Bipolar junction transistor (BJT) based pH sensors of high sensitivity and resolution and fast response time. A sensor is a transducer that receives and responds to a signal or stimulus from a physical system. It produces a signal representing information about the system, which is used by some type of telemetry, information or control system. Specifically, a biosensor is a device that contains a biological, or bio-inspired recognition layer with unique specificity towards target analytes, which can be pH, DNA, proteins, and small molecules like glucose, chemical warfare agents or environmental pollutants. The focus on electrochemical sensors and biosensors is growing exponentially, as these detection systems can be easily miniaturized and mass-produced for point-of-care applications.