What is the net charge on a n-type or a p-type substance?

Answer 1

There is no net charge on either substance. However, an N type substrate contains atoms (such as arsenic or phosphorous) trapped in its lattice that have an electron (called a donor electron) in the outer (valence) band of electrons, which can be easily removed by a relatively weak electromotive force (voltage). A P type substrate, although electrically neutral, contains an element (such as boron) which has one fewer electrons in its valence band that it would like to have, making it an acceptor. The missing electron is often called a "hole". To elaborate slightly, bands of electrons like to have a certain number of electrons to be "stable". A donor atom may have one, or perhaps five, electrons in its valence band, while an acceptor might have three or seven. Both would be happier with zero or four or eight. When an N and a P type substance are in contact, a negative voltage on the N material with respect to the P material will allow electrons to move from the donor material to the vacant holes in the acceptor. If the voltage is applied in reverse, the negative charge on the P material attracts the holes away from the junction, and the (relatively) positive charge on the N material attracts electrons away from the junction, and very little current flows.