Abstract

WIDE BANDGAP semiconductor, particularly Silicon Carbide (SiC), based electronic devices and circuits are presently being developed for use in high-temperature, high-power, and high-radiation conditions under which conventional semiconductors cannot adequately perform. Silicon carbide's ability to function under extreme conditions is expected to enable significant improvements to a far-ranging variety of applications and systems. These range from greatly improved high-voltage switching for energy savings in public electric power distribution and electric motor drives to powerful microwave electronics for radar and communication applications, and to sensors and controls for cleaner-burning more fuel-efficient jet aircraft and automobile engines. Aside from tremendous theoretical advantages yet to be realized in SiC devices, the need for numerical simulation based on accurate models for the design and optimization of these devices is indispensable to the further success of modern power electronics.

First the technology of SiC semiconductor devices is briefly outlined. The 3C-SiC, 4H-SiC, 6H-SiC, and 15R-SiC are the most common polytypes presently being developed for device application. These polytypes are characterized by the stacking sequence of the biatom layers of the SiC structure. Changing of the stacking sequence has a profound effect on the electrical properties. Since the hexagonal polytypes are composed of stacked double layers, several electrical properties are different parallel to the c-axis or perpendicular to the c-axis. This is called anisotropy, and the degree of anisotropy is measured by the quotient of a parameter value parallel and perpendicular to the c-axis.

Currently only the 4H- and 6H-SiC polytypes are available commercially as substrate material. Key crystal growth and device fabrication issues that presently limit the performance and capability of high-temperature, high-power and high-frequency 4H- and 6H-SiC devices are identified. The differences between SiC device technology and well-known silicon VLSI technology are discussed. Projected performance figure of merits of SiC devices are highlighted for several large-scale applications.

A comprehensive and systematic model development based on the recent research findings and published data was performed. Due to the anisotropic nature of the SiC crystal structure, the mobility $\mu$, the dielectric permittivity $\epsilon$, and the conductivity $\kappa$ are tensors along the crystallographic axes of the semiconductor lattice. These tensors are diagonal with only two independent components parallel and perpendicular to the c-axis, respectively. A tensorial formulation of Poisson's equation and the current equations are adapted to make it feasible for use in the general-purpose device simulator MINIMOS-NT applying the same discretization scheme as in the case of conventional current transport equations.

The most common doping impurities in 4H- and 6H-SiC have activation energies larger than the thermal energy ${\mathrm{k_B}}T$ even at room temperature. Inequivalent sites of SiC, one with cubic surrounding and the other with hexagonal surrounding, cause site-dependent impurity levels. Therefore, an appropriate incomplete ionization model which accounts for ionization level dependence on temperature, polytype, and lattice sites is implemented. A variety of other SiC-specific models, including band structure and bandgap narrowing; Shockley-Read-Hall and Auger recombination, temperature- and field-dependent impact ionization; and mobility dependencies on impurity concentration, lattice temperature, carrier concentration, carrier energy, parallel and perpendicular electric fields are few among the many models implemented.

The models are tested on state-of-the-art SiC rectifiers, switches, and RF transistors. Three classes of SiC rectifiers were investigated. The Schottky barrier diodes which offer extremely high switching speed, but suffer from high leakage current; the PiN diodes which offer low leakage current, but show reverse recovery charge during switching and have a large junction forward voltage drop due to the wide band gap of SiC; and the merged PiN Schottky diodes which offer Schottky-like on-state and switching characteristics, and PiN-like off-state characteristics.

Three types of unipolar transistors are simulated. UMOSFET devices which were the first unipolar transistors realized in SiC have shown a good on- and off-state characteristics, but suffered from problems including lower inversion layer mobility and high electric field crowding at its trench corners. The DMOSFET structure formed by using a double ion implantation has avoided the trench problems occurred in UMOSFET, but still has low inversion layer mobility. An ACCUFET structure is proposed by incorporating an n-type counter-doped layer along the oxide/semiconductor interface to restore the low inversion layer mobility observed at both UMOSFET and DMOSFET.

Finally the implemented models are tested on RF transistors. A MESFET fabricated from 4H-SiC was investigated for both DC and high frequency characteristics. Excellent agreement between the simulated and measured data were obtained. These results clearly demonstrate the advantages of SiC for high-power microwave applications.

T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation