Floating Gate ISFET Chemical Inverters: for Semiconductor Based Biomedical Applications

Abstract

Ion sensitive field effect transistors (ISFETs) have long been used as analogue chemical sensors particularly for biomedical applications. However, there are some applications where a "yes" / "no" type answer regarding pH change is sufficient. For example, in DNA sequencing the question is whether a chain extension reaction took place or not. Detecting this at the sensing point reduces the sensing process to pH change threshold detection. It eliminates the need for analogue to digital conversion and facilitates an all digital sensory system. This thesis presents Novel Floating Gate ISFET based Chemical Inverters that were created with semiconductor based biomedical applications in mind. It starts by allowing two ISFETs to share the same ion sensing membrane and a common floating gate. Arranging them in a simple FG inverter configuration, their switching may be triggered by either the reference voltage or chemical pH change. In order to enhance its input noise immunity, a chemical Schmitt Trigger is presented. Using ISFETs for the detection of minute pH changes have been a challenge. A simple method to locally scale input referred chemical signal at the ISFET's floating gate is presented. It is based on using the ratio of capacitive coupling to the floating gate. The chemical signal is coupled via the passivation capacitance (Cpass) while an electrical input (V2) is coupled via a poly capacitance (C2). V2 sees the chemical signal with a scaling of Cpass/C2, which can be designed. Finally, ISFETs suffer from initial trapped charges that cause mismatch between devices in the same die. A fast matching method is presented here, that can be used to hugely reduce mismatch of arrays of FG devices. It is based on using indirect bidirectional tunnelling. Two tunnelling structures are added to each ISFET's FG, one adds electrons to it while the other removes them. It is possible to match all ISFETs' initial FG voltages to a point where both tunnelling currents reach equilibrium.