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FETIS (HEMTS)
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Field Effect Transistor Integrated Simulator (FETIS)—package for simulation of group-III nitride based high electron mobility transistors (HEMTs).

1. Overview

The FETIS software is developed for modeling of group-III nitride-based high electron-mobility field-effect transistors (HEMTs). The module includes a 1D simulator of the band diagram and potential distribution across the device heterostructure and a graphical shell providing comfortable operation with the code and visualization of modeling results. Both quasi-classical and accurate quantum-mechanical consideration of the carrier confinement in a HEMT structure, based on the self-consistent solution of the Poisson and Schrodinger equations, are available in the FETIS package. The code allows predicting such important HEMT characteristics and parameters as the carrier concentration profile, the sheet carrier concentration, the number and energetic position of two-dimensional electron/hole gas subbands, etc., as well as their variation with the gate bias.

The physical models implemented into the FETIS account for specific features of the nitride materials - strong piezoeffect, existence of spontaneous electric polarization, and low efficiency of acceptor activation due to high ionization energy. There is also possible to include in the device heterostructure layers of semiconductor materials different from group-III nitrides, for instance ZnO, MgO or their alloys of a certain composition.


Fig. 1. Band diagram and carrier wave functions
The current version of the FETIS supports 4 types of boundary condition at the heterostructure top:

  • Free surface
  • Free surface passivated by a dielectric layer
  • Schottky contact
  • Metal/dielectric/semiconductor
The first pair describes inter-electrode regions, the latter pair is for gate and field plate regions. A special model of surface trap states is used to account for the band bending near the surface.

The HEMT operation is described within the gradual channel approximation. The user can calculate a distribution of the potential, electric filed, and carrier drift velocity along the channel at given drain and gate voltages, as well as the whole set of I-V characteristics. One can choose between different approximations for electron mobility as a function of the electric field.

2. Capabilities of FETIS 3.0


Fig. 2. Series computation provides the user with electron concentration in the channel as a function of the local bias for both gate and intercontact regions
Calculation of the electric potential distribution in the HEMT heterostructure along the direction normal to epilayers at given gate bias, providing

  • Band diagram
  • Energy levels and wave functions of localized carrier states
  • Carrier concentration and sheet density

HEMT operation analysis within the gradual channel approximation

  • Distribution of the channel potential, electric field, and electron concentration and drift velocity along the channel
  • Current-voltage characteristics


Fig. 3. A set of I-V characteristics can be calculated for a given variation of the gate and drain voltages
3. Brief description of the physical model

Band diagram is calculated by the Poisson equation

  • Spontaneous polarization and piezoeffect are taken into account
  • Carrier density is calculated with account for the quantum nature of the carriers
  • Electroneutrality condition on the bottom heterostructure surface

Two types of boundary conditions on the top contact

  • Schottky barrier on the gate contact
  • Band bending because of the surface trap levels on the intercontact surface


Fig. 4. User-friendly interface for input of the HEMT heterostructure
4. User interface

Friendly user interface allows one to

  • Specify all input data: heterostructure, materials properties, options, etc.
  • Interactively control the computations
  • View results by internal visualizer giving excellent representation of the device operation

The results also can be stored in a number of output files. The FETIS 3.0 supports export either in format of commercial Tecplot graphical package ( Copyright 1988-2005 Tecplot, Inc.) or plain-text data file.


Fig. 5. Materials properties can be easily inspected and edited
5. Database of material properties

The code is provided with an internal database of materials properties of group-III nitrides necessary for simulations. The database can be modified by user.

6. System requirements

  • Operation System—Windows 98/2000/ME/XP
  • RAM—256 Mb
  • Disk Space—2 Mb for the FETIS program files and about 1Mb per typical simulation to save results
  • Display and video card with the support of 1024x768 resolution in the High Color mode
  • Mouse

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