There is an interesting device called unijunction transistor (also known as double-base diode) which is a structure consists of say n-type semiconductor sample with p-type area diffused in it such that the whole structure has only one p-n junction. The terminals of n-type semiconductor are bases B1,B2, the built-in/diffused area terminal is emitter E of the transistor. Here is an example of pulse generator based on equivalent of UJT including a pair of two bipolar transistors of different conductivity type connected as you can see at the diagram. Upper transistor emitter corresponds to UJT emitter E, collector and emitter of the lower one to the bases B1,B2 of UJT respectively. Both BJT-s connected such a way that at the moment when voltage between B2 and E have reached some threshold value the avalanche-like process of both transistors opening starts so that each of them opens another one. RC-circuit leftwards determines time period the capacitor needed to charge up to the voltage sufficient for UJT opening. When UJT is opened the voltage breakdown between Its B2 and E terminals is lower than threshold value so it discharges the capacitor and all process starts from the beginning. The load resistors and 10 Ohm B2-resistor are important, their resistance values are critical for this circuit to run properly. This type generator using real UJT instead of its 2bjt-equivalent may be more stable (not a sarcasm) and less sensitive to resistance values. If it doesn't start - just push the button xD
