Ion sensing device

An ion sensing device comprising:

• a field-effect transistor including a bottom gate and a top gate;
• a reference electrode;
• and
• a driver circuit configured to measure concentration of ions in a sample solution into which the reference electrode and the top gate are immersed,
• wherein the driver circuit includes:
• a constant current source configured to supply a drain of the field-effect transistor with a constant current;
• and
• a voltage follower configured to receive a potential of the drain, and
• wherein the driver circuit is configured to:
• supply the reference electrode with a constant fixed reference potential;
• apply a constant voltage across an output of the voltage follower and the bottom gate so that a voltage across the drain and a source of the field-effect transistor varies;
• and
• output an output potential of the voltage follower.