Abstract

A major problem inherent in the design of any small area photodiode is making electrical contact without greatly increasing the extrinsic capacitance. The planar surface area of the device must be large enough to accommodate both an adequate light collecting area and a metallized contact. We have investigated a scheme where the metallized bonding area is removed from the light collecting surface by extending a 10-µm wide gold finger from the bonding pad to the top surface of the mesa photodiode (15-µm mesa diameter). This is made possible by burying the mesa beneath a layer of transparent passivating dielectric (polyimide) and then patterning the polyimide to produce a hole through to the mesa. The photodiode itself consisted of n-type GaAs (3E16/cm³) which was ion implanted with Zn (3.3E13 Zn/cm² at 100 keV). The small area of the device and passivating qualities of the polyimide yielded devices with dark current typically 0.1 nA at 75% breakdown voltage (33 V). The impulse response of the photodiode at 20-V reverse bias was found to be 48-ps (±2) (FWHM). This was consistent with measured device capacitance (0.325 pF) and a 50-Ω load.

© 1989 Optical Society of America