Electrical Engineering ⇒ Topic : Calculation of n and p in Extrinsic Semiconductor
Calculation of n and p in Extrinsic Semiconductor
In a P-type semiconductor, each acceptor atom accepts one electron and it becomes a negative ion. Similarly, in an N-type semiconductor, each donor atom donates a free electron that moves randomly in the crystal lattice and the donor atom becomes a positive ion fixed in the lattice. The hole created due to the acceptance of electron perambulates freely in the lattice. The crystal as a whole is electrically neutral. Therefore, the positive and negative charges must be equal, i.e.,
ND + p = NA + n ...........(1)
where n is the free electron concentration, p is the hole concentration, ND is the concentration of donor atoms, and NA is the concentration of acceptor atoms.
Under thermal equilibrium,
np = ni2 .............(2)
where ni is the intrinsic concentration of semiconductor.
If the material has both N- and P-type doping, the effective doping will be (ND+ NA). If ND > NA, it will behave like an N-type material. Putting NA = 0 and p = ni Eq. (1), we have
If ND is much larger than ni, ni can be ignored in Eq. (3). Taking +ve sign, we can write
From Eq. (2)
Similarly, in P-type semiconductor, p = NA
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