Active pixel sensors with variable threshold reset
A CMOS image sensor array has rows and columns of active pixels, and column lines in communication with the active pixels in the respective columns. Each active pixel has an output connected to a column line and includes a photodetector that produces a signal proportional to incident light intensity that is coupled to an active pixel output based on column select and row select signals. Each active pixel has a reset transistor for resetting the active pixel, wherein each reset transistor has a first gate terminal and a second gate terminal. The reset transistors have a variable threshold capability that allows increased sensor array dynamic range or mitigation of the effects of temperature or radiation induced transistor threshold voltage shifts. Row select, column select, and sense transistors can also be configured to have variable thresholds.
1. An imaging system, comprising:
an array of pixels, each pixel including:
a sense FET,
a double gated reset FET, and
a radiation detection region electrically coupled to the sense FET,
wherein the sense FET is coupled to a source or drain of the double gated reset FET and at least one of the gates of the double gated reset FET is situated between a channel of the double gated reset FET and the radiation detection region; and
an array controller that hard resets a sense node and soft resets the sense node by applying a hard reset signal and a soft reset signal to one gate of the double gated reset FET in each pixel and that applies a threshold bias signal to the other gate of the double gated reset FET.
2. The imaging system of claim 1 , wherein the hard and soft reset signals are selected to provide a predetermined image lag, dynamic range, and linearity based on an intended signal level in at least one radiation detection region.
3. The imaging system of claim 1 , wherein the array controller provides the threshold bias signal to at least one gate of the double gated reset FETs in each pixel to bias the double gated reset FETs with respect to at least one of the hard reset signal and the soft reset signal.
4. The imaging system of claim 1 , wherein the array controller is configured to provide a threshold bias signal to at least one gate of the double gated reset FETs in each pixel based on a digitized pixel signal associated with a selected pixel.
5. The imaging system of claim 1 , wherein the threshold bias signal is based on charging of an insulating oxide region associated with an ionizing radiation flux incident to the array of pixels.
6. The imaging system of claim 1 , wherein the threshold bias signal applied to the double gated reset FET in a selected pixel is based on a temperature of operation.
7. The imaging system of claim 1 , wherein each of the double gated reset FETs includes a top gate and a bottom gate and the array controller applies a bias to the bottom gates to reduce sensitivity to radiation detection layer substrate bias.
8. The imaging system of claim 1 , wherein the threshold bias signal is a positive or negative signal.