Power MOSFETs based on superjunction technology have become the industry norm in high-voltage switching converters. They offer lower R DS(on) simultaneously with reduced gate and output charges, which allows for more efficient switching at any given frequency.
Prior to the availability of superjunction MOSFETs the dominant design platform for
high-voltage devices was based on planar technology. However, fast switching at high
voltages poses its own challenges in AC/DC power supplies and inverters. Designers
making the transition from planar to superjunction MOSFETs often have to accommodate EMI, voltage spikes, and noise-related concerns by compromising switching speed. Below we will compare the characteristics of the two platforms so that the benefits of superjunction technology are fully understood and utilized.
In order to understand the differences between the two technologies, we need to start
with the basics. Figure 1a shows the simple structure of a conventional planar high-
voltage MOSFET. Planar MOSFETs typically have a high drain-to-source resistance per unit of silicon area, and come with relatively higher drain source resistances. Lower R DS(on) values could be achieved with high cell density and large die sizes. However, large cell densities and die sizes also come with high gate and output charges, which increase the switching losses as well as costs. There is also a limit to how low the total silicon resistance can go. The total R DS(on) for the device can be expressed as the sum of three components: the channel, epi, and the substrate.
RDS(on) = R ch + R epi + R sub
Figure 1a: Conventional Planar MOSFET Structure
Figure 1b: Resistive Components of a Planar MOSFET
Figure 1b shows a breakdown of different components that make up the R DS(on) in a
planar MOSFET. For low-voltage MOSFETs the three components are comparable. However, as the voltage rating is increased, the epitaxial layer needs to be thicker and more lightly doped to block high voltages. For every doubling of