Effect of Junction Capacitance on the Rise Time of LED's and on the Turn-on Delay of Injection Lasers

Among the optical sources presently available for optical communication applications, AlGaAs light-emitting diodes1,2 and injection lasers3,4 are probably the most compatible with low-loss optical fibers. 5,6 The possibility of direct pulse modulation of the optical output of these devices by varying the driving current is a major advantage, and the intrinsic radiative recombination time suggests that very high modulation rates are possible. Fractional-nanosecond rise times and modulation rates to 100 Mb/s for LED'S7,10 and modulation rates to hundreds of Mb/s for injection lasers 11-17 have been reported. Faster rise times and higher modulation rates are more difficult to attain. For an ideal LED, if the injected carriers arrive instantaneously at the recombination (diffusion) region, the rise time of the spontaneous emission is governed solely by the spontaneous recombination time of the carriers. However, in driving a practical diode, the junction capacitance and the stray capacitance cause delay in the arrival time of the injected carriers at the recombination region. Thus, the rise time of the spontaneous emission in an LED and the delay time of the stimulated 53