A photoluminescence lifetime microscope spectrometer (PLµS) based on time-correlated single-photon counting (TCSPC) with a single-photon avalanche diode (SPAD) detector was developed for the investigation of the microscopic spatial fluctuations of the minority-carrier lifetime and other material parameters in gallium arsenide (GaAs) and related compound semiconductors. The high sensitivity of the TCSPC technique is essential for time-resolved photoluminescence (TRPL) measurements with a spatial resolution of the order of a few micrometers. The spatial resolution of the PLµS is compatible with the diffusion length in GaAs substrates or the dimensions of small, highly integrated GaAs devices. The PLµS is the first instrument of its kind based entirely on solid-state components, i.e., a gain-switched diode laser for pulsed excitation and an avalanche diode for single-photon detection. It is explained why, in samples dominated by nonradiative linear recombination, the relatively large excitation pulse width of the diode laser compared to a synchronously pumped dye laser does not adversely affect the timing accuracy of the measured decay time constants. It was previously shown that, by using nonlinear least-squares iterative convolution analysis, a TCSPC instrument with an instrumental response width of 70 ps (FWHM) is capable of accurately measuring fluorescence decay time constants down to 10 ps with ±2 ps timing accuracy. The PLµS is designed to take full advantage of the high repetition rates, up to 100 MHz, typical for mode-locked laser sources or gain-switched diode lasers. The data-acquisition time for a complete decay curve containing typically several hundred thousand counts is thus reduced to a few seconds. Hence, the PLµS is, in principle, suitable for scanning operation and topographical imaging.