Power-switching applications beyond silicon: Status and future prospects of SiC and GaN devices
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Key power-switch requirements and the limitations of silicon devices The generation, transmission, and use of electric power require different forms of power conversion. The following are illustrative examples: The AC main power must be converted into DC power to run consumer electronics many electronic circuits require DC-to-DC conversion from one voltage level to another, and DC power from solar cells needs to be converted into AC power to be connected to electrical appliances or to the power grid. The commonly used electronic circuits for power conversion utilize switching techniques and, consequently, are referred to as switch-mode power electronic circuits. Two types of semiconductor-based switches are used in these circuits: two-terminal rectifiers (diodes) and three-terminal controlled switches (transistors). Three key power-switch requirements are common for both diodes and transistors: (1) high-blocking voltage, (2) high-power efficiency, and (3) high-switching speed. The fourth key requirement is specific to the controlled switches and relates to a strong preference for normally OFF switches—switches that are OFF when no controlled voltage is applied and are turned ON by a positive controlled voltage.
High-blocking voltage The blocking voltage is the voltage that can be applied across a switch in OFF mode, which is the reverse-bias voltage that can be applied either across a diode or a transistor. The maximum achievable blocking voltage corresponds to the breakdown voltage of a planar structure (no corners) and to the lowest possible doping concentration of the semiconductor region supporting the electric field due to the reverse bias. In the case of Si and SiC, this region is n-type doped and referred to as the drift region. For a planar structure, the relationship between the breakdown voltage and the critical field that leads to breakdown can be obtained by solving the one-dimensional Poisson equation:1 VB =
εs Ecr 2 2qN D
(1)
In Equation 1, VB is the breakdown voltage, Ecr is the critical or breakdown electric field, ND is the n-type doping level, εs is the semiconductor permittivity, and q is the value of electron charge. The critical field of Si, which is about Ecr = 3 × 105 V/cm (30 V/µm), is the limiting factor for many applications.
Sima Dimitrijev, Queensland Micro- and Nanotechnology Centre, Griffith University, Australia; s.dimitrijev@griffith.edu.au Jisheng Han, Queensland Micro- and Nanotechnology Centre, Griffith University, Australia; j.han@griffith.edu.au Hamid Amini Moghadam, Griffith School of Engineering and Queensland Micro- and Nanotechnology Centre, Griffith University, Australia; hamid.aminimoghadam@griffithuni.edu.au Amirhossein Aminbeidokhti, Griffith School of Engineering and Queensland Micro- and Nanotechnology Centre, Griffith University, Australia; amirhossein.aminbeidokhti@griffithuni.edu.au DOI: 10.1557/mrs.2015.89
© 2015 Materials Research Society
MRS BULLETIN • VOLUME 40 • MAY 2015 • www.mrs.org/bulletin
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POWER-SWITCHING APPLICATIONS BEYOND SILICON
Assuming the lowest possib
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