New approach for SOI pixel sensor

Abstract

Silicon on Insulator (SOI) is an interesting alternative to bulk silicon for the fabrication of integrated circuits due to its advantages with respect to the junction leakage, low switching noise coupling, high temperature immunity, low voltage and low power applications. Recently, SOI transistors have also been used in high speed CPU's due to their high switching performances and their reduced power consumption. Another application where high performances and even higher densities are needed are dynamic memories (DRAM) where floating body SOI MOSFETS were used as an 1T memory node. Using the floating body as a charge storage reduces the unit cell size and drastically increases the bit density and the storage capacity. However, despite technical advances in SOI technology, it has rarely been exploited in optical sensing and imagery. The main reasons are the expected low optical conversion efficiency due to the relatively thin silicon film thicknesses, well below 1 µm and the slow time constants due to slow recombinations at the junctions. In addition, the slim active region reduces the optical bandwidth of such sensors as longer wavelengths are absorbed deeper (and in the case of SOI probably in the buried oxide layer). Despite these major handicaps, it was shown recently that an SOI MOSFET based phototransistor could detect light intensities as low as 5 mW/m2. However, previous work addressed only low light intensities neglecting the slow transients drawbacks. Moreover, as for most fully and partially depleted SOI MOSFET's based photodetectors, it is the drain current variation due to light absorption that was used as a measure of photon densities (for instance the 5 mW/m2 generates 50 fA of photocurrent). Such variations are hard to measure with the needed resolution as such currents are close to the noise levels of any amplifier. This research project proposes a new measurement technique that does not rely on direct quantification of the photocurrent and hence overcomes the problems inherent to noise and low current variations. In addition to that, this novel technique solves the problem of slow drain current recovery time inherent to the slow recombinations at the junctions. This technique relies on the transient charge pumping used to remove continuously photogenerated charges from the electrically insulated body of the MOSFET. Then, since the transistor is always maintained in equilibrium conditions, this approach will get rid of any transient effect occurring in the partially depleted SOI MOSFET. Also presented in this work is an extension of this technique to any floating body MOSFET. We presented also measurement of bulk P-MOSFET whose n-well was left floating and showed that the behaviour was similar to that of a floating body SOI MOSFET. Still using the transient charge pumping to remove extra charge from the floating n-well. Finally, An SOI circuit implementation of this technique was presented. This circuit takes advantage of some of the properties of the floating body SOI MOSFET to implement a first order delta sigma modulator at the pixel level without substantially reducing the fill factor. The first order delta sigma modulator in each pixel, can improve the resolution and offer a direct digital output without the need of an ADC.