IV (current-voltage) characteristics are used to describe many electronic components, but they are especially appropriate for describing diodes. If you graph the voltage of a resistor on the horizontal axis against its current on the vertical axis, you get a straight line that goes through the origin. A similar graph of a diode is a complex curve that uniquely identifies the diode type and tells you everything you need to know about the behaviour of the diode.
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Semiconductor IV characteristics
Semiconductors come in two types: Positive-type (P-type) and Negative-type (N-type). The N-type have an excess of free electrons and the P-type have an excess of "holes" -- places that are missing electrons. If you put the two different types together you have a PN (or NP) junction. An interesting thing happens to current flow when you apply voltage to a PN (or NP) junction. If you apply a positive voltage to the P-type and a negative voltage to the N-type, the electrons and holes are pushed into each other and current flows. If you apply a positive voltage to the N-type and a negative voltage to the P-type, the electrons and holes are drawn away from each other and no current flows. Diodes consist of just such junctions; current flows freely in one direction through a diode, but not in the other direction.
Diode IV characteristics
If you graph voltage on the horizontal axis and current on the vertical axis of a diode, you get a curve that goes through the origin (no voltage means no current) and increases on the positive voltage side of the graph -- current increases when voltage increases. On the voltage-negative side of the graph, the curve hugs the voltage axis until it reaches a "breakdown point" and then the diode fails. On the positive side (called forward-biasing) the diode acts somewhat like a resistor. On the negative side (called reverse-biasing) the diode acts like an open circuit -- until the breakdown point at which the diode burns out.
Special Diode IV characteristics
Zener diodes have a non destructive breakdown -- different sizes have different breakdown points. Tunnelling diodes have a dip on the forward-biasing side of the IV graph. You can choose the parameters of the dip. Photo-diodes change the IV graph curve depending on ambient light (these diodes are used in automatic doors). Solar cells are a type of photodiodes that introduce voltage and current into the IV characteristics. Varicap diodes change capacitance as the reverse biasing changes -- this effectively decouples I (current) from V (voltage) and makes the IV curve dependent on the history of the surrounding circuits. The constant current diode has a normal negative-bias part of the curve (on the left-hand side of the graph), but quickly grows and then levels off on the forward-biasing half of the IV curve. After a certain point, an increase in voltage causes no significant increase in current.
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