It depends on the property of the materials used in the transistor.It varries in case of si,ge or any other materials used.It depends on the bonding of materials in their crystal structure.

Saptarshi i am not talking about Vbe voltage,i am talking about transistor saturation voltage ,if you see any text book your expalnation is valid for Vbe voltage..but how transistor saturation voltage depends on proprty of material i did not get you...could you explain more 

https://learn.sparkfun.com/tutorials/transistors/operation-modes

http://forum.allaboutcircuits.com/threads/transistor-saturation.26096/

1. A transistor is in saturation when an increase in base current does not produce a change in collector current.

2. That depends on the configuration of the transistor circuit. A transistor breaks into conduction when the B-E voltage reaches some value, typically .7 volts for a silicon transistor. After that, conduction is controlled by base current.

3. Vce is the voltage between the collector and emitter. In the spec sheet, it defines the maximum voltage the transistor can withstand before being destroyed by breaking down.

4. Only if the transistor is conducting.

generally one may say that a transistor breaks into conduction at some value of Vbe. But breaking into might mean a sudden change which is not what it is. If Vbe at 1 ma is 0.7V, then at 0.64V, the transistor still conducts at 100uA and and at .58V, the current is still not zero but 10uA. refer gray and meyers. [Same thing with zeners: they do not break into conduction, there is no abrupt change in current..] But the fact is as mentioned ,  Vsat  depends on material. Just as Vbe does. Both are dependent on material. The ON condition demands that both junctions be forward biased. Get to see waht is "hard" conduction.. One may place a current in base twice as required for making the transistor enter saturation, when hard saturation is desired. What if base current is made five times?... This and other things can be easily seen in a simulator such as circuitmaker (student version, free download). There is also an inverted mode of operation [where roles of collector and emitter are interchanged] that gives a much lesser Vce sat but current gain falls down to 2 or 1. This is sometimes advantageous if one has to use only BJ transistors and cannot use FETs.

Thanks Saptarshi and V.s.V Mani for explanation...i got clear idea