![]() This ratio is called the transistors static or DC transconductance which is short for “transfer conductance” and is given the unit of Siemens (S), as its amps per volt. However, due to the construction and physics of an enhancement type mosfet, there is a minimum gate-to-source voltage, called the threshold voltage V TH that must be applied to the gate before it starts to conduct allowing drain current to flow. Enhancement MOSFET AmplifierĮnhancement MOSFET, or eMOSFET, can be classed as normally-off (non-conducting) devices, that is they only conduct when a suitable gate-to-source positive voltage is applied, unlike Depletion type mosfets which are normally-on devices conducting when the gate voltage is zero. This enhances the electron flow through the channel allowing more channel current to flow from drain to source leading to the name of Enhancement MOSFET. So for our n-type MOS transistor, the more positive potential we put on the gate the greater the build-up of electrons around the gate region and the wider the conductive channel becomes. ![]() The result is that the n-type substrate creates a p-type conductive channel. The reverse is also true for the p-channel MOSFET (PMOS), where a negative gate potential causes a build of holes under the gate region as they are attracted to the electrons on the outer side of the metal gate electrode. This over abundance of free electrons within the p-type substrate causes a conductive channel to appear or grow as the electrical properties of the p-type region invert, effectively changing the p-type substrate into a n-type material allowing channel current to flow. Biasing the gate terminal positive attracts electrons within the p-type semiconductor substrate under the gate region towards it. We can see that for the n-channel MOSFET (NMOS) above the substrate semiconductor material is p-type, while the source and drain electrodes are n-type. ![]() Note that the fundamental differences between a Bipolar Junction Transistor and a FET are that a BJT has terminals labelled Collector, Emitter and Base, while a MOSFET has terminals labelled Drain, Source and Gate respectively.Īlso the MOSFET differs from the BJT in that there is no direct connection between the gate and channel, unlike the base-emitter junction of the BJT, as the metal gate electrode is electrically insulated from the conductive channel giving it the secondary name of Insulated Gate Field Effect Transistor, or IGFET. But first lets remind ourselves of the mosfets basic characteristics and configuration. As with the bipolar transistor common emitter configuration, the common source mosfet amplifier needs to be biased at a suitable quiescent value. There are large variations in the characteristics of different types of mosfets, and hence the biasing of a mosfet must be done individually. In other words, we can control how the mosfet operates by creating or “enhancing” its conductive channel between the source and drain regions producing a type of mosfet commonly called an n-channel Enhancement-mode MOSFET, which simply means that unless we bias them positively on the gate (negatively for the p-channel), no channel current will flow. ![]() An electric field induced around the gate terminal by the application of this gate voltage affects the electrical characteristics of the channel, thus the name field-effect transistor. We can make this conductive channel wider or smaller by applying a suitable gate potential. MOSFETS conduct through a conductive region or path called “the channel”.
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