Low-Light-Level Limitations of Silicon Junction Photodetectors
Abstract
The properties of silicon junction photodetectors have been investigated with the goal of improving low-light-level performance of a reading aid for the blind. This application requires an image-forming mosaic of photodetectors, which may be conveniently fabricated by use of integrated circuit technology. Solid-state imaging is particularly attractive because of the low resolution (100 to 200 elements) needed for the reading-aid system. Attention is focused upon photodetector operation utilizing the charge-storage technique, because of the usefulness of this method in increasing the signal level in scanned image applications. The signal and noise transmission properties of the basic charge-storage circuit are analyzed, with the assumption of an ideal switching element. The primary contributors to output uncertainty are found to be dark current and its associated 1/f noise because of the lowpass nature of the circuit. Practical charge-storage circuits, based upon the available bipolar and metal-oxide-semiconductor (MOS) technologies, are studied. Low-light-level performance is determined to be most severely limited by switch imperfections. The low-light-level operation of the charge-storage phototransistor is analyzed in detail. Performance is shown to be increasingly degraded as the light level is reduced due to influence of the emitter-base junction. The use of an MOS switch is considered, and capacitive feedthrough found to be excessive. A charge integration scheme is proposed to circumvent the feedthrough charge, leaving the most significant noise source to be an unusual 'charge pumping' effect.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 01, 1968
- Accession Number
- AD0834859
Entities
People
- Joseph S. Brugler
Organizations
- Stanford University