To understand how an LED emits light, we need to take a look at how semiconductors and diodes function. This Month’s tip will briefly cover the basics of these and how they apply to LEDs.
Semiconductors – Semiconductors are the foundation for modern electronics. They have a varying ability to conduct electricity, making them a good medium for controlling electrical current. Semiconductor materials exist in two types – elemental materials and compound materials. Although some pure elements and many compounds display semiconductor properties, silicon and germanium are the most widely used in electronic devices.
The conductivity of a semiconductor falls between that of a conductor (such as copper) and an insulator (such as glass).
The specific properties and conductivity of a semiconductor depend on the impurities (atoms from another material) added to it. This process is called doping. There are two types of doped semiconductors: N-type and P-type. A semiconductor with extra electrons is called N-type material because it has extra negatively charged particles. A semiconductor with extra holes is called P-type material because it has extra positively charged particles.
p-n Junction – The p–n junction is one of the primary building blocks of most semiconductor electronic devices such as diodes, transistors, solar cells, and integrated circuits; they are the active sites where the electronic action of the device takes place.
Broadly speaking, the function of a p-n junction the movement/exchange of electrons and holes (the lack of an electron at a position where one could exist in an atom).
p-n junctions are formed by placing a p-type and an n-type material in contact with each other. Since the n-type region has a high electron concentration and the p-type a high hole concentration, electrons diffuse from the n-type side to the p-type side. Filling a hole makes a negative ion and leaves behind a positive ion on the n-side. This exchange will allow a continuous forward current through the junction (if the diode is forward-biased)
Diode - A diode is the simplest form of a semiconductor device. It is an electronic component that restricts current flow to only one direction. Diodes have two electrodes called the Anode (positive wire) and Cathode (negative wire), which are the leads that connect the circuit.
LED – A “Light Emitting Diode”, or LED, is a special type of diode that emits light as electricity flows through it. They are p-n junction diodes. LEDs consist of 3 main components. The anode, the cathode, and a semiconductor crystal between them. The color of the LED is determined by the semiconductor material that is used.
When a suitable voltage is applied, electrons are able to recombine with electron holes, releasing energy in the form of photons. This effect is called electroluminescence, and if the semiconductor is exposed, can produce light.
In this diagram, you can see the process of electrons filling holes, as well the light that is produced. Note that this diode is forward biased based on the orientation of the battery.
If you’d like to learn more about how Semiconductors and LEDs work, I suggest researching topics on conductors & insulators, voltage, current, resistance, and Ohm’s law. If you’d like to learn more about LEDs and how their benefits are applied in the indoor garden, enroll in an Oaksterdam University Horticulture Course, and stay tuned for next month’s tip!