Optoelectronic Semiconductor Devices Principals and Characteristics

Figure 23.: Types of surface-emitting injection lasers. 
[1]
These lasers are quite small. Typically, each emitter is about 6 to 10 µm long and 10 µm in diameter. 
They are often referred to as microlasers. 

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4 LIGHT-EMITTING DIODES 
Light-emitting diodes (LEDs) are a typical pn-junction device used under a forward bias that emit 
incoherent light when current passes through the semiconductor junction. When the minority charge 
carriers recombine radiatively with the majority charge carriers, photons are emitted. This is the basic 
light generation process in semiconductors. 
Depending on the semiconductor material used in the light-emitting layer (active layer), the wavelength 
of the emitted light can be anywhere within the range from visible to infrared. 
Most commercially available LEDs are made from III-V compound semiconductors. Some II-VI 
compound semiconductors such as ZnS and AnSe are used in a few LEDs emitting visible light though 
these materials are not used frequently because of the difficulty of pn-junction formation. 
LEDs have been widely used in various kinds of equipment and systems. LEDs composed of a 
combination of InGaAsP and InP cover the wavelengths from the 1300 to 1550 nm band and are the ones 
usually used in optical fiber communication systems. 
A combination of GaAs and AlGaAs of a combination of different compositions of AlGaAs has been 
used to make LEDs emitting in the 780 to 900 nm band, and those LEDs have been used in optical fiber 
communication systems, data links, remote controllers, etc. 
For display and indicator applications, LEDs emitting from blue to red light are usually applied. 
In a typical display LED, the current density is on the order of 10 A/cm
2
. This corresponds to an injection 
of approximately 6×10
19
carriers/cm
2
s. 
Even for a moderate current injection, such as 10 A/cm
2
, the number of injected minority charge carriers 
far exceeds that of minority charge carriers in thermal equilibrium at room temperature. 
For LEDs designed for communications applications, the current density is typically 10
3
A/cm
2
or higher. 
This means that the injected minority charge carrier density in these LEDs is about two orders of 
magnitude greater than typical display LEDs.