1.1 Need for TCAD in electronics

Technology computer-aided design (TCAD) is a field of electronic design automation (EDA) activity with software, which encompasses the physics-based simulation of semiconductor devices and the associated manufacturing processes, referred to as device TCAD and process TCAD, respectively. Process TCAD simulations predict the structure and material properties resulting from a series of manufacturing steps, like etching, ion implantation and annealing. Device TCAD receives the structure and material composition of a device (provided by process TCAD and/or experimental measurements) as an input and predicts the electrical, optical, thermal and/or mechanical behaviour of the device.

The value of TCAD is two-fold: i) it serves as a predictive tool – as evidenced by its use in the international technology roadmap for semiconductors (ITRS) [1] – to verify concepts and ascertain the effects of changes in the structure and/or process on the performance of a device before manufacturing. This allows the design space, which needs to be explored with experimentally manufactured devices, to be greatly constrained, thereby saving both time and cost. ii) TCAD also aides in the understanding of process and device physics; insight into microscopic physical quantities that cannot be measured or visualized experimentally is provided.

TCAD is increasingly centred around the manufacturing process, where a concurrent optimization of the material system, manufacturing process and device design takes place [2]. Device TCAD for ultra-scaled devices has become very complex, incorporating a multitude of models which describe the electrical, optical and thermal processes at play, operating over different scales [3]. Indeed, there are very few devices that can be modelled completely using a single tool [4]. Therefore, the concurrent consideration of many physical effects – so-called multiscale, multiphysics simulations – and the robust coupling of the various tools and models is a major thrust in (especially commercial) TCAD development. On the other hand, the advancement of models to appropriately describe the physics presents another driver for TCAD development. The charge carrier transport models are fundamental to predicting the electrical performance of devices with TCAD simulation. Quantum transport models are already indispensable to appropriately model certain modern devices and will further gain in relevance.

Only a few devices have exploited quantum mechanical principles up to now, e.g. resonant tunnelling diodes (RTD), whereas other devices merely consider quantum effects to ensure the desired behaviour is retained. With end of downscaling in devices looming, new avenues must be explored to design novel devices for the beyond CMOS era. Quantum considerations will be fundamental in the design of nanoscale structures that will make truly ubiquitous computing and power-efficient sensor networks a reality [5]. Prototypical nano-circuits, which are formed by simple nanostructures, exhibit electrical behaviour that cannot be explained by classical theory. Time-resolved simulations of quantum transport will help to resolve the questions that surround the frequency response of quantum capacitors and resistances.

In the following a brief overview is given of the most common transport models in use for the simulation of existing and novel devices.