Is there any visible difference between thyristor and transistor?

Answer 1

A diode is a solid state passive device that will only allow current to flow in one and only one direction (Anode->Cathode). A Thyristor behaves exactly like a Diode except that in order for current to flow it must "see" current at its gate, if it does not see current (ie logic 0 or ground) then current is not allowed to flow from its anode to cathode.

Note: The Thyristor gate requires little current to trigger its gate, meaning a small amount of current can be used to switch on/off a load require much more current.

Another Note: Since the Thyristor has a gate which controls its output it is not a passive device like the diode

Answer 2

Thyristors are four layer devices and transistors are three layer devices.

Just from looking at them, no, it is not evident whether you are seeing a thyristor or transistor, unless you have some knowledge of the circuit it is in. Both are three terminal devices, meaning there are three wires coming out of them, and at least in the smaller sizes they come in the same footprints. If the part you're looking at has an identifying number, an internet search for that part will quickly tell you what you're looking at.

Answer 3

thyristors have 4 semiconductor layers (3 Junction&terminals)

diode has 2 semiconductor layers(only one junction , 2 terminals)