Silicon unipolar photoconductor

Abstract

A novel silicon photoconductor is presented which is sensitive to light only for one current direction. When the current is reversed its sensitivity to light is drastically reduced. This unique unipolar behavior allows for autocalibration, that is, the dark resistance of the photoconductor can be measured on line simply by reversing the bias current. Regular photoconductors do not show such a unipolar sensitivity and therefore require light chopping for the dark resistance to be measured. This new type of photoconductor can thus be used for applications similar to those for which regular photoconductors are used, but also for applications which require autocalibration. For example, the unipolar principle can be used in photoconductive HgCdTe infrared detectors which require measurement of the dark resistance because the material conducts relatively well even in the absence of infrared radiation. The unipolar photoconductor is a bulk type device consisting of a highly doped p-type silicon point contact on a low-doped p-type silicon substrate. A large-area ohmic contact is made at the bottom of the substrate and serves as a second contact to the device. The principle of operation is based on the effect of minority carrier accumulation2" at high-low junctions (p'p or n ' n junctions). High-low junctions present a small potential step which is a significant barrier for minority carriers but does not restrict the flow of majority carriers. Minority carriers flowing towards the highly doped region meet the potential barrier and thus accumulate near the junction lowering the local resistivity of the material. This effect is exploited in the photoconductor as follows. When the bias current is directed such that the holes (majority carriers) flow away from the point contact, which forms a high-low junction with the substrate, and the electrons (minority carriers) flow towards it, photogenerated excess electrons near the point contact will flow towards the point contact where they accumulate and lower the resistance of the device. When the current is reversed the photogenerated electrons flow away from the point contact into the bulk where they have little influence on the device resistance. The device was realized having a 6x6 pm square point contact with a doping level of approximately 1019 cm-3 on a 525 pm thick substrate with a doping level of about loi5 ~ m ~ , The dark resistance was about 10 kQ for both current directions. Sensitivity measurements were performed in a dark room using a 550 nm wavelength monochromatic light source varying the illumination power between 0.01 and 30 W/m2. Since the accumulation effect is strongly current dependent, the sensitivity was measured with currents varying from l FA up to 2 mA for both current directions. Optimal performance was observed for a bias current of 200 pA at which the resistance at 10 W/m2 illumination power decreased by about 75% for the sensitive current direction and only about 5% for the insensitive current direction.