For several applications, such as for example laser distance ranging or time gating, very fast electronic shutters, fast electrically programmable changing of sensitivity volume of a pixel or of sensitivity of a pixel, it is needed that impinging radiation is alternatingly visible (detector is sensitive for impinging radiation) and not visible (detector is insensitive for impinging radiation) during a single integration time. Making the detector alternatingly sensitive and insensitive to the impinging radiation is also called “time-gating the radiation”. It has the advantage that it allows to read out a total accumulated charge of many smaller time periods with the read noise of one single readout operation.
Time-gating of radiation can be obtained by using a mechanical shutter, or by pulsing a grid in an image amplification tube (also called photomultiplier tube or valve amplifier).
Other techniques can be solid state based.
The most well-known solid state techniques are the so-called “electronic shutter” methods. In the classical electronic shutters, a pixel can be made effectively sensitive during a precise part of a total frame time, called the “integration time”, which integration time is then typically determined as the time between the end of the pixel reset, and the moment of readout or of “sample & hold”. However, such methods are only suitable when the radiation must be gated as one single continuous time period or integration time.
Another solid state solution is disclosed by M. Lehman & al. in “Smart pixels for future 3D TOF sensors”, IEEE Workshop on CCD and advance image sensors, Karuizawa Japan, Jun. 9-11 2005, proceedings p. 193. This solution is based on a CCD principle, which is non-standard and expensive. Furthermore, it shows difficulties when working with thick (hence sensitive) silicon layers.
Yet another solid state solution is disclosed by D. Van Nieuwenhove et al. in “A novel Standard CMOS detector using majority current for guiding Photo-generated electrons towards detecting junctions”, proceedings symposium IEEE/LEOS Benelux chapter, 2005, p. 229-232. This solution is based on majority current assisting movement of charge carriers. It is a disadvantage of this solution that the assisting majority current consumes high peak currents and power, and that the on/off ratio is limited.
Still another solution is based on the work of Rudolf Schwarte, and is for example disclosed in WO 02/33817. This document essentially describes a method to demodulate radiation that is modulated. The teaching of this document is based on the concept that electrical field lines are such that they start at one diode and end at another. This is effective and needed when the signal charges are assumed to emerge in the volume where these electrical field lines reign.
Electric domain demodulation is described in “A CMOS photosensor array for 3D imaging using pulsed laser”, Jeremias, R.; Brockherde, W.; Doemens, G.; Hosticka, B.; List I, L.; Mengel, P. IEEE International Solid-State Circuits Conference, 2001. Digest of Technical Papers. ISSCC. 2001 Session 16, 2001 Page(s):252-253, 452-453: demodulations in the electric signal are performed, i.e. after the light signal or photo charge has been converted to an electrical signal or voltage. However, the applicability is limited. The electric domain demodulation is inherently very noisy, as the differencing operation (that is essentially demodulation) now happens after noise has already entered into the signal.
Each of the above solutions has manufacturing and operational disadvantages.