from the valence band to the conduction band in an intrinsic semiconductor is . eV for Ge and . eV for Si, while the energy required to set free a donor electron is only . eV for Ge and .
eV for Si. The V group pentavalent impurity atoms donate electrons to the conduction band and are called donor impurities. Therefore, - - - - - - - - Unit electronics and Communication The n -type and p -type semiconductors are neutral because only neutral atoms are doped to the intrinsic semiconductors. DIODES .
. P-N Junction formation i) Formation of depletion layer A single piece of semiconductor crystal is suitably doped such that its one side is p -type semiconductor and the other side is n -type semiconductor. The contact surface between the two sides is called p-n junction. Whenever p-n junction is formed, some of the free electrons diffuse from the n -side to the p -side while the holes from the p -side to the n -side.
The diffusion of charge carriers happens due to the fact that the n -side has higher electron concentration and the p -side has higher hole concentration. The diffusion of the majority charge carriers across the junction gives rise to an electric current, called diffusion current . When an electron leaves the n -side, a pentavalent atom in the n -side becomes a positive ion. The free electron migrating into p -side recombines with a hole present in a trivalent atom near the junction and the trivalent atom becomes a negative ion.
Since such ions are bonded to the neighbouring atoms in the crystal lattice, they are unable to move. As the diffusion process continues, a layer of positive ions and a layer of negative ions are created on either side of the junction accordingly. The thin region near . Figure .
Thermally generated electron in the conduction band and the holes generated by the dopants in the valence band ( p -type semiconductor) Hole from dopant Termally generated electron E V E g E c Acceptor energy