View 2n2646 2n2647 datasheet:

2N2646 2N2647 SILICON UNIJUNCTION TRANSISTORS Silicon Planar Unijunction Transistors have a structure resulting in lower saturation voltage, peak-point current and valley current as zell as a much higher base-one peak pulse voltage. In addition, these devices are much faster switches. The 2N2646 is intended for general purpose industrial applications where circuit economy is of primary importance, and is ideal for use in firing circuits for Silicon Controlled Rectifiers and other applications where a guaranteed minimum pulse amplitude is required. The 2N2647 is intended for applications where a low emitter leakage current and a low peak point emitter current (trigger current) are required and also for triggering high power SCR’s. ABSOLUTE MAXIMUM RATINGS Tj=1