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Infrared (IR) gas sensors are a minority of the general gas-sensing instrument market. They compete with devices based upon gas interaction with surfaces such as electrochemical sensors. They di¬er in that IR devices are selective, can`fail to safety' and are potentially intrinsically safe. However, they tend to be more expensive, using existing technology. We may expect that in due course the impact of all solid-state semiconductor mid-IR technology should revolutionize this situation and lead to signi cant growth in IR gas sensing.

The saturation of the photoconductivity due to Is-2p+ shallow donor transitions in n-GaAs has been investigated using a high power cw-FIR-laser. Magnetic field strengths were chosen in such a way that 2p+ energy levels were either below or above the N = O conduction band Landau level. In the former case Is-2p+ transitions are found to be inhomogeneously broadened with a saturation intensity as low as 0.84 mW/cm 2, giving an effective lifetime of the 2p+ state of 1.5 ~s. Above the band edge the integrated photoconduetivity does not saturate though the intensity-normalized peak photosignal decreases and the linewidth increases with raising intensity. This strange behaviour is tentatively attributed to optical excitations of 2p+ electrons to higher lying electron Landau states.Key word:semiconductors, epitaxial GaAs, shallow donors, nonlinear photoconductivity, power broadening.

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