Apparatus and method for operating an ISFET at multiple drain currents and gate-source voltages allowing for diagnostics and control of isopotential points

An apparatus for measuring ion concentration of a solution, comprising:

• an ion sensitive field effect transistor (ISFET) having a drain, a source and an ion sensitive gate region;
• a reference device separated from the gate region;
• an ISFET control circuit coupled to the ISFET for applying a drain-source voltage V.sub.DS across the drain and source and for operating the ISFET at n successive drain currents I.sub.Di and corresponding gate-source voltages V.sub.GSi, wherein i is an integer variable from 1 to n and n is an integer greater than 1, wherein the drain currents flow through the drain of the ISFET and the gate-source voltages are referenced between the reference device and the source, and wherein the ISFET has an iso potential ion concentration point which is different for each of the n successive drain currents I.sub.Di and corresponding gate-source voltages V.sub.GSi ;
• a memory which stores the n successive drain currents I.sub.Di and gate-source voltages V.sub.GSi ;
• and
• measurement means for making n successive ion concentration measurements of the solution as a function the n successive drain currents I.sub.Di and gate-source voltages V.sub.GSi and for providing an ion concentration output based on an average of the n successive ion concentration measurements.

A method of measuring ion concentration of a solution with an apparatus comprising a reference device and an ion sensitive field effect transistor (ISFET) having a drain, a source, an ion sensitive gate region and a plurality of device characteristics, wherein the ion sensitive gate region and the reference device are exposed to and separated by the solution, the method comprising:

• applying a drain-source voltage V.sub.DS across the drain and source;
• operating the ISFET at n successive drain currents I.sub.Di and corresponding gate-source voltages V.sub.GSi such that the ISFET has an isopotential ion concentration point which is different for each of the n successive drain currents I.sub.Di and corresponding gate-source voltages V.sub.GSi, wherein i is an integer variable from 1 to n and n is an integer greater than 1, and wherein the drain currents flow through the drain of the ISFET and the gate-source voltages are referenced between the reference device and the source;
• storing the plurality of device characteristics and the n successive drain currents I.sub.Di and gate-source voltages V.sub.GSi ;
• measuring ion concentration of the solution as a function of at least one of the n successive drain currents I.sub.Di and gate-source voltages V.sub.GSi and the stored device characteristics;
• and
• measuring at least one of the device characteristics of the ISFET as a function of the n successive drain currents I.sub.Di and gate-source voltages V.sub.GSi.