What is a Light Emitting Diode?

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p-type and n-type materials

Inorganic Light emitting diodes have become very common in the last decade. They are used for traffic lights and taillights. If you look inside you see a very small inorganic multi-layer diode material that is driven in forward bias. That light is emitted isotropically and a cone is used to focus the light in the forward direction. In the last decade we gone from red being readily available to green and blue. You can get these quite inexpensively. The diode laser is ubiquitous.

Here is a simple model for diode. We take a piece of p type semiconductor material that is created with excess dopants such as boron. This is assembled with an type semiconductor which is created with silicon that has been doped with phosphorous. When you bring these two together you have the potential for charge transfer between these two materials. This creates a PN diode. You deplete the n part of interfacial area of electrons and you deplete the p portion of the interface of holes. This creates a built-in potential, V_bi.

Properties of Diodes

Diodes provide rectification. Current only flows in one direction. When apply voltage and pass current through a resistor you get a linear plot known as Ohms law. When you apply voltage and pass current through a diode you get an asymmetry in the voltage current response. If we add a positive bias the current is exponentially increasing. With a reverse bias there is very little current flowing. A good diode should have very low turn on voltage Vd with exponential increase, and very low current with negative bias. The Shockley equation describes the current through the diode at any bias. The reverse saturation current Js is related to the purity of the device and should be as low as possible. The applied potential, the diode quality factor Boltzman constant, absolute temperature and charge on the electron. This results in current flowing across the depletion layer region. Interesting things happen when you go from bias to bias, especially in systems that can produce light.

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This is an energy level diagram as a function of distance across a PN diode. The system is at equilibrium therefore the Fermi level in both the N region and the P region are exactly the same. This is the definition of equilibrium in the condensed phase. We can think of the Fermi level as the average level of an electron entering or leaving the solid. There is significant energy difference as we move from the N region to the P region for both electrons and holes because the depletion of the majority carriers that have occurred in that system.

In the case of reverse bias we make that depletion even more significant. We increase the built-in voltage by V_d . The deplete region thickness increases. As long as it is a purified material we see very little current flowing.

In reverse bias the Fermi levels are no longer aligned because the system is not in equilibrium. There is an energy barrier that makes it very difficult for electrons in the N region to transit to the P region and similarly it is difficult for the holes to move from the P region to the N region. This is exactly the region in a organic photovoltaic cell where the application of light causes absorbs energy and causes the charge to move. However in LEDs or OLEDs you do not want current flow when there is reverse bias.

Forward Bias in a LED

In the forward biased diode the current flows and the depletion region is narrowed or eliminated. Majority carriers move from one region to the other. The energy bands are not in equilibrium and the energy level of the P region is lower than the N region. Electrons move from the N region to the P region and holes move from P region to N region. Forward bias in an LED moves the electron and holes. Recombination events occur at the junction and excess free energy is dissipated as light coming out of the center region. The color of light from an inorganic diode is controlled by the band gap energy for those semiconductors. The first LED were gallium arsenside that has a low bandgap that gives the red light of early calculators. As people began to tailor the bandgap of 3-5 semiconductors they have achieved orange, green and most recently blue color emission.

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