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.