Investigation of Gallium Nitride Based on Power Semiconductor Devices in Polarization Super Junction Technology

Over the last decade, gallium nitride (GaN) has emerged as an excellent material for
the next generation of power devices. GaN transistors, switching losses are very low,
thanks to the small parasitic capacitances and switching charges. Device scaling and
monolithic integration enable a high-frequency operation, with consequent
advantages in terms of miniaturization. For high power/high voltage operation, GaN�based Polarization Super-Junction (PSJ) architectures demonstrate great potential.
The aim of this thesis is devoted to the development of PSJ technology. Detailed
analysis of the on-state behaviour of the fabricated Ohmic Gate (OG) and Schottky
Gate (SG) PSJ HFETs is presented. Theoretical models for calculating the sheet
densities of 2DEG and 2DHG are proposed and calibrated with numerical simulations
and experimental results. To calculate the R (on, sp) of PSJ HFETs, two different gate
structures (Ohmic gate and Schottky gate) are considered herein.
The scaling tendency of power devices enables the emergence of multi-channel PSJ
concepts. Therefore, lateral and vertical multi-channel PSJ devices based on practical
implementation are also investigated. Presented calculated and simulated results
show that both lateral and vertical multi-channel PSJ technologies can be well suited
to break the unipolar one-dimensional material limits of GaN by orders of magnitude
and achieve an excellent trade-off between R (on, sp) and voltage blocking capability
provided composition and thickness control can be realised.
A novel multi-polarization channel is applied to realize normally-off and high�performance vertical GaN device devices for low voltage applications based on the
multi-channel PSJ and vertical MOSFET concepts. This structure is made with 2DHG
introduced to realize the enhancement mode channel instead of p-GaN as in
conventional vertical GaN MOSFETs. As the 2DHG depends upon growth conditions,
p-type doping activation issues can be overcome. The Mg-doped layer is only used to
reduce the short-channel effects, as the 2DHG layer is too thin. Two more 2DEG layers
P a g e | iv
are formed through AlGaN/GaN/AlGaN/GaN polarization structure, which
minimizes the on-state resistance. The calculation results show this novel vertical GaN
MOSFET – termed SV GaN FET - has the potential to break the GaN material limit in
the trade-off between R (on, sp) and breakdown voltage at low voltages. The
comprehensive set of development based on the PSJ concept gives a comprehensive
overview of next-generation power electronics.