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The number desnity of donor atoms which ...

The number desnity of donor atoms which have to be added to an intrinsic germanium semiconductor to produce an n-type semiconductor of conductivity `5 ohm^(-1) cm^(-1)` is `axx10^(15)cm^(-3)`. Given that the mobility of electron in n-type Ge is `3900 cm^(2)//Vs`, Neglect the contribution of holes to conductivity. What is the integer value of a?

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The correct Answer is:
8
Here, `sigma = 5Omega^(-1)cm^(-1) ,mu_(e) = 3900 cm^2//Vs, n_e = ?`
We know that, `sigma =1/rho=en_emu_e`
or `n_e=sigma/(emu_e)=5/((1.6xx10^(-19)xx3900))~~8xx10^(15)=cm^(-3)`
Since, one donor atom provides one free electron to the germanium semiconductor, therefore, number density of donor atoms `= 8xx10^(15) cm^(-3)`
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