bias, but the exception of Zener diodes.
Typical diodes have a white stripe on them, shown in the video, the determines it's cathode or negative side.
Video of Diodes in action
Schematic of Forward and reverse diodes in action
Diodes Do not conduct until their Forward Voltage is reached, kinda like an LED... LEDs won't light until it's forward voltage is reached.
Normally Silicon Rectifier Diodes Drop between 0.6 and 0.7 volts to turn itself on in forward bias.
Zener Diodes are specifically doped to turn on at certain voltages to "clip" a signal or regulate a voltage.
