Carrier transport
Drift-Diffusion Model
The continuity equations in the presence of generation \(G\) recombination \(R\) of electron-hole pairs read
where the current is proportional to the gradient of quasi Fermi levels \(E_{\text{F},n/p}(\mathbf{x})\)
Here the charge current has the unit of (area)\(^{-1}\)(time)\(^{-1}\). \(\mu_{n/p}\) are the mobilities of each carrier. In nextnano++, \(\mu_{n/p}\) are determined using the mobility model specified in the input file under currents{ }.
Hereafter we consider stationary solutions and set \(\dot{n}=\dot{p}=0\). The governing equations then reduce to
which we call current equation.
We can also say that the current equation governs the relationship between the carrier densities \(n(\mathbf{x})\), \(p(\mathbf{x})\) and quasi Fermi levels \(E_{\text{F},n/p}(\mathbf{x})\).
The nextnano++ tool solves this equation and Poisson equation (and also Schrödinger equation) self-consistently.
In their solution, the corresponding calculation of the carrier densities \(\big(n(\mathbf{x}, \phi, E_{\text{F},n}),p(\mathbf{x}, \phi, E_{\text{F},p})\big)\) and Poisson equation are firstly iterated for a given quasi-Fermi levels until the carreir densities converge. Then the resulting carrier densities are substituted into the current equation and the quasi-Fermi levels are updated. This whole cycle is iterated until the quasi-Fermi levels satisfies the convergence criteria, which can be tuned by the users from run{ current_poisson{ } } or run{ quantum_current_poisson{ } }.
Last update: 04/12/2024