## 8 Challenges and Research Needs of Matrix Converters  
As briefly approached on this work, developments on semiconductor technologies will have significant impact on the successful industrial penetration of matrix converter-based solutions for the promising future applications described throughout this work, to tackle the MC main challenges of higher efficiency and higher voltage operation capability. As a converter mainly composed by semiconductor switches, it is expected that matrix converters will benefit the most from breakthroughs in semiconductor technologies. One such breakthrough is the wideband-gap device which is now widely available on the power semiconductor market. Silicon carbide (SiC) MOSFETs/IGBTs and gallium nitride (GaN) JFETs are two semiconductor technologies that enable the use of matrix converters with higher efficiencies, higher withstand voltages while improving power density. Bidirectional JFETs [155], as depicted in Figure 21, could be the ultimate semiconductor switch for matrix converters since they provide a four-quadrant controlled switch using only one channel and a dual gate drive. This effectively addresses the problem of having two semiconductor devices (transistors or diodes) in series with the current path, resulting in reduced conduction losses. A matrix converter with 98% efficiency using GaN Bi-directional switches has been demonstrated in [156] where the characteristics of this switch are compared to the typical IGBT + Diode-based switch. While GaN JFETs are candidates to improve Matrix converter efficiency, new compound semiconductor materials such as Ga\({}_{2}\)O\({}_{3}\) may strongly increase the high voltage capability of matrix converters. Gallium oxide bandgap is nearly 5 electron volts, therefore larger then GaN (3.4 eV) and well above silicon (1.1 eV). Thus, while the silicon critical field strength is typically hundreds of kV/cm, and about 3.3 MV/cm in GaN, in Ga\({}_{2}\)O\({}_{3}\) it now attains 5.3 MV/cm and may reach 8 MV/cm (800 V/\(\upmu\)m) [157]. Gallium oxide may achieve high operating voltages using just a few \(\upmu\)m of Ga\({}_{2}\)O\({}_{3}\) (7 kV would require 10 \(\upmu\)m, while a comparable Si LGBT needs above 100 \(\upmu\)m). Although semiconductor materials such as diamond and aluminium nitride present even larger bandgaps, unfortunately they do not allow ion-implantation of dopants during epitaxial growth like Ga\({}_{2}\)O\({}_{3}\). This feature of Ga\({}_{2}\)O\({}_{3}\) allows the use of well established commercial lithography and processing techniques in use for silicon devices. Moreover, it is possible to make large wafers (150 mm) of crystalline Ga\({}_{2}\)O\({}_{3}\), and the oxide thermal stability allows its production using high-volume commercial techniques developed to make silicon wafers, such as edge-defined, or film-fed crystal growth [157]. Recently, Ga\({}_{2}\)O\({}_{3}\) JFETs with operation like the high-electron mobility transistor (HEMT) were developed showing on-state resistances similar to GaN HEMTs. The main weaknesses of Ga\({}_{2}\)O\({}_{3}\) are the low thermal conductivity, one-tenth of SiC and one-fifth that of Silicon, and the lack of p-type doped semiconductors. While the lack of p-types is not a problem for power matrix converters, the low thermal conductivity may be resolved by using high thermal conductivity dielectric fillers to shunt heat from the contact electrodes and the top \(\upmu\)m of the material (where most of the heat is generated) to a suitable heat sink. Compared to a 1200 V, 3 kW Silicon matrix converter switching at 20 kHz, a Silicon Carbide matrix converter switching at 150 kHz, and operating at higher temperature can reduce the packaging volume to one-third of the silicon size, while it may be expected that  
Figure 21: Bi-directional gallium nitride (GaN) high-electron mobility transistor (HEMT) junction field effect transistors (JFET): (**a**) structure, (**b**) symbol voltages and currents (adapted from [155]).