junction. The pinch-off voltage (Vp) (also known as threshold voltage or cut-off voltage) varies considerably, even among devices of the same type. For example, VGS(off) for the Temic J202 device varies from to . Typical values vary from to . (Confusingly, the term pinch-off voltage is also used to refer to the VDS value that separates the linear and saturation regions.) To switch off an n-channel device requires a negative gate–source voltage (VGS). Conversely, to switch off a p-channel device requires positive VGS. In normal operation, the electric field developed by the gate blocks source–drain conduction to some extent. Some JFET devices are symmetrical with respect to the source and drain. Schematic symbols The JFET gate is sometimes drawn in the middle of the channel (instead of at the drain or source electrode as in these examples). This symmetry suggests that "drain" and "source" are interchangeable, so the symbol should be used only for those JFETs where they are indeed interchangeable. The symbol may be drawn inside a circle (representing the envelope of a discrete device) if the enclosure is important to circuit function, such as dual matched components in the same package. In every case the arrow head shows the polarity of the P–N junction formed between the channel and the gate. As with an ordinary diode, the arrow points from P to N, the direction of conventional current when forward-biased. An English mnemonic is that the arrow of an N-channel device "points in". Comparison with other transistors At room temperature, JFET gate current (the reverse leakage of the gate-to-channel junction) is comparable to that of a MOSFET (which has insulating oxide between gate and channel), but much less than the base current of a bipolar junction transistor. The JFET has higher gain (transconductance) than the MOSFET, as well as lower flicker noise, and is therefore used in some low-noise, high input-impedance op-amps. Additionally the JFET is less susceptible to damage from static charge buildup. Mathematical model Linear ohmic region The current in N-JFET due to a small voltage VDS (that is, in the linear or ohmic or triode region) is given by treating the channel as a rectangular bar of material of electrical conductivity : where ID = drain–source current, b = channel thickness for a given gate voltage, W = channel width, L = channel length, q = electron charge = 1.6 C, μn = electron mobility,