From analyzing the discontinuous electrical behaviors of laser diodes(LDs) at lasing threshold in detail, it can be confirmed that the discontinuous electrical behaviors are the necessary results of the lasing of LDs.The sudden changes of junction voltage, series resistance and ideal factor of LDs at threshold could be directly deduced from the experimental IdV/dI-I curve.Compared with the ac small-signal method, the conventional electrical methods are proved to be very rough,and could not accurately charac...