The thyristor is different from a transistor because the thyristor 'latches' on. This means that the thyristor is switched on in the same way as the transistor; by making electrons flow into the gate. However the thyristor will now stay on even if we stop electrons flowing into the gate. The only way to switch off the thyristor is to disconnect it from the battery (normally by adding another switch). We can use thyristors where we want the output to stay on until there is another input. For example an alarm that sounds until someone comes to reset it.
