![]() This diode, the "body diode" is a consequence of the manufacturing process. MOSFETs only switch current flowing in one direction they have a diode between source and drain in the other direction (in other words, if the drain (on an N-channel device) falls below the voltage on the source, current will flow from the source to the drain). As demonstration, one can wire up a MOSFET normally, except connecting nothing to the gate pin, and then touch the gate while holding either ground or a positive voltage - even through your body’s resistance, you can turn the FET on and off! To ensure that a MOSFET remains off even if the pin is not connected (ex, after Espruino is reset), a pull-down resistor can be placed between gate and source. The voltage on the gate determines whether current can flow from the drain to the load - no current flows to or from the gate (unlike a bipolar junction transistor) - this means that if the gate is allowed to float, the FET may turn on, or off, in response to ambient electrical fields, or very tiny currents. ![]() The source is connected to ground (or the positive voltage, in a p-channel MOSFET), the drain is connected to the load, and the gate is connected to a GPIO pin on the Espruino. MOSFETs have three pins, Source, Drain, and Gate. MOSFETs can only be used to switch DC loads. In their off state, MOSFETs are non-conducting, while in their on state, they have an extremely low resistance - often measured in milliohms. ![]() These are useful for controlling loads that draw more current, or require higher voltage, than a GPIO pin can supply. A MOSFET (Metal Oxide-Semiconductor Field Effect Transistor) is a semiconductor device that can be used as a solid state switch.
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