Sensors dark current

Because dark current limits the integration times obtainable at room temperature, Peltier cooling (to -150C) is used to reduce thermal population of the conduction band. In contrast to the silicon vidicon and the SIT [vide infra], where the presence of intense radiation may bloom out the entire sensor, blooming is greatly reduced with photodiode arrays even when intense lines saturate individual diodes. 

Light sensors made from a-Si H are either p-i-n or Schottky barrier structures. Unlike crystalline silicon, a p-n jimction is ineffective without the undoped layer, because of the high defect density in doped a-Si H. Illumination creates photoexcited carriers which move to the junction by diffusion or drift in the built-in potential of the depletion layer and are collected by the junction. A photovoltaic sensor (solar cell) operates without an externally applied voltage and collection of the carriers results from the internal field of the junction. When the sensor is operated with a reverse bias, the charge collection generally increases and the main role of the doped layers is to suppress the dark current. A Schottky device replaces the p-type layer with a metal which provides the built-in potential. 

A pH sensitive photoconductor based on PPV has been reported. The detection of local changes in pH by small-scale sensors is of particular interest for a variety of medical, biological, and environmental applications. In thin sheets of 25-30 nm of PPV, an increase in dark current and photocurrent is observed upon exposure to aqueous solutions. The change in photocurrent is a function of the pH. 

Calibration instability in the detector is typically caused by dark current changes over time and temperature as well as cutoff wavelength shifts with temperature which can cause small changes in apparent responsivity. Instability in the ROIC is primarily limited to offset changes in MOSFETs over time, temperature, and environmental exposure. The extent to which these impact the calibration frequency of a system is often determined through test and validation of the ROIC at the sensor-chip level, and can take minutes to days of data collection depending on the needs of the application. 

Fig. 10.3 illustrates the current-voltage characteristics of a p-i-n sensor in the dark and in light. The photocurrent rapidly saturates in reverse bias when there is full collection of the incident absorbed photon flux. The photovoltaic properties in slight forward bias are characterized by the short circuit current J, the open circuit voltage, Vog, and the fill factor, F. The maximum power delivered by the device is the product of the three terms... 

Fig. 10.3. Schematic of the current-voltage characteristics of a p-i-n sensor in the dark and under illumination.

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