Time-of-flight pixel with vertical photogates
Examples are disclosed that relate to time-of-flight camera systems comprising vertical photogates. One example provides a time-of-flight camera comprising a plurality of addressable pixels configured for backside illumination, each addressable pixel comprising a first vertical photogate and a second vertical photogate. The time-of-flight camera further comprises a processor and a storage device storing instructions executable on the processor to, during an integration period, apply a first relative bias to the first vertical photogate and the second vertical photogate to collect charge at a first pixel tap, apply a second relative bias to the first vertical photogate and the second vertical photogate to collect charge at a second pixel tap, and determine a distance value for the addressable pixel based at least upon the charge collected at the first pixel tap and the charge collected at the second pixel tap.
1. A time-of-flight (ToF) camera comprising:
an image sensor comprising a plurality of addressable pixels configured for backside illumination, each addressable pixel comprising a photoelectron generation region, a first vertical photogate, and a second vertical photogate spaced apart from the first vertical photogate with the photoelectron generation region therebetween such that photoelectrons are generated between the first vertical photogate and the second vertical photogate during operation of the addressable pixel, wherein the first vertical photogate and the second vertical photogate are separated by a distance less than a thickness of the photoelectron generation region of the addressable pixel;
a processor; and
a storage device storing instructions executable by the processor to, for each addressable pixel
during an integration period, apply a first relative bias to the first vertical photogate and the second vertical photogate to collect charge at a first pixel tap,
during the integration period, apply a second relative bias to the first vertical photogate and the second vertical photogate to collect charge at a second pixel tap, and
determine a distance value for the addressable pixel based at least upon the charge collected at the first pixel tap and charge collected at the second pixel tap.
2. The ToF camera of claim 1 , wherein the first pixel tap and the second pixel tap each comprises a storage gate.
3. The ToF camera of claim 1 , wherein the first pixel tap and the second pixel tap each comprises a storage diode.
4. The ToF camera of claim 1 , wherein the vertical photogates have a length that is between 50% and 95% of the thickness of the photoelectron generation region of the addressable pixel.
5. The ToF camera of claim 1 , wherein the photoelectron generation region comprises a region of germanium.
6. The ToF camera of claim 1 , wherein applying a first relative bias comprises applying a bias to the first vertical photogate while maintaining the second vertical photogate at ground.
7. The ToF camera of claim 1 , wherein applying a first relative bias comprises applying a positive bias to the first vertical photogate and a negative bias to the second vertical photogate.
8. The ToF camera of claim 1 , wherein the addressable pixels comprise a pitch of 1 mm to 3 mm.
9. A method for determining a distance value for an addressable pixel of an image sensor of a ToF camera, the addressable pixel comprising a first vertical photogate and a second vertical photogate spaced from the first vertical photogate, the addressable pixel further comprising a photoelectron generation region disposed between the first vertical photogate and the second vertical photogate, wherein the first vertical photogate and the second vertical photogate are separated by a distance less than a thickness of the photoelectron generation region of the addressable pixel, the method comprising:
illuminating a scene with amplitude modulated light;
generating photoelectrons in the photoelectron generation region between the first vertical photogate and the second vertical photogate from light reflected from the scene;
during an integration period, applying a positive bias to the first vertical photogate and a negative bias to the second vertical photogate to collect charge at a first pixel tap from photoelectrons generated in the photoelectron generation region;
during the integration period, applying a negative bias to the first vertical photogate and a positive bias to the second vertical photogate to collect charge at a second pixel tap from photoelectrons generated in the photoelectron generation region; and
determining a distance value for the addressable pixel based at least upon the charge collected at the first pixel tap and the charge collected at the second pixel tap.
10. The method of claim 9 , wherein collecting charge at the first pixel tap and collecting charge at the second pixel tap comprises collecting charge from a germanium region.
11. The method of claim 9 , wherein collecting charge at the first pixel tap and collecting charge at the second pixel tap comprises collecting charge from an epitaxial silicon photoelectron generation region.
12. The method of claim 9 , wherein collecting charge at a first pixel tap comprises collecting charge at a first storage gate and wherein collecting charge at a second pixel tap comprises collecting charge at a second storage gate.
13. The method of claim 9 , wherein collecting charge at a first pixel tap comprises collecting charge at a first storage diode and wherein collecting charge at a second pixel tap comprises collecting charge at a second storage diode.
14. A computing device, comprising:
a light source configured to illuminate a scene by emitting amplitude-modulated light;
an image sensor comprising plurality of addressable pixels, each addressable pixel comprising a first vertical photogate and a second vertical photogate, each addressable pixel further comprising a photoelectron generation region disposed between the first vertical photogate and the second vertical photogate such that photoelectrons are generated between the first vertical photogate and the second vertical photogate during operation of the addressable pixel, the first vertical photogate separated from the second vertical photogate by a distance less than a thickness of the photoelectron generation region of the addressable pixel, each of the first vertical photogate and the second vertical photogate comprising a length that is between 70% and 95% of the thickness of the photoelectron generation region of the addressable pixel;
a processor; and
a storage device comprising instructions executable on the processor to
control the light source to emit amplitude-modulated light,
for each phase sample of a plurality of phase samples, modulate a bias applied between first vertical photogate and the second vertical photogate to integrate charge at a first pixel tap and a second pixel tap of the addressable pixels,
for each addressable pixel, determine a distance value for the addressable pixel based at least upon the charge integrated at the first pixel tap and the second pixel tap for each phase sample, and
based on the distance value for each addressable pixel, output a depth image of the scene.
15. The computing device of claim 14 , wherein the photoelectron generation region of each addressable pixel comprises a germanium region.
16. The computing device of claim 14 , wherein each addressable pixel further comprises a storage gate.
17. The computing device of claim 14 , wherein each addressable pixel further comprises a storage diode.
18. The computing device of claim 14 , wherein modulating the bias comprises applying ground to the first vertical photogate and applying a nonzero voltage to the second vertical photogate.
19. The computing device of claim 14 , wherein modulating the bias comprises applying a positive voltage to the first vertical photogate and a negative voltage to the second vertical photogate.