transistors - Electromagnetic waves

I have a problem to understand how electromagnetic waves works. Why do transistors create EM waves when they're switching? And difference between antenna and transformer with secondary coil. When I have a antenna, the EM field is changing and electrons go forward and backward and the circuit with antenna isn't closed, so we can turn on a light bulb with two wires. Transformer uses also AC, so why does the circuit need to be closed to flow current?