Comparison of Single Heterostructure and Double Heterostucture GaAs - GaAIAs LEDs for Optical Data Links

Burrus-type, 1 single heterostructure (SH) light-emitting diodes ( L E D S ) are presently fabricated in our laboratory for use in optical data links. Their performance requirements are 45 MHz bandwidth and a minimum butt-coupled power of 10 juW into a 0.36 numerical aperture (NA), 55-jiim core, graded-index optical fiber at 60 mA forward current. 2 S H L E D S were chosen for the simple growth procedure and exceptional 3 reliability. After optimization of the active layer width and carrier concentration for maximum efficiency while maintaining the required bandwidth, the SH L E D S butt-couple 10 to 15 juW of power into the fiber. However, initially fabricated, unoptimized double heterostructure (DH) L E D S were found to couple 7 to 8 times more power than the optimized S H LEDS. The purpose of this paper is to explain the difference between the SH and DH LED performance. We begin with an 1579 explanation of the factors influencing the efficiency and bandwidth of the two structures. Figure 1 is a schematic of our Burrus-type SH and DH LEDS. These L E D S use current crowding by contact area restriction to increase current density for increased brightness. In addition, the localized emission region couples the light more efficiently into the fiber. The contact diameter for both structures is chosen to be 50 fim, slightly smaller than the fiber core, for our performance comparison. Figure la shows the design of our SH LED. The active layer is confined on one side by the n-GaAlAs/p-GaAs heterojunction and on the other side by the p-ohmic contact.