Level shifter circuit and method
A circuit includes a photodiode electrically coupled to a first node, the first node configured to be charged by a first power supply voltage. A second node is configured to be charged by a second power supply voltage lower than the first power supply voltage, a source follower transistor is electrically coupled between the second node and a column line, and a level shifter is electrically coupled between the first node and the second node.
1. A circuit comprising:
a photodiode electrically coupled to a first node, the first node configured to be charged by a first power supply voltage;
a second node, the second node configured to be charged by a second power supply voltage lower than the first power supply voltage;
a source follower transistor electrically coupled between the second node and a column line; and
a level shifter electrically coupled between the first node and the second node.
2. The circuit of claim 1 , wherein the level shifter comprises a capacitor.
3. The circuit of claim 2 , wherein a signal level at the second node is based on
a number of signal electrons at the photodiode,
a level shifter capacitance of the capacitor,
a first node capacitance of the first node, and
a second node capacitance of the second node.
4. The circuit of claim 1 , wherein
the photodiode is electrically coupled to the first node by a transfer transistor, the transfer transistor comprising a first gate having a first oxide thickness, and
the source follower transistor comprises a second gate having a second oxide thickness less than the first oxide thickness.
5. The circuit of claim 1 , wherein the circuit is one circuit of a plurality of circuits of a pixel array, each circuit of the plurality of circuits being electrically coupled to the column line.
6. The circuit of claim 5 , wherein
the column line is one column line of a plurality of column lines of the pixel array, and
the plurality of circuits is one plurality of circuits of multiple pluralities of circuits, each circuit of a given plurality of circuits being electrically coupled to a column line of the plurality of column lines corresponding to the given plurality of circuits.
7. A circuit comprising:
a first node on a first substrate, the first node configured to be charged by a first power supply voltage;
a second node on a second substrate, the second node configured to be charged by a second power supply voltage lower than the first power supply voltage;
a photodiode on the first substrate and electrically coupled to the first node;
a source follower transistor on the second substrate and electrically coupled between the second node and a column line; and
a level shifter electrically coupled between the first node and the second node.
8. The circuit of claim 7 , wherein
the first substrate is bonded to the second substrate at a bonding boundary, and
the level shifter comprises a capacitor, the capacitor comprising:
a first electrode on the first substrate at the bonding boundary, and
a second electrode on the second substrate at the bonding boundary.
9. The circuit of claim 8 , wherein a signal level at the second node is based on
a number of signal electrons at the photodiode,
a level shifter capacitance of the capacitor,
a first node capacitance of the first node, and
a second node capacitance of the second node.
10. The circuit of claim 7 , wherein
the circuit is one circuit of a plurality of circuits of a pixel array,
each circuit of the plurality of circuits comprises a photodiode on the first substrate, and
each circuit of the plurality of circuits is electrically coupled to the column line.
11. A method comprising:
charging a photodiode and a first node to a first power supply voltage;
charging a second node to have a second node voltage equal to a second power supply voltage, the second power supply voltage being lower than the first power supply voltage, the second node being electrically coupled to the first node by a level shifter;
electrically connecting the photodiode to the first node; and
outputting a voltage with a source follower transistor electrically coupled to a column line, the voltage being based on the second node voltage and indicative of exposure of the photodiode to light.
12. The method of claim 11 , wherein the level shifter comprises a capacitor.
13. The method of claim 12 , wherein outputting the voltage comprises outputting the voltage based on
a number of signal electrons at the photodiode,
a level shifter capacitance of the capacitor,
a first node capacitance of the first node, and
a second node capacitance of the second node.
14. The method of claim 11 , wherein charging the photodiode and the first node to the first power supply voltage comprises using a signal having the first power supply voltage to control a first transistor electrically coupled to the first node.
15. The method of claim 14 , wherein
the first transistor comprises a first gate having a first oxide thickness, and
outputting the voltage comprises outputting the voltage with the source follower transistor comprising a second gate having a second gate thickness less than the first gate thickness.
16. The method of claim 11 , wherein outputting the voltage with the source follower transistor comprises using a signal having the second power supply voltage to control the source follower transistor.
17. The method of claim 11 , wherein outputting the voltage with the source follower transistor comprises outputting the voltage having a value based on a photodiode voltage reduced by the level shifter.
18. The method of claim 17 , wherein the photodiode voltage having the first power supply voltage indicates that the photodiode has not been exposed to light.
19. The method of claim 17 , wherein the photodiode voltage having a value below the first power supply voltage indicates that the photodiode has been exposed to light.
20. The method of claim 11 , further comprising outputting another voltage with another source follower transistor electrically coupled to the column line, the another voltage being indicative of exposure of another photodiode to light.