Transistor active to saturation question?

Your questions are very confusing and indicate you don't have a clue what you are talking about. I suggest you read your book again, or talk to your instructor.

Hi,Friend., As you can see from the basic diode characteristics .7 voltage will be farely enough to make the junction forward bias. basically transistor is made of diodes. Diode is having a forward bias characteristics in exponential fashion.so until .7 voltage the junction is reverse biased .As and when the voltage is attained the junction will give a fairly large current for a small voltage as it is having exponential manner. So it will draw a large current which will drive it to saturation. Thank you.

Think of the base emitter circuit as the controlling input, and the collector emitter circuit as the controlled output. The base circuit can be seen as something like a diode junction. It is forward biased by the external circuit, in the region of 0.7V, so current can flow into the base emitter circuit. That is the voltage between base and emitter in the conducting direction. When saturated, the collector emitter voltage can be a lot less than 0.7V. It is best not to look at it as some sort of junction like a diode, or even as being connected to the base emitter circuit except through the transistor action described as beta or DC current gain. The idea of a transistor being two diodes back to back is not relevant, except as a very basic ohm-meter test method using only two wires at a time. Just think of the transistor as two separate circuits with current flowing in them, the base emitter circuit and the collector emitter circuit. The collector emitter circuit is like a resistor, controlled by base current, though it may not be a linear relationship, so I hesitate to call it ohmic. The control action is due to base emitter current, rather than voltage, though there is a small change in voltage too, it is not the issue of interest. The base emitter junction has to be biased in the forward direction (about 0.7V) so it is conducting, and then if sufficient current flows through this junction the collector emitter circuit can become saturated. This occurs when the base current multiplied by the DC current gain (beta) of the transistor is more than the collector current. The collector current is limited by the external part of the collector emitter circuit. If the collector emitter circuit is 12V through a 12W lamp and the transistor, a maximum current of 1A can flow with the transistor fully saturated. If the beta is 12, then a current of at least 1A/12 = 83mA is required in the base circuit to ensure saturation. Normally the base is reduced well below the forward bias voltage for the "off" state. This is the equivalent of connecting the base to the emitter. It is so any current leaking from the collector into the base circuit cannot cause base emitter current. Saturation just means that a further increase in base current no longer increases collector current, for the circuit involved. As a side issue, current gain can change with temperature, individual transistors and with collector current, so a wide margin is allowed to ensure saturation occurs.