8.13 Sspd_chapter 6_part 7_device simulation1  (Page 2/5)

• Small-signal calculation techniques that converge at all frequencies.

• Stable and accurate time integration.

ATLAS is often used in conjunction with the ATHENA process simulator. ATHENA predicts the physical structures that result from processing steps. The resulting physical structures are used as input by ATLAS, which then predicts the electrical characteristics associated with specified bias conditions. The combination of ATHENA and ATLAS makes it possible to determine the impact of process parameters on device characteristics.

The electrical characteristics predicted by ATLAS can be used as input by the UTMOST device characterization and SPICE modeling software. Compact models based on simulated device characteristics can then be supplied to circuit designers for preliminary circuit design. Combining ATHENA, ATLAS, UTMOST, and SMARTSPICE makes it possible to predict the impact of process parameters on circuit characteristics.

7.2. Physically based device simulator.

ATLAS is a physically-based device simulator. Physically-based device simulation is not a familiar concept for all engineers. This section will briefly describe this type of simulation.

Physically-based device simulators predict the electrical characteristics that are associated with specified physical structures and bias conditions. This is achieved by approximating the operation of a device onto a two or three dimensional grid, consisting of a number of grid points called nodes. By applying a set of differential equations, derived from Maxwells laws, onto this grid you can simulate the transport of carriers through a structure. This means that the electrical performance of a device can now be modelled in DC, AC or transient modes of operation.

The Maxwell Laws are:

1. Divergence of electric flux from the control volume = Charge Density in the control volume (Poisson’s Equation) or Divergence of electric flux = zero (Laplace’s Equation);
2. Divergence of electrons from a control volume = difference in generation rate and recombination rate of electrons in the control volume;
3. Divergence of holes from a control volume = difference in generation rate and recombination rate of holes in the control volume;
4. Total electron current density = drift electron flux density + diffusion electron flux density;
5. Total hole current density = drift hole flux density + diffusion hole flux density;

First three are coupled , non-linear second order partial differential equations and last two

are transport equations.

Solution of these five equations give the simulated predictions.

The intersection of the grid lines give the nodes. At node (i,j) there are three unknowns v _ij , n _ij and p _ij .Initial intelligent guess is made of v _ij , n _ij and p _ij . Using these values we solve the Poisson equations and the continuity equations for electron and holes until we get converging results.

There are two methods for solving for these three unknowns: Coupled Method and Uncoupled Method or Sequential Method.

Table 7.1. Comparative study of the Coupled and Uncoupled Methods.