Photoelectric Characteristic Research Of Semiconductor Lasers

The study on electrical characteristics of semiconductor diodes has been a valuable subject. However, in the past years, an accurate method to determine electrical characteristics of a diode at forward bias voltage did not exist.In recent years, our group found that the forward alternating-current (ac) behavior, involving both real and imaginary parts of admittance at different frequencies, can provide more information of diodes. Thus we developed a new method based on ac measurements together with a direct-current (dc) I-V plot to accurately characterize the electrical behavior of a semiconductor diode. Using this method, the accurate forward electrical behavior of multi-quantum-well laser diodes (MQW LDs) has been characterized. The synchronous step offsets of various electrical characteristics at the onset of lasing were observed and characterized. In particular, the junction voltage jumps abruptly to a saturated value just at lasing threshold. None of the phenomena can be explained by existing theories about semiconductor laser. All the sudden changes correspond to broken symmetry, in other words, the laser is in a new, highly ordered state on a macroscopic scale. Furthermore, a jump of ideality factor and a drop of resistance in magnitude at lasing threshold were observed; according to the traditional theory, the ideality factor and resistance are constants, but actually they are nonlinear, especially at low current this nonlinearity is obvious. All these properties can not be characterized by conventional methods. In order to compare the characteristics of different semiconductor devices, the properties of AlxGa1-xAs-GaAs strip Double-Heterostructure (DH) lasers and light-emitting diodes (LEDs) were characterized. Similar to MQW LDs, all electrical parameters of DH LDs show discontinuity at lasing threshold; however, all electrical parameters of LEDs vary continuously with increasing current, which displays that lasing is a reason of discontinuity of electrical parameters of LDs. Complete saturation of junction voltage and nonlinearities of ideality factor and resistance of LEDs were also observed. From the above research, a convenient and accurate method for confirming lasing threshold and a new evoluation method of electrical characteristics of LEDs are presented.At present, the sudden changes of properties of LDs at the onset of lasing were studied theoretically. First, after analyzing the rationality of discontinuity of electrical parameters of LDs, we confirmed that the discontinuity of electrical parameters at the onset of lasing is a necessary result of lasing. Second, the jump of junction voltage at threshold could be explained from theory of dissipation structure, and the change scope of concentration of carries was calculated from the variational value of junction voltage at threshold. Finally, the sudden changes of electrical characteristics of LDs at threshold were discussed using theory of phase transition. When studying the transition-region lamp laser, Haken H. concluded that the laser shows the features of a second-order phase transition. However, an electrical injection semiconductor laser involving carriers of electron and hole as well as photons is a far more complex system than a light pumping laser. So author thought that semiconductor laser may show the features of a first-order phase transition under certain condition.