47 The types of semiconductors most widely used in transistors are silicon and germanium, but for some specific applications, gallium compounds are also employed. Who invented the transistor and when? They are analogous, respectively, to collector, emitter, and base. TRANSISTORS NPN Transistor Transistors are electronic components which regulate or control the current or voltage flow; for these characteristics, they are called active electronic components. They are made of three layers of semiconductors , two of the same type (P or N-type) and the one between them of the opposite type. So, a transistor has a PNP configuration when an N-type semiconductor is placed between two P-type layers, and an NPN configuration when a P-type layer is placed between two N-type ones. Transistors are considered one of the most significant achievements in the history of electronics; their invention made it possible to gradually reduce the size of the circuits running electronic devices, while constantly improving their performances. Classification Transistors are classified into two main types: ONLINE RESOURCES Gordon Moore and his law field effect: effetto di campo gain: guadagno quantity: grandezza BJTs (Bipolar Junction Transistors), which can be either PNP or NPN, consist of three terminals, called the emitter, the collector, and the base. Emitter and collector are connected to an output circuit, while the base is connected to a circuit providing current; the base controls the flow of current from emitter to collector in the PNP transistor, and from collector to emitter in the NPN one. BJTs are mainly used for applications requiring low current and provide a high power gain. 114 ELEcTRONIcS AND ELEcTROTEcHNOLOGY PNP Transistor FETs (Field Effect Transistors), which also have three terminals: source, drain, and gate . In a FET, when voltage is applied to the gate, this creates an electric field that makes electrons flow from the source to the drain and can be blocked or unblocked by closing or opening the gate. Some field effect transistors are coated with layers of metal oxide and are therefore called MOSFETs (MetalOxide Semiconductor Field Effect Transistors). FETs usually have a more compact size, consume less power, and have a better thermal stability than BJTs; however, they do not provide a high power gain. MOSFET Functions A transistor can perform two basic functions; it can work: as a switch, blocking the flow of current in an electronic circuit or allowing it to pass. FETs are more suitable to be used as such; as an amplifier, making a current bigger and increasing signal strength; BJTs are more suitable to amplify signals.