Editing Silicon controlled rectifier (section)

== Modes of operation ==
[[File:Scr curve.jpg|thumb|Characteristic curve of a silicon-controlled rectifier]]
There are three modes of operation for an SCR depending upon the biasing given to it:
# Forward blocking mode (off state)
# Forward conduction mode (on state)
# Reverse blocking mode (off state)

===Forward blocking mode===
In this mode of operation, the anode (+, p-doped side) is given a positive voltage while the cathode (&minus;, n-doped side) is given a negative voltage, keeping the gate at zero (0) potential i.e. disconnected. In this case junction '''J1''' and '''J3''' are [[P–n diode|forward-bias]]<nowiki/>ed, while '''J2''' is [[P–n junction|reverse-bias]]<nowiki/>ed, allowing only a small leakage current from the anode to the cathode. When the applied voltage reaches the breakover value for '''J2''', then '''J2''' undergoes avalanche breakdown. At this breakover voltage '''J2''' starts conducting, but below breakover voltage '''J2''' offers very high resistance to the current and the SCR is said to be in the off state.

===Forward conduction mode===
An SCR can be brought from blocking mode to conduction mode in two ways: Either by increasing the voltage between anode and cathode beyond the breakover voltage, or by applying a positive pulse at the gate. Once the SCR starts conducting, no more gate voltage is required to maintain it in the '''ON''' state. The minimum current necessary to maintain the SCR in the '''ON''' state on removal of the gate voltage is called the latching current.

There are two ways to turn it '''off''':
# Reduce the current through it below a minimum value called the holding current, or
# With the gate turned '''off''', short-circuit the anode and cathode momentarily with a push-button switch or transistor across the junction.

=== Reverse blocking mode ===
When a negative voltage is applied to the anode and a positive voltage to the cathode, the SCR is in reverse blocking mode, making J1 and J3 reverse biased and J2 forward biased. The device behaves as two diodes connected in series. A small leakage current flows. This is the reverse blocking mode. If the reverse voltage is increased, then at critical breakdown level, called the reverse breakdown voltage (V<sub>BR</sub>), an avalanche occurs at J1 and J3 and the reverse current increases rapidly.
SCRs are available with reverse blocking capability, which adds to the forward voltage drop because of the need to have a long, low-doped P1 region. Usually, the reverse blocking voltage rating and forward blocking voltage rating are the same. The typical application for a reverse blocking SCR is in current-source inverters.

An SCR incapable of blocking reverse voltage is known as an '''asymmetrical SCR''', abbreviated '''ASCR'''.  It typically has a reverse breakdown rating in the tens of volts.  ASCRs are used where either a reverse conducting diode is applied in parallel (for example, in voltage-source inverters) or where reverse voltage would never occur (for example, in switching power supplies or DC traction choppers).

Asymmetrical SCRs can be fabricated with a reverse conducting diode in the same package.  These are known as RCTs, for [[Thyristor#Reverse conducting thyristor|reverse conducting thyristors]].