A semiconductor device used in amplifiers, oscillators, and control circuits in which current flow is modulated by voltage or current applied to electrodes. A device used to amplify a signal or open and close a circuit. In a computer, it functions as an electronic switch.
Transistors are tiny electrical devices that can be found in everything from radios to robots. Most transistors are based on the use of silicon including some thin film ones but these are not strictly printed because they have to be vacuum deposited at present.
Technically, a transistor is a semiconductor component that acts like a variable switch, controlling the flow of an electric current. Typically it is a component with an electrode, the gate, that controls the flow of current between two other electrodes, the source and drain. The gate does this by having a potential applied to it as appropriate. Transistors have two key properties: 1) they can amplify an electrical signal and 2) they can switch on and off, letting current through or blocking it as necessary.
See the IDTechEx report Introduction to Printed Electronics