Ion-sensing charge-accumulation circuits and methods
An ion-sensitive circuit can include a charge accumulation device, to accumulate a plurality of charge packets as a function of an ion concentration of a fluid, and at least one control and readout transistor, to generate an output signal as a function of the accumulated plurality of charge packets, the output signal representing the ion concentration of the solution. The charge accumulation device can include a first charge control electrode above a first electrode semiconductor region, an electrically floating gate structure above a gate semiconductor region and below an ion-sensitive passivation surface, a second charge control electrode above a second electrode semiconductor region, and a drain diffusion region. The first control electrode can control entry of charge into a gate semiconductor region in response to a first control signal. The ion-sensitive passivation surface can be configured to receive the fluid. The second charge control electrode can control transmission of the plurality of charge packets out of the gate semiconductor region and into the drain diffusion region in response to a second control signal. The drain diffusion region can receive the plurality of charge packets from the gate semiconductor region via the second electrode semiconductor region.
1. A method of detecting an ion concentration of a fluid, comprising:
passing the fluid over an ion-sensitive passivation surface above an electrically floating gate structure of a charge accumulation device of an ion sensitive circuit, wherein the floating gate includes a plurality of conductors electrically coupled to one another and separated by dielectric layers;
applying a control signal to a first charge control electrode disposed on a gate oxide layer and formed over a first electrode semiconductor region of the charge accumulation device to control entry of charge from the ion concentration of the fluid to form a packet of charge in a gate semiconductor region below the electrically floating gate structure;
applying a control signal to a second charge control electrode disposed on the gate oxide layer and formed over a second electrode semiconductor region of the charge accumulation device to control transmission of the charge packet out of the gate semiconductor region and into a drain diffusion region;
accumulating a plurality of charge packets at the drain diffusion region of the charge accumulation device as a function of the ion concentration in the fluid at a selectable charge packet accumulation frequency; and
generating an output signal using at least one control and readout transistor as a function of the accumulated plurality of charge packets at the drain diffusion region at a selectable output signal generation frequency, wherein the output signal represents a measure of the ion concentration in the fluid.
2. The method of claim 1 , further comprising individually transmitting, one at a time, the plurality of charge packets through the second electrode semiconductor region in response to the applying the control signal to the second charge control electrode.
3. The method of claim 1 , further comprising removing any charge packets previously accumulated at the drain diffusion region prior to accumulating the plurality of charge packets.
4. The method of claim 1 , wherein the output signal generation frequency is less than the charge packet accumulation frequency.