X'tors and their symmetry

[shekhar_dandya]

Hi All,
Why is X'tor not a symmetrical device i.e if c-b is forward biased & b-e is reverse biased will it function the same and give same characteristics?
I heard that FET's are symmetrical when used as switches.Is it true?
rgds,
-Shekhar

[audioguru]

Hi Shekhar,
Transistors are not symmetrical to make them have lots of gain. At a low supply voltage, a transistor will work with its emitter and collector reversed, but the gain will not be much more than one.
Look at the maximum voltage ratings of a transistor. Its collector to base voltage is very high and is the rating of the transistor. But the emitter to base voltage (reversed biased) is only 5V to 7V for all silicon transistors.

Many FETS are completely symmetrical. A 2N5457 and the rest of that family can amplify with its drain and source reversed just as well as the "normal" way. But its normal pinout reduces capacitive coupling between its drain and gate at high frequencies.