Doping Semiconductors

Doping involves the process of adding enough impurity atoms to increase carrier concentration without changing the crystal properties of silicon.

P-type Doping

P-type doping adds trivalent atoms to a silicon crystal lattice, typically boron. The atom accepts a free electron, which generates a hole. The boron is an acceptor and negatively charged, but no free electron is generated as the charge is fixed. More positive charges are free to move, hence the name p-type.

N-type Doping

N-type doping adds pentavalent electron atoms to a silicon crystal lattice, typically phosphorus. The extra valence electron will not form a covalent bond, and thus becomes a free electron. The phosphorus is a donor and positively charged, but no hole is generated as the charge is fixed. More negative charges are free to move, hence the name n-type.

Properties

Doping changes the charge held by the majority of carriers. N-type doping increases electron carriers, and P-type doping increases hole carriers. While the added impurity holds the opposite charge, it is fixed and does not move. Thus, doping does not affect the overall charge of the silicon.

P-type Acceptor Concentration Relation to Intrinsic Carrier Concentration
$N_A\gg n_i$
$p_p\cong N_A$
$p_p{n}_p=n_i^2$
$n_p\cong \frac{n_i^2}{N_A}$

N-type Donor Concentration Relation to Intrinsic Carrier Concentration
$N_D\gg n_i$
$n_n\cong N_D$
$p_n{n}_n=n_i^2$
$p_n\cong \frac{n_i^2}{N_D}$