Device simulation in a TCAD framework
The increasing importance of integrated TCAD frameworks creates the demand for simulation tools which are fit to work in such frameworks. Two new main design goals for a device simulator arise from this demand: The ability to work on automatically created input data, and the refrainment of any user interaction during the simulation. In this work, a new device simulation tool is presented which complies with these goals and covers the state of the art of device simulation by the solution of coupled differential equations.
A very general and flexible design of the new simulator allows the analysis of arbitrarily complex device topologies (provided that the models for the materials are present and the device structures can be gridded). This is necessary to analyze input created automatically by process simulation.
The partial differential equations are composed from elementary parts which are provided by different physical models, implemented in so-called model functions. The set of equations is easily extensible, and new physical models can be added without much effort. The standard set of equations for semiconductors is either a drift-diffusion set, consisting of the Poisson equation and the carrier continuity equations, or a hydrodynamic set, which additionally contains the carrier temperature equations. All equations have been extended to inhomogeneous band edge energy offset and density of states, to enable the simulation of compound semiconductor materials. Separate carrier equations can be used for different sorts of electrons or holes.
The core of the new simulation tool is an apt method of equation assembly. This method allows to implement boundary and interface conditions separately and independently from the differential equations. Arbitrary types of interface conditions can be specified in a numerically stable way to connect regions with different materials. Contact current integration is contained in the equation system, which allows to apply arbitrary linear combinations of contact voltage, contact current, and contact charge as boundary conditions.
Distinct devices can be connected, and external linear elements (resistances, capacitances, and inductances) may be used to perform coupled device simulation of small circuits. A simulation of a 9-stage ring oscillator in ultra low power CMOS technology is presented to show the benefits of coupled device simulation. From the wave forms of the transient signals, very accurate data about power dissipation and delay time are obtained.
The simulation of a HEMT (high electron mobility transistor) structure emphasizes the necessity of careful discretization of the interface conditions at the hetero junctions. The abrupt hetero junction requires consistent treatment of the carrier concentration and carrier temperature at the interface point, each of them being represented by two distinct values describing the two sides of the interface. In the typical operation regime, the carrier heating process in the channel shows to be the major limiting factor to the current transport, and the hydrodynamic model proves absolutely necessary.