Ion controlled diodes (ICD)

There has been a growing need for chemically sensitive semiconductor devices (CSSDs) for such applications as biomedical instrumentation, chemical process control and pollution detection. Recent work on ion sensitive field effect transistors has shown that field effect structures are well suited to these tasks. We have designed and built a new type of CSSD based on the operation of a gate controlled diode. This structure provides both electrical and chemical shielding of the contact regions from the solution which drives the chemically sensitive layer. The ICD is biased into inversion from a reference electrode. The admittance of the ICD is then measured at sufficiently high frequencies that it is transit time limited. Under these conditions, the capacitance-gate voltage response becomes frequency dependent and, at constant capacity, the pH response becomes simply dependent on the frequency.