Thin-film pinned photodiode boost for SWIR sensors

Adding the pinned photogate and a transfer gate improves the absorption of thin-film image sensors beyond 1µm for more cost effective SWIR sensors.

Applications include cameras in autonomous vehicles to ‘see’ through smoke or fog and cameras to unlock a smartphone via face recognition. Sensors using thin-film absorbers such as quantum dots have recently emerged as a promising alternative to silicon and II-V sensors. These have better absorption characteristics and potential for integration with conventional (CMOS) readout circuits but suffer from inferior noise performance, which leads to poorer image quality.

The pinned photodiode (PPD) structure was introduced for silicon-CMOS image sensors in the 1980s and adding an extra transistor gate and a special photodetector structure so that the charge can be completely drained before integration begins. This allows a reset operation without thermal noise or the effect of the previous frame.

However incorporating this structure beyond silicon was not possible up until now because of the difficulty of hybridizing two different semiconductor systems.

A SWIR quantum-dot photodetector was monolithically hybridized with an indium-gallium-zinc oxide (IGZO)-based thin-film transistor into a PPD pixel. This array was subsequently processed on a CMOS readout circuit to form a superior thin-film SWIR image sensor.

“The prototype 4T image sensor showed a remarkable low read-out noise of 6.1 e-, compared to over 100 e- for the conventional 3T sensor, demonstrating its superior noise performance,” said Nikolas Papadopoulos, project leader ‘Thin-Film Pinned Photodiode’ at imec. This allows infrared images to be captured with less noise, distortion or interference, and more accuracy and detail he says.

Future steps include optimization of the technology in other types of thin-film photodiodes, as well as broadening its application in sensors beyond silicon imaging.