Electrical Engineering ⇒ Topic : Calculation of n and p in Extrinsic Semiconductor

Sunita
 
Calculation of n and p in Extrinsic Semiconductor In a Ptype semiconductor, each acceptor atom accepts one electron and it becomes a negative ion. Similarly, in an Ntype 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., N_{D} + p = N_{A} + n ...........(1) where n is the free electron concentration, p is the hole concentration, N_{D} is the concentration of donor atoms, and N_{A} is the concentration of acceptor atoms. Under thermal equilibrium, n_{p} = n_{i}^{2} .............(2) where n_{i }is the intrinsic concentration of semiconductor. If the material has both N and Ptype doping, the effective doping will be (N_{D}+ N_{A}). If N_{D }> N_{A}, it will behave like an Ntype material. Putting N_{A} = 0 and p = ni Eq. (1), we have
........... (3) If N_{D} is much larger than n_{i}, n_{i} can be ignored in Eq. (3). Taking +ve sign, we can write
From Eq. (2) ............ (4) Similarly, in Ptype semiconductor, p = N_{A} ............. (5)  
 
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