In semiconductor physics, the Haynes–Shockley experiment was an experiment that demonstrated that diffusion of minority carriers in a semiconductor could result in a current. The experiment was reported in a short paper by Haynes and Shockley in 1948, with a more detailed version published by Shockley, Pearson, and Haynes in 1949. The experiment can be used to measure carrier mobility, carrier lifetime, and diffusion coefficient. In the experiment, a piece of semiconductor gets a pulse of holes, for example, as induced by voltage or a short laser pulse.
Equations To see the effect, we consider a n-type semiconductor with the length d. We are interested in determining the mobility of the carriers, diffusion constant and relaxation time. In the following, we reduce the problem to one dimension. The equations for electron and hole currents are:
j e = + μ n n E + D n ∂ n ∂ x {\displaystyle j_{e}=+\mu _{n}nE+D_{n}{\frac {\partial n}{\partial x}}}
j p = + μ p p E − D p ∂ p ∂ x {\displaystyle j_{p}=+\mu _{p}pE-D_{p}{\frac {\partial p}{\partial x}}}
where the js are the current densities of electrons (e) and holes (p), the μs the charge carrier mobilities, E is the electric field, n and p the number densities of charge carriers, the Ds are diffusion coefficients, and x is position. The first term of the equations is the drift current, and the second term is the diffusion current.
Derivation We consider the continuity equation:
∂ n ∂ t = − ( n − n 0 ) τ n + ∂ j e ∂ x {\displaystyle {\frac {\partial n}{\partial t}}={\frac {-(n-n_{0})}{\tau _{n}}}+{\frac {\partial j_{e}}{\partial x}}}
∂ p ∂ t = − ( p − p 0 ) τ p − ∂ j p ∂ x {\displaystyle {\frac {\partial p}{\partial t}}={\frac {-(p-p_{0})}{\tau _{p}}}-{\frac {\partial j_{p}}{\partial x}}}
Subscript 0s indicate equilibrium concentrations. The electrons and the holes recombine with the carrier lifetime τ. We define
p 1 = p − p 0 , n 1 = n − n 0 {\displaystyle p_{1}=p-p_{0}\,,\quad n_{1}=n-n_{0}}
so the upper equations can be rewritten as:
∂ p 1 ∂ t = D p ∂ 2 p 1 ∂ x 2 − μ p p ∂ E ∂ x − μ p E ∂ p 1 ∂ x − p 1 τ p {\displaystyle {\frac {\partial p_{1}}{\partial t}}=D_{p}{\frac {\partial ^{2}p_{1}}{\partial x^{2}}}-\mu _{p}p{\frac {\partial E}{\partial x}}-\mu _{p}E{\frac {\partial p_{1}}{\partial x}}-{\frac {p_{1}}{\tau _{p}}}}
… excerpt ends here. Continue reading the full article.
