The kinetics of generating ultrashort light pulses by gain switching unbiased semiconductor lasers emitting relaxation oscillations is theoretically modeled and described using phase portraits. Biomolecular recombination processes and realistic injection current pulse shapes are incorporated in the model. Approximate analytical solutions of the rate equations are derived for high current injection. Laser pulse widths, pulse peak power, electrical to optical pulse delay times, and time difference to subsequent relaxation oscillations are computed. Their dependence on injection current to threshold current density ratio ( ) and on material and laser design parameters is explicitly derived and is in good agreement with experiment. In particular the remarkable observation that the laser pulse width is broadly independent of the injection current rise and fall time can thus be understood.