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Engineering LibreTexts

3.1: Theory Overview

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  • The basic diode is an asymmetric non-linear device. That is, its current-voltage characteristic is not a straight line and it is sensitive to the polarity of an applied voltage or current. When placed in forward bias (i.e. positive polarity from anode to cathode), the diode will behave much like a shorted switch and allow current flow. When reversed biased the diode will behave much like an open switch, allowing little current flow. Unlike a switch, a silicon diode will exhibit an approximate 0.7 volt drop when forward biased. The precise voltage value will depend on the semiconductor material used. This volt drop is sometimes referred to as the knee voltage as the resulting I-V curve looks something like a bent knee.

    The effective instantaneous resistance of the diode above the turn-on threshold is very small, perhaps a few ohms or less, and is often ignored. Analysis of diode circuits typically proceeds by determining if the diode is forward or reversed biased, substituting the appropriate approximation for the device, and then solving for desired circuit parameters using typical analysis techniques. For example, when forward biased, a silicon diode can be thought of as a fixed 0.7 volt drop, and then KVL and KCL can be applied as needed. The polarity of the device is typically denoted by a band placed closest to the cathode.

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