Subsequent to the calibration, the peak emission energy is measured as a function of the DC This method makes use of the dependence of the bandgap energy (and thus the peak emission wavelength) on temperature. The method consists of a calibration measurement and a junction- temperature measurement. In the calibration measurement, the peak energy is […]

The Boltzmann distribution of carriers, applicable to the high-energy part of the emission spectrum, results in an exponential dependence of the emission intensity on energy, i. e. I к exp [-hvl(kTc) (6.1) where Tc is the carrier temperature. The high-energy slope of the spectrum is given by d(lnI) -1 к (6.2) d(hv) Thus, the carrier […]

The temperature of the active region crystal lattice, frequently referred to as the junction temperature, is a critical parameter. The junction temperature is relevant for several reasons. Firstly, the internal quantum efficiency depends on the junction temperature. Secondly, high — temperature operation shortens the device lifetime. Thirdly, a high device temperature can lead to degradation […]

The emission intensity of LEDs decreases with increasing temperature. This decrease of the emission intensity is due to several temperature-dependent factors including (i) non-radiative recombination via deep levels, (ii) surface recombination, and (iii) carrier loss over heterostructure barriers. Near room temperature, the temperature dependence of the LED emission intensity is frequently described by the phenomenological […]

The index contrast between the light-emitting material and the surrounding material leads to a non-isotropic emission pattern. For high-index light-emitting materials with a planar surface, a lambertian emission pattern is obtained. Figure 5.4 illustrates a point-like light source located a short distance below a semiconductor-air interface. Consider a light ray emitted from the source at […]

All LEDs have a certain radiation pattern or far-field pattern. The intensity, measured in W/cm2, depends on the longitudinal and azimuth angle and the distance from the LED. The total optical power emitted by the LED is obtained by integration over the area of a sphere. P = jA lx IWdX dA (5.25) where I(X) […]

5.3 The light escape cone Light generated inside a semiconductor cannot escape from the semiconductor if it is totally internally reflected at the semiconductor-air interface. If the angle of incidence of a light ray is close to normal incidence, light can escape from the semiconductor. However, total internal reflection occurs for light rays with oblique […]

The physical mechanism by which semiconductor LEDs emit light is spontaneous recombination of electron-hole pairs and simultaneous emission of photons. The spontaneous emission process is fundamentally different from the stimulated emission process occurring in semiconductor lasers and superluminescent LEDs. Spontaneous recombination has certain characteristics that determine the optical properties of LEDs. The properties of spontaneous […]

5.1 Internal, extraction, external, and power efficiencies The active region of an ideal LED emits one photon for every electron injected. Each charge quantum-particle (electron) produces one light quantum-particle (photon). Thus the ideal active region of an LED has a quantum efficiency of unity. The internal quantum efficiency is defined as П _ number of […]

The energy of an injected electron is converted into optical energy upon electron-hole recombination. Thus, conservation of energy requires that the drive voltage or forward voltage of a light-emitting device is equal to (or larger than) the bandgap energy divided by the elementary charge. The diode voltage is thus given by V = hv/ e […]