Semiconductor lasers- Under forward biased
This section describes the variation of inversion layer width under forward biaesd.
Variation of Inversion Region:
- The condition for population inversion in semiconductors must take into account the distribution of energies available for transitions between the bands.
- The basic definition of population inversion holds—for dominance of stimulated emission between two energy levels separated by energy hv, the electron population of the upper level must be greater than that of the lower level.
- The unusual aspect of a semiconductor is that bands of levels are available for such transitions.
- Population inversion obviously exists for transitions between the bottom of the conduction band Ec and the top of the valence band Ev.
- In fact; transitions between levels in the conduction band up to Fn and levels in the valence band down to Fp take place under conditions of population inversion.
- For any given transition energy hv in a semiconductor, population inversion exists when
( Fn-Fp) > hv
- For band-to-band transitions, the minimum requirement for population inversion occurs for photons with hv = Ec- Ev = Eg
( Fn-Fp) > Eg
- When Fn and Fp lie within their respective bands, stimulated emission can dominate over a range of transitions, from hv = (Fn- Fp) to hv = Eg.
- As the dominant transitions for laser action are determined largely by the resonant cavity and the strong recombination radiation occurring near hv = Eg.
- In choosing a material for junction laser fabrication, it is necessary that electron-hole recombination occur directly, rather than through trapping processes such as are dominant in Si or Ge.
- Gallium arsenide is an example of such a "direct" semiconductor.
- Furthermore, we must be able to dope the material n-type or p-type to form a junction.
- If an appropriate resonant cavity can be constructed in the junction region, a laser results in which population inversion is accomplished by the bias current applied to the junction .