Hello dear student,
Uni Junction Transistor (UJT) is also known as double base diode . It acts as a switching transistor instead of amplifying the signals. In UJT the PN junction is formed by lightly doped N-type silicon with heavily doped P-type material on one side. The UJT is a three terminal device and the diode is a two terminal device. In diode there are p-type and n-type regions whereas in UJT emitter type region is also there. UJT also acts a voltage breakdown device when the input is applied between emitter and one base terminal which shows that emitter is responsible for current flow in the device as the current varies.
I hope this is helpful to you.

Best Wishes!