ABSTRACT
The central microscopic process in all optoelectronic devices is the absorption or the generation of a photon by an electron. The photon generation can happen spontaneously or can be triggered by another photon. We distinguish the following three key processes and corresponding device types:
Photon absorption in photodetectors Spontaneous photon emission in light-emitting diodes Stimulated photon emission in laser diodes
• • •
Typically, photon absorption and generation involve the generation and recombination of an electron-hole pair, respectively. Electrons and holes carry electric charges and are often called carriers. Figure 2.1 shows all three processes schematically. The photon energy hn must be equal or larger than the energy gap Eg between valence and conduction band (h, Planck’s constant; and n, light wave frequency). Correspondingly, the maximum light wavelength l = c/n is equal to the so-called gap wavelength
(nm) (eV)
= =
hc
E E
(2.1)
where c denotes the free-space light velocity. Transition rates are used to quantify the number of generation or recombination events per unit
volume and per second. The carrier generation rate Gopt due to photon absorption is proportional to the photon fl ux Φph per area and per second
G
P
= × = ×
a a n
Φ
(2.2)
where the material parameter a is called absorption coeffi cient (unit: m−1). The photon fl ux is equal to the optical power Popt divided by the area A and the photon energy hn.
