Lattice Matched HEMT Breakdown Simulation

hemtex02.in : Lattice Matched HEMT Breakdown Simulation

Requires: Blaze
Minimum Versions: Atlas 5.22.1.R

This example demonstrates simulation of avalanche breakdown in a single quantum-well lattice matched AlGaAs/GaAs HEMT. It shows:

  • Construction of the heterojunction structure using Atlas syntax
  • Material and models parameter specification
  • Simulation of Id-Vds and breakdown characteristic
  • Display of the results in TonyPlot

The device under consideration consists of the highly doped (1.e18/cm3) AlGaAs layer with the Al composition fraction of 0.3, the GaAs channel layer 250 A thick with the donor concentration of 1.e15 /cm3, and AlGaAs buffer layer with the same doping. The parts of the channel under the source and drain are heavily doped, and source and drain electrodes are located vertically, touching the channel region. This technique accounts for the alloying of contact material and the tunneling of carriers through the heterojunction. The channel length is 0.5 micron.

In the first part of the input file, the device is described using the Atlas structural syntax. The description includes the mesh, regions locations, electrodes locations, and doping distribution. The region statements are used to define the AlGaAs and GaAs regions. The Al composition fraction (x.composition=0.3) is defined here as well. Note that an artificial oxide layer (region num=4) is defined here providing the possibility to specify AlGaAs surface states using the interface statement.

After the device description, the first material statement is used to specify the electron and hole SRH lifetimes applied to both GaAs and AlGaAs materials. In the second material statement the low field mobilities, electron affinity, and density of states in the conduction band Nc are defined for AlGaAs. In this example the band alignment is calculated by using energy band gaps and electron affinities of the materials forming heterojunctions. If not explicitly specified in the material statements the default values of other material parameters are applied.

The models statements are used to specify the following set of models : field dependent mobility, and SRH. The two carrier transport model is specified here as well (the default). In addition, the concentration dependent mobility model is activated.

In order to simulate avalanche breakdown the impact ionization-generation model should be turned on. This is done using the impact selb statement in which the Selberherr impact ionization model is activated. All basic impact ionization parameters are user-accessible and can be modified by the user. As an example the appropriate set of parameters is defined in this statement. The parameters apply to both GaAs and AlGaAs materials.

The gate electrode in HEMT structures is of Schottky type. To indicate this the contact statement is used to define the workfunction of the gate electrode, which gives the Schottky barrier height of approximately 0.9V.

The initial solution is performed automatically if not specified. The gate voltage is set to zero, the structure under zero bias is displayed using TonyPlot, and the drain voltage is ramped up to 0.3V. The solutions are obtained using the combined Gummel-Newton algorithm specified in the statement method gummel newton. The algorithm implies that if the solution does not converge in the course of the decoupled Gummel iterations, the program will automatically switch over to the fully coupled Newton algorithm.

Next the drain voltage is ramped until the drain current reaches the predefined value of 9.e-5 A/um well within the breakdown region, using the compliance limits on the {solve} solve statement. These calculations are performed using the Newton method :
method newton.
The results of the simulation are saved in the log file and the Id-Vds breakdown characteristics are displayed using TonyPlot.

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