Accurate, High Speed Physics in FastBlaze

Ultra-Fast Simulation Technique

FastBlaze uses novel device simulation techniques to simulate typical MESFET and HEMT characteristics in less than a minute. To be a truly useful engineering tool this extremely high speed must be achieved without compromising the accuracy of the physical models. In practice the speed of FastBlaze allows the use of more advanced models than are usually considered for conventional device simulation.

Four areas of modeling in FastBlaze are highlighted here to demonstrate the necessity of using the most accurate available device physics when simulating MESFETs and HEMTs.

 

Impact Ionization Models

In conjunction with the DC and RF analysis, FastBlaze also simulates breakdown by including impact ionization models. The Selberrherr and the second and third order Crowell-Sze, Toyabe and Concannon models are supported with tuned coefficients for GaAs. FastBlaze includes an automated DC IV point algorithm which samples IV points only in regions where the current is changing. This algorithm has been enhanced to handle the sharp turning points produced as breakdown occurs. Figure 1 shows breakdown occurring in a double-recessed GaAs MESFET.

Figure 1. Breakdown occurring in a double-recessed GaAs MESFET

 

Ionized Traps

FastBlaze includes state-of-the-art charge models, namely the inclusion of quantum mechanics via the self-consistent solution of Schrodinger's equation. Further, incomplete impurity ionization is included for both dopants and traps allowing accurate simulation of DX centers and surface pinning. Figure 2 illustrates the carrier profile across an InGaAs/InP heterojunction generated by the quantum simulation.


Figure 2. Comparison of complete and incomplete donor ionization. As the conduction band edge dips below the Fermi-level the shallow donor sites trap electrons, reducing the net doping level.

 

Multi-Layer Model Specification

FastBlaze implements a sophisticated multi-layer transport technique that includes a wide selection of default models. The conventional analytic field dependent models are supplemented by a comprehensive Monte-Carlo derived transport database providing users with unequalled resources important for accurate simulation. In addition core device models can be replaced with user-defined code using the C-Interpreter allowing device engineers unparalleled freedom.

FastBlaze permits different mobility, energy-field and impact-ionization models to be declared in different epitaxial layers. Figure 3 illustrates the effect of limiting a device simulation to one transport model. Here the DC-IV family of curves are compared for an AlGaAs/GaAs pseudomorphic HEMT simulated using a simple one-model approach and the more accurate multi-layer models. Note in the multi-layer simulation the material transport parameters are extracted from the Monte Carlo transport database.

Figure 3. FastBlaze incorporates multi-layer transport models. Here an
AlGaAs/InGaAs/GaAs pseudomorphic HEMT is simulated using the
Monte-Carlo derived transport models and compared with the simple
one-mobility model results. The effect on breakdown is illustrated.

 

C-Interpreter

FastBlaze incorporates an updated version of the C-language Interpreter used by other Silvaco products. This allows users to add their own models for:

  • mobility as a function of doping, field and lattice temperature
  • impact ionization
  • relation of hydrodynamic energy (carrier temperature) to electric field equivalent to energy relaxation time
  • DC gate conduction
  • doping distribution
  • incomplete ionization

 

The new C-Interpreter incorporates a graphical debugger and a set of debugging syntax for efficient prototyping of models. The interpreter pre-processes the user supplied code resulting in only a small decrease in overall simulation time compared with the built-in models.

 

 

 Model  Simulation Options
mobility: Monte Carlo Database standard "Silicon type" analytic velocity overshoot analytic C-interpreter
hydrodynamic energy-field relationship (equivalent to energy relaxation time): Monte Carlo Database standard field-dependent analytic C-interpreter
DC gate leakage current: none (perfect insulator) thermionic emission C-interpreter
impact ionization: Selberrherr Crowell-Sze (2nd and 3rd order) Toyabe Concannon C-interpreter
Table 1. Summary of physical models in FastBlaze.

 

 

Conclusion

The four models described above demonstrate the standard of models required for accurate device simulation of III-V FET devices; note FastBlaze only simulates devices using the full hydrodynamic energy - balance models. FastBlaze incorporates these features and is able to model most DC characteristics inside 30 seconds and RF characteristics inside 2 minutes. This combination of speed and accuracy makes FastBlaze an ideal tool for interactive TCAD.