Diodes
A diode is an electrical circuit component that allows current to flow in only one direction. A diode consists of two electrodes sealed in a vacuum. One electrode, the cathode, is attached to a negative electric potential, the other electrode, the anode, is attached to a positive electrical potential. The word diode is derived from the term di-electrode (i.e., two electrodes).
The diode was first developed by English scientist Sir John Ambrose Fleming. Historically the original diode was an evacuated glass bottle or tube with two metal plates (electrodes) inside, each connected to opposite electrical charges. The negatively charged plate (cathode) was heated to a glowing red temperature and the positively charged electrode (anode) was left cold. Thermionic diode tubes (originally thermionic valves) were the mainstay of the electronics industry until the late 1950s when transistors and, later, integrated circuit (IC) chips became the norm. Today, thermionic diode tubes still have use, particularly when large electrical currents are needed.
The principle of the thermionic diode is based on the fact that in a vacuum a hot conductor will expel free electrons from its surface. If the electron emitter is connected to a negative potential, the free-floating electrons will experience an electrical force directed away from the negative cathode. If an additional electrode attached to a relatively positive electrical potential (i.e., the anode) is placed some distance away from the cathode, the electrons will experience an attractive force toward the positive anode. Accordingly, as the charged particles migrate, a current of electricity is said to flow across the space between the cathode and the anode.
If the electrical charge on the two electrodes is now reversed the flow of electrons stops. This is because the hot cathode is still expelling electrons but the electron cannot cross to the original anode because of the repulsive negative charge. They are also pulled back to the original cathode, which is when it becomes positively charged. In other words, in a thermionic diode, current can flow only one way across the space between the cathode and anode, and that is when the hot cathode is negatively charged with respect to the cold relatively positive anode.
Most electrical current in electronic equipment is alternating in two opposite directions. Circuit designers often need to rectify this current from an alternating current (AC) to a direct current (DC). The diode achieves this effect. When an alternating potential is applied across a conductor, an alternating current results; when it is placed across a diode, current flows in one direction only.
The transistor eventually replaced the thermionic diode with what became known as the p-n junction diode. This diode had no glass bottle, no heated cathode, and no vacuum, instead it used two substances termed p-type and n-type semiconductors that have opposite charge flow characteristics. One substance, n-type germanium, allows its electrons to move through itself relatively easily. The other semiconductor, p-type germanium, tends to allow its positive charges or, more accurately, positive holes (vacancies formerly occupied by electrons) to move much more efficiently one direction than in the other.
In a p-n junction diode, these two substances are fused together. Their junction becomes a place where the drifting positive charges or holes from the p-type germanium and the drifting electrons from the n-type germanium can combine (in the process forming new positive holes elsewhere in the p-type germanium and new free electrons elsewhere in the n-type germanium). This drifting of the positive holes and the negative electrons towards each other at the junction is similar to an electric current through the bonded materials. The current is accentuated by applying a positive charge to the p-type germanium thus repelling the positive holes more strongly towards the junction. A corresponding negative charge applied to the n-type germanium strongly repels the electrons towards the junction and a strong electrical current appears to flow through the junction.
Applying the opposite charges to the materials has the same effect as in the thermionic diode, basically there will be no electric current. This is because the reversed forces on the positive holes and the electrons will pull them both away from the junction where they can combine. Pulling the holes and electrons apart effectively stops the current. Accordingly, the thermionic diode and the p-n junction diode produce the same effect. Applying an electric potential one way across them will result in current flow. Reversing that potential stops current flow and allows rectification of alternating current.
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