For a doped semiconductor, the density of electrons in the conduction band is,
$$n = N_c(T)\exp\left(\frac{E_F-E_c}{k_BT}\right).$$The density of holes in the valence band is,
$$p = N_v(T)\exp\left(\frac{E_v-E_F}{k_BT}\right).$$The density of ionized donors is,
$$N_D^+=\frac{N_D}{1+2\exp\left(\frac{E_F-E_D}{k_BT}\right)}.$$and the density of ionized acceptors is,
$$N_A^-=\frac{N_A}{1+4\exp\left(\frac{E_A-E_F}{k_BT}\right)}.$$The factor of 4 is valid in the formula for the acceptors if the semiconductor has a light hole and a heavy hole band as Si and Ge do.
The four quantities $n$, $p$, $N_D$, and $N_A$ can only be determined if the Fermi energy, $E_F$, is known. Typically, $E_F$ must first be determined from the charge neutrality condition,
$$n+N_A^- = p+N_D^+.$$The Fermi energy can be found by solving the charge neutrality condition numerically. One way to do this is to program the formulas for n, p, Nd+, and Na- in a spreadsheet. Then choose a temperature and calculate n, p, Nd+, Na- for every value of the Fermi energy between Ev and Ec. For one of these Ef values, the charge neutrality condition will be satisfied.
When n + Na- and , p + Nd+ are ploted as a function of EF, the Fermi energy is where the two lines cross. A new plot like the one below can be generated by pressing the 'Replot' button.
|
Normally it is not necessary to determine Ef numerically and the following approximation is sufficient.
n-type Nd > Na:
n = Nd - Na
p = ni²/n
Ef = Ec - kBTln(Nc/(Nd - Na))
p-type Na > Nd:
p = Na - Nd
n = ni²/p
Ef = Ev + kBTln(Nv/(Na - Nd))
The plot is generated with the following code: for (i=0; i<500; i++) { Ef = i*Eg/500; n=Nc*pow(T/300,1.5)*exp(1.6022E-19*(Ef-Eg)/(1.38E-23*T)); p=Nv*pow(T/300,1.5)*exp(1.6022E-19*(-Ef)/(1.38E-23*T)); Namin = Na/(1+4*exp(1.6022E-19*(Ea-Ef)/(1.38E-23*T))); Ndplus = Nd/(1+2*exp(1.6022E-19*(Ef-Ed)/(1.38E-23*T))); }