Parasitic structure
Encyclopedia
In a semiconductor device
Semiconductor device
Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced thermionic devices in most applications...

, a parasitic structure is a portion of the device that resembles in structure some other, simpler semiconductor device, and causes the device to enter an unintended mode of operation when subjected to conditions outside of its normal range. For example, the internal structure of an NPN bipolar transistor
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...

 resembles two PN junction
P-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...

 diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...

s connected together by a common anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

. In normal operation the base-emitter junction does indeed form a diode, but in most cases it is undesirable for the base-collector junction to behave as a diode. If a sufficient forward bias is placed on this junction it will form a parasitic diode structure, and current will flow from base to collector.

A common parasitic structure is that of an SCR. Once triggered, an SCR conducts for as long as there is a current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

, necessitating a complete power-down to reset the behavior of the device. This condition is known as latchup
Latchup
Latchup is a term used in the realm of integrated circuits to describe a particular type of short circuit which can occur in an improperly designed circuit...

.
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