Electron quasi-Fermi Level Research Articles

Electrical model of the injection laser

The simplest electrical model of the injection laser assumes that the active layer, the layer in which spontaneous and stimulated radiative recombination take place, is a homogeneous layer. The only parameters of such an electrical model are the thickness of the active layer, and the electron and hole concentrations and quasi-Fermi levels. If such a model is combined with an optical model with no k -selection rule, one obtains reasonable agreement with experimental threshold current densities, particularly if energy band tails are taken into account. The theory, however, predicts a superlinear relation between gain constant and current density near room temperature, while the observed relation is linear. An alternative electrical model by Pikus assumes that the quasi-Fermi levels of electrons and holes are constant in the active layer. This model, when combined with an optical model having a k -selection rule, gives a strong superlinear dependence of gain on current and is therefore also in disagreement with experiment. A realistic electrical model of the injection laser requires a knowledge of the impurity concentration at each point and solution of the equations governing the motion of electrons and holes. We are carrying out such calculations, using a method of solution developed by J. W. Cooley and G. Hachtel. Preliminary results show that at room temperature the spontaneous emission rate and the stimulated emission rate may have substantially different spatial dependences. The calculations will give information about the effect of different impurity profiles on injection laser performance and on the effect of different current densities on the electron and hole distributions. A detailed account of this work Will be submitted for publication later.

Effect of Magnetic Field on the Threshold Current in InAs and InSb Laser Diodes†

ABSTRACT The lowering in a magnetic field of the electron quasi-Fermi level with respect to the bottom of the conduction band in a highly degenerate semiconductor is shown to contribute to two effects which affect the threshold current in InAs and InSb laser diodes. One of the effects is the narrowing of the linewidth of spontaneous emission while the other one is the reduction of the width of the active region in which radiative recombination takes place. Unlike the reduction of the width of the active region only in a transverse magnetic field due to the change of the carrier diffusion length, these effects would more or less be independent of the orientation of the magnetic field.

Current Noise and Distributed Traps in Cadmium Sulfide

The high-frequency portion of the current noise spectrum observed in lightly doped CdS single crystals under uniform 5200 A illumination is characteristic of electron trapping transitions to shallow levels. In many crystals the noise spectra have a characteristic $\frac{1}{f}$ trend when the electron quasi-Fermi level is not located near a discrete trap. The $\frac{1}{f}$ trapping noise observed in one crystal at temperatures of 10, 27, and 52\ifmmode^\circ\else\textdegree\fi{}C and for 20 different positions of the electron quasi-Fermi level between 0.5 and 0.3 ev below the conduction band can be represented by a single expression of the form $(\frac{1}{\ensuremath{\omega}})invtan\ensuremath{\omega}\ensuremath{\tau}$, where $\ensuremath{\tau}$ is determined by the low-frequency turnover of the $\frac{1}{f}$ trapping noise. From these experimental values of $\ensuremath{\tau}$, trap depths are calculated which are in good agreement with the positions of discrete trapping levels determined from other measurements. Since the low-frequency turnovers of the $\frac{1}{f}$ spectra are thus related to the discrete traps, rather than to the position of the electron quasi-Fermi level directly, it appears that the $\frac{1}{f}$ noise may not be associated with a postulated continuous distribution of traps in energy, but rather with a dispersion of capture and release times into the discrete traps.