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Diode
Diodes are electrical components that have a reverse direction and a forward direction, which means that current can pass through these components in one direction, while the diodes react like resistors in the other direction. With 4 interconnected diodes you can also force a rectification of AC voltage (bridge rectifier). A comparison between the operation of a diode and a check valve in a water pipe is often made too clearly. Diodes are very sensitive to light, which is why they are usually coated with light insulation, exceptions are of course diodes that are supposed to react sensitively to this, such as photodiodes. Semiconductor diodes consist either of a pn-doped semiconductor crystal or of a metal-semiconductor junction. How conductive such a transition ultimately is depends on the operating voltage at the anode and cathode and the polarity. The p-n transition is an area that is neutral, i.e. free of mobile charge carriers, since the p and n layers have balanced each other out here. The electric field that prevents the combination of stationary charges prevents the charge transport and is called anti-diffusion voltage. However, this field can also be bridged by externally applied voltages, this creates the forward direction and, on the contrary, the barrier layer, the resistance. In the case of the check valve and the water pipe, one would find the following parallels: If pressure (voltage) is applied to this valve (diode) in the reverse direction, the current flow is blocked. In the opposite direction, the pressure must be large enough to overcome the spring pressure of the valve (flow stress). The valve then opens and the current can flow. The voltage, which is necessary in this mechanical model to overcome the spring pressure, corresponds to the so-called forward voltage or flow voltage in a diode. With ordinary silicon diodes, this necessary forward voltage is around 0.7 V. Diodes are a standard electronic component and, like resistors and capacitors, can be found in almost every electrical circuit, of course primarily in power supplies as bridge rectifiers for alternating current to make DC voltage.
