Silicon Carbide Diode Characteristics

diodeex04.in : Silicon Carbide Diode Characteristics

Requires: Blaze
Minimum Versions: Atlas 5.28.1.R

In this example, a SiC diode is simulated to demonstrate the Atlas capabilities to handle wide band gap semiconductor devices under room and elevated temperature conditions. The interest toward SiC technologies is growing due to the thermal and electronic properties of the material potentially leading to very high figures of merit for high-power, high-speed, high-temperature, and radiation hard applications.

The p+/n/n+ diode structure considered is a device based on one of the SiC polytypes called alpha-SiC (or 6H-SiC), and is constructed using Atlas syntax. The input file consists of the following main parts:

  • Mesh, regions, electrodes and doping specification using Atlas syntax
  • Material and models definition for SiC
  • Calculation of forward I-V characteristic under the room temperature conditions
  • Calculation of forward I-V characteristics under elevated temperature 623K
  • Calculation of reverse and breakdown characteristics at 623K

The simulations at elevated temperatures are performed in a separate Atlas run within the same input file with the respective resetting/changing of the material and models definition.

The input file starts by defining the mesh. The location and grid spacing along x and y directions are specified in the x.m and y.m statements. This simulation employs cylindrical symmetry and is therefore quasi-3-dimensional. To activate this feature, the parameter cylindrical is included in the mesh statement.

The structure consists of 3 regions: p+ emitter, n base, and n+ emitter regions, each uniformly doped. The dimensions of the regions and their doping are defined in the region and doping statements respectively. The anode and cathode contacts are specified in the electrode statements.

Basic material parameters of alpha-SiC are defined in the material statements. These include dielectric permittivity, energy band gap, parameters related to the band gap narrowing, auger recombination coefficients, saturation velocity, and parameters describing mobility and lifetime temperature dependencies. The low field mobilities and lifetimes are specified on a region-by-region basis taking into consideration the level of doping in the respective regions.

The set of physical models in the model statement includes electric field mobility dependence, Shockley-Read-Hall and Auger recombination, and band gap narrowing. The temperature is also specified in the model statement.

The first run is completed by the solve statement in which the anode bias is stepped from 0.1 up to 4 V to calculate the diode forward characteristic under room temperature conditions. The I-V data is saved in the log file.

The second Atlas run simulates the forward, reverse, and breakdown characteristics under elevated temperature conditions. It starts by reading in the mesh and structure data from the file produced by the first run. In order to use the same cylindrical symmetry as the first run the cylindrical parameter must be specified whilst loading the mesh. The same material and model statements are used in this input file with the temperature set at 623K. Then the solving procedure is repeated to obtain the forward I-V characteristic which is saved in the log file.

The set of physical models needs to be extended for simulating reverse and breakdown characteristics to include the impact ionization-generation model. This is done in the following part of the input file, in the impact statement. The Selberherr impact ionization model is used with the parameters changed to reflect the properties of alpha-SiC. The other material and model statements are repeated here to ensure proper reinitialization of the material and models parameters. Since the breakdown in diode structures is typically very sharp the curvetrace feature of Atlas is used to bias the curve. Note that the parameter step.init is negative. This is sufficient to force a negative voltage sweep on the anode. The reverse/breakdown characteristic is saved in the log file. To plot the breakdown the anode int.bias should be used as the x-axis.

The graphs with the forward I-V characteristics at the room and elevated temperatures, and the reverse and breakdown characteristic at 623K are displayed using TonyPlot.

Since cylindrical coordinates were used, Note that in all I-V plots the units of current is Amps and not Amps/micron.

To load and run this example, select the Load button in DeckBuild > Examples. This will copy the input file and any support files to your current working directory. Select the Run button in DeckBuild to execute the example.