You need to define e. If I define it as something, like 1, it works:
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e=1;
u1=68.5;
umax=1414;
u2=56.1;
Cr=9.2e16;
Cs=3.41e20;
alfa=0.711;
beta=1.98;
elec = 1.602e-19;
N1=1e17;
N2=1e17;
N3=1e21;
index=0;
k = N1:N2:N3;
for Ne = 1:length(k)
index = index + 1 ;
u(index) = u1 + ((umax-u1)/(1+(k(Ne)/Cr)^alfa)) - ((u2)/(1+(Cs/k(Ne))^beta));
p(index) = (k(Ne)*e*u(index)).^-1;
f(index) = k(Ne);
x(index) = f(index) ;
y(index) = p(index);
end
semilogx(x,y)
