thyristor

I'm not going to worry about the semantic difference between "silicon-controlled rectifier (SCR)" and "thyristor."

A thyristor is a semiconductor which creates a switchable #diode. Unlike a transistor, a thyristor is ONLY on or off, no active region, no analog amplification.

uses

controllable rectification

e.g. for HVDC - because thyristors can deal with a lot of current
And since #AC has a zero crossing point, you can be sure that they won't remain latched for longer than $\frac{1}{60}$ of a second when you don't want them to be open.

thyristor states

For this discussion, voltage from the anode to the cathode is $V_{A K}$ , voltage from the gate to the cathode is $V_{G}$ , and (forward) current through the diode is $I_{A K}$ .

switching characteristics

The latching of a thyristor is driven by the charge required to cause a breakdown in the internal PN junction, so both Voltage and Current over the threshold $(V_{G T} and I_{G T})$ is required to produce sufficient charge in the region.

Once the gate is triggered, current begins to flow from anode to cathode like a diode. The gate will remain latched if a current greater than the latching current $(I_{A K} > I_{L})$ flows through the diode.

Once latched, there is no way to stop current flow through the thyristor until the current is less than the holding current $(I_{A K} < I_{H})$ .

The latching current $(I_{L})$ is always greater than the holding current $(I_{H})$

Reverse blocking

When $0 > V_{A K} > V_{B R}$ reverse (cathode to anode) flowing current is blocked

Forward Blocking

When $V_{G} < V_{G T}$ and $I_{G} < I_{G T}$

current flow from anode to cathode will be blocked
unless both voltage $(V_{G})$ and current $(I_{G})$ at the gate are sufficient to

Forward conducting

The thyristor latches in the on state: it will conduct current from anode to cathode after it has been triggered by a current flow greater than the gate trigger current $(\geq I_{G T})$ at the gate
Unlike a transistor, once the thyristor is energized, it will continue to conduct ==regardless of if there is still current supplied to the gate== as long as current through the Anode to Cathode does not fall below the threshold holding current $(I_{H})$ .

construction

Canonically, a thyristor has 4 layers of doped silicon junctions -PNPN- between the anode and cathode. The gate acts at the middle P-layer.

This means you can build a thyristor with two transistors arranged so that a change in state causes the gate to latch in the open position.