Summary:

In a BMDO SBIR contract, Astralux, Inc. has developed a high-temperature heterojunction bipolar transistor (HBT) made from gallium nitride and silicon carbide, two wide bandgap materials that operate under higher temperatures than conventional materials. Astralux, along with Sterling Semiconductor and the University of Colorado at Boulder, are now working on an Air Force STTR Phase I research project to refine the transistor for fighter radars. Such transistors may also be used to control the power delivered to motors in future electric cars, eliminate the need for expensive and heavy cooling systems in space electronics, allow electric motors to replace hydraulic systems, and permit devices to be packaged more closely together in high-temperature digital circuits.

Technology Description:

In a BMDO SBIR contract, Astralux, Inc. (Boulder, CO), has developed the first transistor to operate above 500°C with a current gain over 100. (In comparison, the maximum operating temperature of conventional silicon transistors is 150°C.) At room temperature, this heterojunction bipolar transistor (HBT) had a current gain of over 10 million, and at 535°C, the highest temperature reached by Astralux's equipment, it had a gain of 100. The device was operated at a current density of 1,600 amps per cm2 and a power density of 30 kilowatts per cm2. Since this work was done, the SiC technology has achieved a breakdown voltage greater 5 kV and the HBT has operated at 2 kA/cm2 . Hence a power handling capability of 10 MW/cm2 should be possible.

To achieve high-temperature operation, Astralux built the HBT out of gallium nitride (GaN) and silicon carbide (SiC). Both of these materials have a wide bandgap, a property of semiconductors that allows operation at high temperatures and reduces leakage current at heterojunctions.

As in all bipolar transistors, the HBT consists of an emitter, where the input current enters the transistor; a collector, where the output current leaves the device; and a base, which controls current amplification. In this device, the collector and base are made of an SiC p-n junction and the emitter is made of n-type GaN. Because GaN has a wider bandgap than SiC, the GaN emitter prevents current from leaking in from the base (the difference in bandgap energies creates a barrier to current flow in the reverse direction). Lower leakage currents result in a higher emitter efficiency and a higher current gain. GaN is also an attractive emitter material because it has a close lattice match to SiC and a high thermal conductivity, though not as high as SiC.

The main barrier to mass production at present is obtaining production quantities of silicon carbide with sufficient purity. According to primary investigator Dr. Jacques Pankove, no company now produces the substance. However Astralux has teamed up with Sterling Semiconductor, Inc. (Sterling, VA), to produce a transistor that employs 4H-silicon carbide, a type of silicon carbide with a different atomic arrangement than the 6H-silicon carbide used in earlier HBT prototypes. To this end, Sterling has built a dedicated furnace to produce 4H-silicon carbide (Sterling was funded by BMDO in two SBIR Phase IIs to develop SiC growth processes). While the bandgap ratio of GaN/4H-SiC HBT is not as favorable as in GaN/6H-SiC, they did produce workable results: this model exhibited a current gain of 15 at room temperature and 3 at 300°C.

MDA Origins:

Electronics capable of operating at high temperatures could provide significant cost and weight savings for missile defense control systems. As a result, the research to demonstrate the high-temperature amplifier was funded by a Phase II BMDO SBIR contract.

Spinoff Applications:

Astralux's HBTs will permit electronics that monitor and control the performance of systems to operate at high temperatures, making them useful in automotive, aviation, and utility applications. More specifically, they may be used to control the power delivered to motors in future electric cars, eliminate the need for expensive and heavy cooling systems in space electronics, allow electric motors to replace hydraulic systems, and permit devices to be packaged more closely together in high-temperature digital circuits. Another commercial application that would benefit from HBTs would be microwave ovens. Combining the transistor with a microwave source would eliminate the need for the bulkier magnetrons now used.

Commercialization:

At present, Astralux is looking for a strategic partner that can help market the devices. The company will also consider licensing devices for specific applications. Astralux, Sterling, and the University of Colorado at Boulder are now working on an Air Force STTR Phase I research project to refine the transistor for high frequency (8-12 Ghz) transmitters to be placed in fighter radars.

Company Profile:

Astralux is based in Boulder, CO, and is run by Dr. Jacques Pankove. It has received multiple BMDO SBIR awards for work in nanostructures, high temperature optoelectronics, and electronics.

Contact Information:

Dr. Jacques Pankove
Astralux, Inc.
2500 Central Avenue
Boulder CO 80301-2845
Tel:303-413-1440 (business) or 303-492-5470 (university) or 303-494-0670 (home)
Fax:303-413-1465 (business) or 303-492-2758 (university) or 303-494-7414 (home)