Unijunction transistors are three-lead transistors that act exclusively as electrically controlled switches. This differs from other transistors in that general transistors usually provide the ability to act as a switch and also as an amplifier. But a unijunction transistor does not provide any decent type of amplification because of the way it is constructed. It's simply not designed to provide a sufficient voltage or current boost.
The three leads of a unijunction transistor are base leads (B1 or B2) and an emitter lead, which is the lead which receives the input current. The basic operation of a unijunction transistor is relatively simple. When no potential voltage exists between its emitter and either of its base leads, only a very small current flows from B2 to B1. However, if a sufficiently large positive trigger voltage-relative to its base leads is applied to the emitter, a larger current flows from the emitter and combines with the small B2-to-B1 current, thus giving rise to large B1 output current. Unlike other transistors where the control leads provide little additional current, the unijunction transistor is just the opposite. Its emitter current is the primary source of current for the transistor. The B2-to-B1 current is only a very small amount of the total combined current. That’s why unijunction transistors are not suitable for amplification purposes, but only for switching.