Therefore, datasheets will specify threshold voltage according to a specified measurable amount of current (commonly 250 μA or 1 mA). In fact, there is a current even for gate biases below the threshold ( subthreshold leakage) current, although it is small and varies exponentially with gate bias. If the gate voltage is below the threshold voltage (left figure), the "enhancement-mode" transistor is turned off and ideally there is no current from the drain to the source of the transistor. In the (red) bulk the number of holes p = N A making the bulk charge neutral. The depletion layer dopant is labeled N A − to indicate that the ions in the (pink) depletion layer are negatively charged and there are very few holes. In the figures, the source (left side) and drain (right side) are labeled n+ to indicate heavily doped (blue) n-regions. 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.ĭepletion region of an enhancement-mode nMOSFET biased above the threshold with channel formed
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Likewise for a p-channel "depletion-mode" MOS transistor a positive gate-source voltage will deplete the channel of its free holes, turning it “OFF”. This ease-of-flow threshold also applies to p-channel depletion-mode devices, in which a negative voltage from gate to body/source creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions.įor the n-channel depletion MOS transistor, a negative gate-source voltage will deplete (hence its name) the conductive channel of its free electrons switching the transistor “OFF”. Accordingly, the term threshold voltage does not readily apply to turning such devices on, but is used instead to denote the voltage level at which the channel is wide enough to allow electrons to flow easily. In contrast, n-channel depletion-mode devices have a conductive channel naturally existing within the transistor. The application of a negative gate voltage to the p-type "enhancement-mode" MOSFET enhances the channels conductivity turning it “ON”. When V GS = 0 the device is “OFF” and the channel is open / non-conducting.
The reverse is true for the p-channel "enhancement-mode" MOS transistor. Further gate-to-source voltage increase will attract even more electrons towards the gate which are able to create a conductive channel from source to drain this process is called inversion. But first, enough electrons must be attracted near the gate to counter the dopant ions added to the body of the FET this forms a region with no mobile carriers called a depletion region, and the voltage at which this occurs is the threshold voltage of the FET. The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel. In n-channel enhancement-mode devices, a conductive channel does not exist naturally within the transistor, and a positive gate-to-source voltage is necessary to create one such. Unlike pinch off, the term threshold voltage is unambiguous and refers to the same concept in any field-effect transistor. This is somewhat confusing since pinch off applied to insulated-gate field-effect transistor (IGFET) refers to the channel pinching that leads to current saturation behaviour under high source–drain bias, even though the current is never off.
When referring to a junction field-effect transistor (JFET), the threshold voltage is often called pinch-off voltage instead. It is an important scaling factor to maintain power efficiency. The threshold voltage, commonly abbreviated as V th or V GS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (V GS) that is needed to create a conducting path between the source and drain terminals.
Note that the y-axis is logarithmic, showing that only extremely small amounts of current pass below the threshold voltage (around 0.45 V) when the conducting channel connects.
Simulation result for formation of inversion channel (electron density) and attainment of threshold voltage (IV) in a nanowire MOSFET. Minimum source-to-gate voltage for a field effect transistor to be conducting from source to drain
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