diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. In other words, a diode is a semiconductor that essentially acts as a one-way current switch. It allows current to flow in one direction, but strictly prevents it from flowing in the opposite direction.

Diodes are also called rectifiers because they change alternating current (AC) into pulsating direct current (DC). Diodes are classified according to their type, voltage and current capacity.

Diodes are characterized by polarity, which is determined by an anode (positive terminal) and a cathode (negative terminal). Most diodes only allow current to flow when positive voltage is applied to the anode. When a diode allows current flow, it is directly biased. When a diode is reverse biased, it acts as an insulator and does not allow current flow.

Strange but true: the diode symbol is an arrow pointing in the opposite direction of the electron flow. Reason: The symbol was devised by engineers and in their schematics the current flows from the positive (+) pole to the negative (-) pole of the voltage generator. This is the same convection used for semiconductor symbols that include arrows pointing in the “traditional” direction of flow and opposite the allowed direction of electron flow.

Ideal diode

In many applications of interest, the voltage-current characteristic of an ideal diode, obtained under static conditions, can be approximated by a linear function with dashes. In such an approximation, the current can be considered zero if the voltage between the anode and cathode is less than or equal to a precise voltage value Vγ (threshold or knee voltage); if, however, the voltage is greater, the diode can be approximated to a voltage generator of value Vγ, whose current is imposed by the circuit to which it is subjected. The value of Vγ is a voltage value such that the actual current is greater than a well-defined current useful for circuit analysis, and, in general, it is customary to use the conventional value of 0.6 V that can characterize with sufficient accuracy the voltage drop at the ends of a silicon diode when significant currents flow in it.

When the values of the circuit voltages are sufficiently high (tens, hundreds of V and more), so as to consider negligible the voltage drop Vγ at the ends of the diode, is commonly adopted an even stronger approximation using an ideal diode that in conduction is assimilated to a short circuit. This approximation allows further simplification of the circuit analysis with virtually no significant error.

When the potential difference at the ends of the ideal diode is greater than 0 (i.e. when the electric current flows in the conventionally positive direction, from the anode to the cathode), the diode is said to be in a condition of direct polarization. Whereas, when the potential difference is negative, this is said to be inversely polarized. These definitions are also used to identify the regions of operation of real diodes.


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