17-11-2012, 04:51 PM
INFRARED LED'S
Gallium arsenide is a direct-gap semiconductor with an energy gap of 1.4eV at room temperature. A typical GaPs LED is made by solid-state impurity diffusion with zinc as the p-type impurity diffused into an n-type substate doped with tin, tellurium or silicon. The external efficiency at room temperature is typically 5 percent.
A GaAs diode can also be fabricated by liquid-phase epitaxy with silicon as both its n and p dopants. If a silicon atom replaces a Ga atom, it provides one additional electron, thus the resulting GaAs in as n-type. If a silicon atom replaces arsenic atoms, an electron is missing and the resulting GaAs is a p-type. In Si doped GaAs diode, the emission peak shifts down to 1.32eV. Since the emission is in infrared region, GaAs light sources are suitable for application such as the optical isolator. The high switching speed, with a recovery time between 2 and 10ns, makes them ideal for data transmission.
The disadvantages of the GaAs emitter are emitted wavelength and the associated attenuation an dispersion. A critical issue of using an LED for the fibre optics is the coupling of light from the semiconductor to the fibre. Because of the larger refractive index of GaAs relative to air, the internal efficiency of LED can be quite low.