Integration of high-gain double heterojunction GaAs bipolar transistors with a LED for optical neural network application

A 10×10 array of optical neurons consisting of monolithically integrated DHPTs (double heterojunction phototransistors), DHBTs (double heterojunction bipolar transistors), and LEDs (light-emitting diodes) was fabricated in an AlGaAs/GaAs/AlGaAs double heterostructure. A single DHBT exhibited a current gain as high as 500 with an ideality factor of 1.4. A Darlington transistor pair showed a combined current gain of 4000. The power density of the LED was about 300 W/cm². The integrated structure, however, showed SCR (silicon controlled rectifier) characteristics, which was attributed to the coupling of a parasitic p-n-p transistor to the n-p-n DHBT. This problem was eliminated by first etching a groove in the semi-insulating substrate between the LED and the Darlington transistor pair and then employing metallization to provide proper connection. However, overall gains for the Darlington transistor pair were low, probably due to the leakage currents caused by surface contamination and the Zn diffusion process