Read before you solve · 3 min
Read Doping: Count Donor Atoms Against the Crystal
A donor fraction is the added carrier density divided by the density of lattice atoms. Convert material density to atoms per cubic metre first.
NSEP · Electronics · 1 linked question · 100% coverage
Know this first
Doping and Carrier Density is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Study deeper: NCERT Physics · Electronics
Start with the condition
Use number density = rho N-A/M for silicon, then compare the required additional conduction-electron density with that atomic density
Build the governing relation
Subtract the intrinsic carrier density when it is not negligible compared with the doped value.
Finish with a physical check
Check the limiting case before selecting an option: remove the special feature in the diagram or make the relevant parameter very large or very small. The result should reduce to a familiar relation.
OlympIQ exam lens
- ✓State the physical condition before choosing a formula.
- ✓Keep direction, sign and constraint information explicit.
- ✓Test one limiting case before committing to an option.
OlympIQ trap
Using molar mass in grams while density is in SI units creates an error of a million in the donor fraction.
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