Single-photon avalanche diode image sensor with photon counting and time-of-flight detection capabilities
A back side illuminated image sensor may operate using the single-photon avalanche diode (SPAD) concept in a Geiger mode of operation for single photon detection. The image sensor may be implemented using two layer stacking with a silicon on insulator (SOI) chip. The chip-to-chip electrical connections between the top level image sensing chip and the second level ASIC circuit chip may be realized at each pixel with a single bump connection per pixel. A light level signal may be obtained from pixels that have photon counting capabilities while a distance measurement signal for 3-dimensional imaging may be obtained from pixels that have time-of-flight (ToF) detection capabilities. Both types of pixels may be integrated within the same array and use the same SPAD structure placed on the top chip.
1. An image sensor comprising:
a substrate;
a single-photon avalanche diode in the substrate; and
a time-of-flight to voltage converter circuit that is coupled to the single-photon avalanche diode and that includes:
a capacitor;
a pass gate, wherein a global ramp pulse is configured to discharge the capacitor through the pass gate and wherein a pulse generated by the single-photon avalanche diode is configured to disconnect the global ramp pulse from the capacitor;
an additional capacitor that is discharged in response to the pulse generated by the single-photon avalanche diode; and
an inverter coupled to the additional capacitor, wherein an output of the inverter is supplied to the pass gate.
2. The image sensor defined in claim 1 , wherein the inverter is configured to supply signals to the pass gate to disconnect the global ramp pulse from the capacitor in response to the additional capacitor being discharged in response to the pulse generated by the single-photon avalanche diode.
3. The image sensor defined in claim 1 , wherein the time-of-flight to voltage converter circuit comprises a source follower transistor and wherein the capacitor is coupled to a gate of the source follower transistor.
4. The image sensor defined in claim 3 , wherein the time-of-flight to voltage converter circuit further comprises a row select transistor and a column output line and wherein the row select transistor is interposed between the source follower transistor and the column output line.
5. The image sensor defined in claim 1 , wherein the global ramp pulse is configured to start at the same time as a scene illumination pulse.