Zener diode
A Zener diode is a solid state device having two contiguous regions of opposite conductivity type (P-type and N-type) and a PN junction at the interface of the two regions. Typically, one of the regions is more lightly doped than the other, and is the region chiefly affecting breakdown voltage of the PN junction. Zener diodes are commonly formed in a semiconductor process that utilizes a separate doping step to define the relative doping concentrations. The separate doping step typically forms the n or the p region of the diode, or adds a predefined amount of dopant to an existing n or p region, to thereby define the reverse breakdown voltage of the diode. Zener diodes are a well known electronic components which have the property of acting as normal diodes in a circuit with conduction taking place in only one direction. Zener diodes are characterized in that they have sufficient power dissipation capabilities to operate in the breakdown region under a reverse bias. When a reverse bias voltage sufficient to cause breakdown is applied to a Zener diode, the Zener diode conducts current, clamping the applied voltage to a constant level. The level of reverse bias voltage required to cause a Zener diode to enter the breakdown region is controllable depending upon the physical characteristics of the diode junction. Thus, Zener diodes are commonly used as voltage regulators and in other circuits requiring a constant reference voltage. Zener diodes have a number of uses in semiconductor integrated circuits, including in an electrostatic discharge (ESD) protection circuit that provides ESD protection for variable power supply lines. Zener diodes are used to protect circuit components from an over-voltage condition by preventing a circuit node from exceeding the breakdown voltage. A zener diode allows substantially no current flow through the diode when the diode is reverse biased (when the voltage on the n region of the diode is greater than the voltage on the p region of the diode.) However, when the n-to-p voltage difference is positive and exceeds a reverse breakdown voltage, a large breakdown current flows through the diode.
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