Figure 4.

Energy bands and semiconductor‐related devices. a) Energy‐band diagrams for insulators, conductors, and semiconductors. The current in semiconductors are either generated from electrons (−) or electron holes (+). b) A transistor is basically made from three different semiconductors. The current can only run in one direction in a transistor, and the current that passes through it is typically enhanced with a sustainable gain factor, making them suitable for various sensing applications. c) A qualitative illustration of the working principles behind a sensor that is based on a field‐effect transistor made from PEDOT:PSS, which is an organic semiconductor capable of absorbing electrolytes (anions) from a solution. The uptake of anions abolishes the mobile holes within PEDOT:PSS and thus changes the current that passes through it; it thereby enables it to sense biological processes that either diminish or increase the amount of electrolytes in the surrounding environment.