The first thyristor devices were released commercially in 1956. Because
thyristors can control a relatively large amount of power and voltage
with a small device, they find wide application in control of electric
power, ranging from and electric motor speed control to power transmission.
Originally thyristors relied only on current reversal to turn them off,
making them difficult to apply for direct current; newer device types
can be turned on and off through the control gate signal. A thyristor is
not a proportional device like a transistor. It operates only fully on
or fully off, making it unsuitable as an analog amplifier