Recessed Gate Pseudomorphic HEMT DC Characterization

hemtex05.in : Recessed Gate Pseudomorphic HEMT DC Characterization

Requires: DevEdit/Blaze
Minimum Versions: Atlas 5.22.1.R

In this example a pseudomorphic HEMT structure based on the GaAs-AlGaAs-InGaAs-InP material system is constructed using DEVEDIT. The structure is then passed to Atlas for electrical testing. The input file consists of the following main portions:

  • Construction of the device in DEVEDIT
  • Electrical simulation of a family of Id/Vds curves
  • Simulation of Id/Vgs characteristics
  • Basic parameter extraction

The first part of the input file constructs the HEMT geometry, material regions, doping profiles, and electrodes in DEVEDIT. The structure considered is non- planar with a non-rectangular recessed gate. This demonstrates the important capability of DEVEDIT to generate arbitrarily shaped geometries. The device is created in DEVEDIT by drawing the device regions in interactive mode and specifying 2D doping distributions. The device is based on a GaAs substrate. It employs a double channel HEMT concept where the InGaAs channel is sandwiched between two AlGaAs regions. The structure also employs 2 delta- (or pulse-) dopings above and below the channel in both AlGaAs regions. These are modeled by narrow, 10 angstrom thick layers. The delta-doping usually expressed in terms of planar concentration (cm-2) must be recalculated into bulk doping (cm-3) given the thickness of the layers.

The delta-doped regions play an important role and are typical for modern HEMT technologies. They are used as additional carrier suppliers to the channel and for better control of the threshold voltage and other device parameters. The source and drain cap regions are made of GaAs. In practical applications the source and drain contact alloys often penetrate deep into the structure well below the channel. This is modeled by heavily doped areas under the source and drain in which the vertical doping distribution is assumed to be Gaussian, and the horizontal one is approximated by the complimentary error function. The means and the set of functions for specifying arbitrary 2D doping distributions are provided by the DEVEDIT. The composition fractions of AlGaAs (0.22 for Al) and InGaAs (0.78 for Ga) are specified in DEVEDIT as respective region attributes. The mesh was generated automatically by specifying basic mesh constraints and refining it along the x- and/or y-directions in the important areas of the device.

DEVEDIT then generates two types of files: a DEVEDIT input file and the structure file. The first can be run in DeckBuild to produce the corresponding structure file and is included here as the first part of the input file. The second can be read in directly by Atlas in the MESH statement. Note that the DEVEDIT input file can be edited as any other input file. It is straightforward to change the type and value of the doping associated with each region or resize regions. More importantly, DEVEDIT input files can also be read directly into the graphical user interface of DEVEDIT to provide all the menu options used to construct the structure.

The Atlas simulation begins from reading in the structure from DEVEDIT. DeckBuild provides autointerface between DEVEDIT and Atlas so that the structure produced by DEVEDIT is transferred to Atlas without having to indicate the MESH statement (commented out in this example). Without the automatic DEVEDIT/Atlas interface under DeckBuild, the MESH statement is needed to load the structure and the mesh.

The first active statements in the Atlas portion of the input file are the contact, material and model definitions. The gate workfunction is set up in the contact statement. The material parameters and physical models are specified in the material and models statements respectively on material-by- material basis. Shockley-Read-Hall recombination and electric field dependent mobility models are applied to all the material/regions. For GaAs and AlGaAs regions doping dependent mobilities and recombination parameters (lifetimes) are also applied. Conversely for InGaAs regions low field mobilities and carrier lifetimes are explicitly specified in the material statement. The band alignment is defined here using the align parameter, which defines the portion of the energy band gap difference applied to the conduction band. This parameter is given in the material statements.

The simulation is first performed to obtain the condition of the structure for 3 different gate biases, Vgs=0, -0.2, and -0.4 V, with the source and drain grounded. The respective states of the structure are saved in three separate solution files. Then the family of three Id-Vds characteristics is calculated in three separate drain voltage sweeps from 0 up to 2 V. Each series of the drain biasing is performed after loading the solutions with the respective gate bias. The Id-Vds characteristics are saved in three separate log files. The final conditions of the structure with 2 V on the drain are also saved in separate solution files. The zero bias condition is loaded again and the Id-Vgs characteristic is calculated.

At the end of simulation, parameter extraction statements are used to extract the following parameters using the extract feature of DeckBuild:

  • Threshold voltage
  • Maximum saturation current (Idss)
  • Gate voltage at Id=0.3*Idss

The results are also displayed using TonyPlot:

  • Id/Vds characteristics
  • Id/Vgs characteristic

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.