The characteristic curve for the UJT shows a negative resistance region. A common use of the device is to make a relaxation oscillator.
An n-type unijunction transistor (left symbol) is made by implanting a small p-type emitter probe in a bar of n-type silicon. The emitter probe is offset from the center of the bar and there are two bases as shown below.
The negative resistance region in the characteristic of the unijunction transistor makes it useful for constructing relaxation oscillators.