ISFET´s threshold voltage control using bidirectional electron tunnelling

Ion sensitive field effect transistors (ISFETs) built in standard CMOS technology are effectively floating gate devices. They suffer from random trapped charges that alter their threshold voltage (V_t). This paper describes one way to program an ISFET´s floating gate charges in order to bring its V_t to a desirable value using bidirectional electron tunnelling. Two inputs are capacitively coupled to the ISFET´s floating gate via matched capacitors. They are used to indirectly tunnel opposite currents across their oxide isolation to the ISFET´s floating gate in a controlled way. The floating gate charges are programmed using a balanced combination of these two tunnelling currents. Measured experimental results demonstrated programmability and removal of positive and negative trapped charges.