CEA-Leti develops a new light-sensing device that integrates photodiodes below the buried oxide (BOX) of FDSOI transistors, making the transistors very sensitive to visible light. Photodiodes are co-integrated in the SOI substrate, replacing conventional FDSOI transistor backgate. Potentially, this device architecture may lead not only to very small pixels with maximized fill factor, but also to more complex light-detection functions, due to complementary effects observed depending on diode polarity and FET type.
Presented in IEDM 2016 32.6 paper “Extending the Functionality of FDSOI N- and P-FETs to Light Sensing”, the new device architecture uses capacitive coupling, which doesn’t necessarily require an electrical connection between the transistor and the diode. Leti said preliminary results show that sensitivity in the visible spectrum is already better than 0.1pW/µm2, with a wide dynamic range (seven orders of magnitude, i.e. similar to most advanced CMOS image sensors).
“FDSOI is a very versatile technology that already has been shown to be ‘faster, cooler, and simpler’ than FinFET, and which also may become smarter for More than Moore applications such as imaging,” said Lina Kadura, who presented the paper. “In fact, it may be smarter for sensing generally, because FDSOI transistors can be considered as very small footprint probes that are sensitive to the electric potential below the BOX.”
In addition to embedding more light-sensing functionality in circuits, potential future applications include leveraging pixel size in image sensors.
In other results of the study, Leti demonstrated for the first time that SRAM cell characteristics can be controlled by light illumination. Leti also said that with capacitive coupling, light absorption in the diode integrated below the BOX leads to light-induced Vth shift of the transistor above the BOX, which means that forward optical back-biasing and reverse optical back-biasing are possible, depending on the diode polarity. In addition, the response of the system is logarithmic with light illumination, similar to the response of human vision.