MAKING A ZENER DIODE
Sometimes a zener diode of the required voltage is not available. Here are a number of components that produce a characteristic voltage across them. Since they all have different voltages, they can be placed in series to produce the voltage you need. A reference voltage as low as 0.65v is available and you need at least 1 to 3mA through the device(s) to put them in a state of conduction (breakdown).
source : http://www.talkingelectronics.com.au/projects/200TrCcts/200TrCcts.html