Finding Nc

Consider a gold Schottkey diode at T=300k formed on a n-type GaAs doped at Nd=5x10^16 cm^-3.  Among other things find Phi|n.

Phi|n=KT/e*ln(Nc/Nd)

I don't understand how to get Nc from the information given here.

What is NC given the information stated in the problem?

Thanks.