Diodes are the simplest of semiconductor devices.
Electrons travel through a diode much more easily in one direction than the other.
Diodes are therfore most commonly used for converting AC (Alternating Current) to DC (Direct Current).
Semiconductors are usually made from crystals of Silicon (although sometimes Selenium or Germanium).
Man-made crystals are grown into round inguts several inches wide that are then sliced into thin wafers.
A pure Silicon wafer is a poor conductor of electricity, but it becomes a much better conductor when later doped
with impurities such as Boron or Phospherous. (There are many other impurities that produce a variety
of effects.) Boron produces a P-type semiconductor. Phosphorous produces an N-type semiconductor. When P-type
material is in contact with N-type material, it forms a P-N junction diode. Electrons flow fairly easily from
the "N" (Negative) side to the "P" (Positive) side. Note that electrons have a negative charge, but in
electronics, current is always defined as flowing in the "positive" direction, so the electrons actually
travel in the oposite direction to "current". Positive current will flow from the "P" side (Anode) to the
"N" side (cathode).
Note that current will not flow across the P-N junction until a 0.7 bias voltage is applied across the junction.
Once this bias is applied, current flows freely as if across a short-circuit. Note that since a diode can act like a short-circuit,
you must always limit the current somehow to avoid a high current that burns out the diode. Resistors are often used to limit current.
Diode model showing a forward-biased P-N junction:
Diode Experiment:
Using aligator clip test leads, connect a 1K (1000) ohm 1/4 watt resistor in series with a 1N4004 diode and apply 9 VDC (9 volts direct current) across the resistor and diode using a 9 VDC transistor radio battery. Using a DMM (Digital Multi-Meter) (or more often referred to as DVM for Digital Volt meter), measure the voltage across the diode and across the resistor. If the battery is connected so that the diode is forward-biased, you should measure 0.7 VDC across the diode and 8.3 VDC (9 - 0.7 = 8.3) across the resistor. If you set the meter to current-mode (amps) and connect it in series with the diode and resistor, you should measure 8.3 milliamps. According to Ohm's law, 8.3v/1000ohm = 8.3 milliamps. milliamps). You may get slightly different readings depending on the actual battery voltage. Set the DMM back to volts and measure the voltage across the battery terminals. A fresh alkaline battery will read a bit higher than 9.0 volts. (Be carefull not to put the leads of the DMM across the battery when the DMM is in current-mode; the resulting short-circuit and high current can damage the DMM.) Try reversing the connection to the battery so that the diode is reverse-biased. Under this condition there should be 9 volts across the diode, zero volts across the resistor and zero milliamps of current.