what is biult in potential for p-n junction??

Answer 1

The built-in potential is the delta in Fermi levels of the two sides of the junction

For silicon diodes, it's about 0.6 V

Answer 2

Around 0.7 volts for silicon and around 0.3 for germanium pn junctions.

Actually it is not as if the voltage can be utilised as in a battery. It is the potential difference at the junction which is required to overcome to let current pass.

Answer 3

By connecting a voltmeter, the motion of electrons in the connecting wires of the voltmeter, creates more holes in the N type material near the junction and more negative ions in the P type materials near the junction. This widens the depletion layer. It means that the barrier became stronger than it was. As there is no closed path for the passage of charges through voltmeter, the voltmeter reads zero voltage. ==========================

Answer 4

.....and if you need the detailed spiel, then read http://ece-www.colorado.edu/~bart/book/book/chapter4/ch4_2.htm#4_2_3