The ESFET: Evaporated Sensing Field Effect Transistor

ESFET (Evaporated Sensing Field Effect Transistor), a new type of ion-sensitive transistor is developed. This device is capable of measuring the solution pH level and also the concentration of ionic species in electrolytes. The ESFET is a modified thin-film transistor with CdSe as the semiconductor on a glass substrate. A model adopting the surface ionization and complexation of hydroxyl groups and other ions on the gate insulator surface in conjunction with TFT theory is proposed to arrive at the terminal behavior of the ESFET in response to the variation of electrolyte parameters. Good agreement with experimental results is found. Electrical studies of CdSe/insulator interface using conductance technique are described and trap characteristics in thin evaporated CdSe films analyzed using the TSC method. Design parameters of the ESFETs to enhance sensitivity, selectivity over interfering ions, stability, response time, operating life, and reliability are presented.