Solid State Physics- Finding concentration of donor atoms (Nd)

[Matt1234]

Homework Statement

Silicon at T= 300K contains acceptor atoms at a concentration of Na = 5*10^15 cm-3. Donor atoms are added forming an n-type compensated semiconductor such that the fermi level is 0.215 eV below the conduction band edge. What concentration of donor atoms are added?

Homework Equations

I have one equation byt i still have one unknown:

The Attempt at a Solution

here is what i have:
(ni^2) = (Nd- 5*10^15) * po

 

[member 553083]

where ni is the intrinsic carrier concentration and po is the hole concentration. Nd is the donor atoms added, but I don't know what this is. I'm not sure how to proceed. Any help would be much appreciated. Thank you!