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Wide Bandgap Semiconductor Technology Initiative (WBGS-HPE)
There exists a critical need for solid state switching devices and integrated circuits that can meet the high-current, high-voltage, and speed requirements of electric components and sub-systems in emerging military applications. The High Power Electronics (HPE) program is investigating novel technical approaches to enable high-power solid-state electronics, with the main focus being on power electronics technologies for 10 kV class devices. Applications include power distribution and electro-magnetics weapons in future hybrid-electric combat vehicles, naval ship propulsion, and electric aircraft (Figure 1-Technology Applications).
The leading candidate semiconductor material for 10 kV class high power devices and circuits is SiC in the 4H polytype. SiC has a unique combination of a high critical electrical breakdown field, good majority carrier transport, long minority carrier lifetimes due to its indirect bandgap, and high thermal conductivity. These attributes combine to give SiC the potential to significantly exceed the current-carrying density, temperature and voltage-blocking capabilities of existing silicon power semiconductor devices (Figure 2- SiC Capabilities). 4H SiC, one of over 175 crystal structures that SiC can form, is preferred for power devices due to it having the highest critical field and mobility of all the polytypes.
Research in the HPE program has been concentrated in the following technical areas: (1) 4H SiC semiconductor materials and processes, (2) high-power device structures, and (3) high power integrated circuit technology. In Phase I, the program focused on developing 4H SiC conducting substrates, thick, lightly-doped epitaxial technology, and device processes. Low defect density bulk and epitaxial growth techniques of 4H silicon carbide semiconductors are required for the successful development of the new structures and, ultimately, to make high-power devices. (See Table1 for Phase I milestones and accomplishments.) Phase I of WBG-HPE has completed and demonstrated commercial "high quality" three-inch n-type 4H-SiC wafers as shown in Figure 2a(Three inch n-type 4H-SiC wafers). In addition, the growth of high quality thick epitaxial layers was demonstrated (Figure 2b-Growth of thick epitaxial layers). Furthermore, phase I efforts has also enabled SiC devices to achieved remarkable advances in PiN diodes and MOSFETs. 9kV SiC PiN diodes with forward voltage drops <3.8V, and excellent low on-state resistance of 0.123 _-cm2 for a blocking voltage of 10 kV MOSFETs was fabricated.
| Metrics | Program Start | Phase I Go / No Go |
Program Goal | Today |
|---|---|---|---|---|
| SiC micropipe density (per cm2) | 15 MP / cm2 | 1.0 | 0.2 | 0.19 |
| Epi SiC thickness and doping uniformity | N/A | Ft > 1GHz, 30V | Ft > 5GHz, 50V | Ft > 10GHz, 50V |
| Temperature of operation | N/A | 100 µm±5%, doping±5% | 150 µm±5%, doping<5% | 100 µm±1.6%, doping<5% |
| Total electrically active defects (per cm2) | N/A | ≤ 1.5 | ≤ 0.5 | 1.4 |
| MOSFET on-state performace (_-c,m2) (10kV blocking) | N/A | 0.3 | 0.1 | <0.25 |
| Bipolar on-state performance (Vr) (10kV blocking, 100A/cm2) | >6 | 5.0 | 3.5 | 3.8 |
For the ongoing phase II program, the goal is to fabricate 10-20kV, 100A/cm2 power devices using the higher quality materials enabled during phase I. More specifically, 10kV PiN diodes with Vf = 4V with a die yield of > 30%, 10kV MOSFETs with on-state resistance of = 0.25 _-cm2 and die yields of = 30%, and 10-20kV P- and N- channel IGBTs with Vf = 5V and die yield of = 50%, all at junction temperatures of 200oC will be developed. Additionally, since a complete power system requires packaging and assembly of discrete and multi-chip switches, diodes, and modules, Phase II effort will also investigate approaches for packaging and assembly for aforementioned devices. Table 2 summarizes the milestones and program goals for phase II devices and modules.
The ultimate focus of the program at Phase III is to build a 2.7MVA solid-state transformer stepping down 13.8kV to 4160Vac at 20kHz. A Memorandum of Agreement was signed on November of 2004 between DARPA and the Navy. Under this collaboration reliable, cost-effective SiC devices and power modules will be developed leading to a laboratory demonstration of SSPS. The US Navy will subsequently take over the direction of the program optimizing the SSPS for integration into an aircraft carrier. A savings of > 50% in volume and weight can be observed by replacing an existing 2.7MVA, 60Hz transformer with a solid-state converter. Figure 4 (Conventional Transformer) depicts a conventional transformer and its circuit and illustrates one circuit design concept of a 20kHz SSPS converter integrating SiC devices. Improved power quality factor, digital control, reconfigurability, and superior control of power sag, flicker, and harmonics are additional features