Mocasim is an ensemble Monte Carlo semiconductor transport parameter generator. It calculates the electron transport properties of Diamond, Zincblende and Wurtzite crystal semiconductors. The results can be used in FastBlaze for the accurate simulation of MESFETs and HEMTs.

Valid Semiconductors

Mocasim simulates the conduction band using a non-parabolic three-valley model. Mocasim is ideal for simulating the transport properties of Diamond and Zincblende crystal semiconductors (such as Silicon and GaAs) which have a well known G, L, X conduction band structure. The conduction band of Wurtzite semiconductors (such as GaN) is more complex but can also be simulated by defining the properties of the three lowest valleys.

Input Parameters

Mocasim requires the basic physical properties of the crystal: density, speed of sound, dielectric constant, optical phonon energy, deformation potential. A conduction band valley is described by an effective mass, non-parabolicity, degeneracy and its energy relative to the lowest valley. A maximum of three conduction band valleys can be defined. The parameters of several semiconductors are pre-defined (e.g. SiGe, GaAs, GaN, etc.) and Mocasim also supports user-defined materials.

Output Parameters

Mocasim calculates velocity, energy and momentum relaxation time, kinetic and potential energy, effective mass and the fraction of electrons in each conduction band valley. These parameters are calculated as a function of electric field, doping and temperature.

Initially the electrons are in a Boltzmann distribution in the lowest conduction band valley. The equilibrium distribution is attained by running the simulation for a “burn in” period. Once the electrons are in their equilibrium distribution the results are extracted by averaging over many time steps.

 

Mocasim generates the scattering rates for a variety of mechanisms allocated by the user. The figure above illustrates the variation of the total and individual scattering rates in the G valley of GaAs at 300K as a function of energy.

 

Scattering Mechanisms

The electrons are scattered by imperfections in the crystal and by the thermal motion of the atoms.

Any number of scattering mechanisms may be defined for simulation. Mocasim has functions for the most common scattering events which occur in bulk semiconductors.

Built-in Scattering Mechanisms Intravalley: polar optical phonon non-polar optical phonon deformation potential acoustic phonon piezo-electric acoustic phonon ionized impurity scattering neutral impurity scattering   Intervalley: deformation potential intervalley phonon

 

If other scattering mechanisms, such as alloy scattering or plasmon scattering, are deemed important they can be defined by the user and included through the Silvaco C-Interpreter.

 

The electron velocity for GaAs at 300K is shown as a function of net impurity density and electric field.

 

The electron momentum relaxation time in Si at 300K is shown as a function of doping at three different electric fields.

 

The electron velocity in GaN at 300K is shown as a function of electric field at doping of 10¹⁵, 10¹⁷, 10¹⁸, and 10¹⁹ cm^-3.

 

 

This figure shows the velocity, energy, energy relaxation time and momentum relaxation time as a function of electric field and doping for GaN at 300K.