Mass action law (electronics)

For other uses, see Mass action.

In electronics and semiconductor physics, the law of mass action relates the concentrations of free electrons and electron holes under thermal equilibrium. It states that, under thermal equilibrium, the product of the free electron concentration $n$ and the free hole concentration $p$ is equal to a constant square of intrinsic carrier concentration $n_{\text{i}}$. The intrinsic carrier concentration is a function of temperature.

The equation for the mass action law for semiconductors is: $$np=n_{\text{i}}^{2}$$

Carrier concentrations

In semiconductors, free electrons and holes are the carriers that provide conduction. For cases where the number of carriers are much less than the number of band states, the carrier concentrations can be approximated by using Boltzmann statistics, giving the results below.

Electron concentration

The free-electron concentration n can be approximated by $$n=N_{\text{c}}\exp \left[-{\frac {E_{\text{c}}-E_{\text{F}}}{k_{\text{B}}T}}\right],$$ where

• E_c is the energy of the conduction band,
• E_F is the energy of the Fermi level,
• k_B is the Boltzmann constant,
• T is the absolute temperature in kelvins,
• N_c is the effective density of states at the conduction band edge given by ${\textstyle N_{\text{c}}=2\left({\frac {2\pi m_{\text{e}}^{*}k_{\text{B}}T}{h^{2}}}\right)^{3/2}}$, with m*_e being the electron effective mass and h being the Planck constant.

Hole concentration

The free-hole concentration p is given by a similar formula $$p=N_{\text{v}}\exp \left[-{\frac {E_{\text{F}}-E_{\text{v}}}{k_{\text{B}}T}}\right],$$ where

• E_F is the energy of the Fermi level,
• E_v is the energy of the valence band,
• k_B is the Boltzmann constant,
• T is the absolute temperature in kelvins,
• N_v is the effective density of states at the valence band edge given by ${\textstyle N_{\text{v}}=2\left({\frac {2\pi m_{\text{h}}^{*}k_{\text{B}}T}{h^{2}}}\right)^{3/2}}$, with m*_h being the hole effective mass and h being the Planck constant.

Mass action law

Using the carrier concentration equations given above, the mass action law can be stated as $$np=N_{\text{c}}N_{\text{v}}\exp \left(-{\frac {E_{\text{g}}}{k_{\text{B}}T}}\right)=n_{i}^{2},$$ where E_g is the band gap energy given by E_g = E_c − E_v. The above equation holds true even for lightly doped extrinsic semiconductors as the product $np$ is independent of doping concentration.

See also

• Law of mass action

External links

• Doping, Carrier Concentration, Mobility, and Conductivity
• Semi-conductor tutorial