Definitions for Threshold voltage
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The threshold voltage, commonly abbreviated as Vth, of a field-effect transistor is the value of the gate–source voltage when the conducting channel just begins to connect the source and drain contacts of the transistor, allowing significant current to flow. In wide planar transistors the threshold voltage is essentially independent of the drain–source voltage and is therefore a well defined characteristic, however it is less clear in modern nanometer-sized MOSFETs due to drain-induced barrier lowering. The threshold voltage of a junction field-effect transistor is often called pinch-off voltage instead, which is somewhat confusing since "pinch off" for an insulated-gate field-effect transistor is used to refer to the channel pinching that leads to current saturation behaviour under high source–drain bias, even though the current is never off. The term "threshold voltage" is unambiguous and refers to the same concept in any field-effect transistor. In an n-MOSFET the substrate of the transistor is composed of p-type silicon, which has positively charged mobile holes as carriers. When a positive voltage is applied on the gate, an electric field causes the holes to be repelled from the interface, creating a depletion region containing immobile negatively charged acceptor ions. A further increase in the gate voltage eventually causes electrons to appear at the interface, in what is called an inversion layer, or channel. Historically the gate voltage at which the electron density at the interface is the same as the hole density in the neutral bulk material is called the threshold voltage. Practically speaking the threshold voltage is the voltage at which there are sufficient electrons in the inversion layer to make a low resistance conducting path between the MOSFET source and drain. When the voltage between transistor gate and source exceeds the threshold voltage, it is known as overdrive voltage.−
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"Threshold voltage." Definitions.net. STANDS4 LLC, 2015. Web. 26 Nov. 2015. <http://www.definitions.net/definition/Threshold voltage>.