Materials Science Forum Vols. 527-529

Abstract: We evaluate the performance capabilities and limitations of high voltage 4H-SiC based Bipolar Junction Transistors (BJTs). Experimental forward characteristics of a 4kV BJT are studied and simulations are employed to determine the factors behind the higher than expected specific onresistance (Ron,sp) for the device. The impact of material (minority carrier lifetimes), processing (surface recombination velocity) and design (p contact spacing from the emitter mesa) parameters on the forward active performance of this device are discussed and ways to lower Ron,sp, below the unipolar level, and increase the gain (β) are examined.

Abstract: The performance prospects for 4H-SiC Bipolar Junction Transistors (BJTs) and Insulated Gate Bipolar Transistors (IGBTs) are theoretically evaluated. The total power dissipated (Ptotal) for both devices is calculated as a function of lifetime in the drift region and blocking voltage and used as a figure of merit to compare and contrast the effectiveness of different semiconductor materials for bipolar device applications. Assuming a maximum of 300W/cm2 for the total permissible power dissipation due to heat sink constraints we estimate an upper limit of 5kV for SiC BJT operation.

Abstract: This paper summarizes the recent demonstration of 3200 V, 10 A BJT devices with a high common emitter current gain of 44 in the linear region, and a specific on-resistance of 8.1 mΩ- cm2 (10 A at 0.90 V with a base current of 350 mA and an active area of 0.09 cm2). The onresistance increases to 40 mΩ-cm2 at 350°C, while the DC current gain decreases to 30. A sharp avalanche behavior was observed with a leakage current of 10 μA at a collector voltage of 3.2 kV.

Abstract: For 1-kV, 30-A 4H-SiC epitaxial emitter npn bipolar junction transistors, the dependence of the common-emitter current gain β on the collector current IC were measured at elevated temperatures. The collector-emitter voltage was fixed (at 100 V voltage) to provide an active operation mode at all collector currents varying in a wide range from 150 mA to 40 A (current densities 24 - 6350 A/cm2). The maximum room temperature current gain was measured to be βmax = 40 (IC = 7 A) while βmax = 32 (IC = 10 A) at 250oC. The β-IC dependences were simulated using a model which takes into account the main processes affecting the current gain. Minority carrier lifetimes and surface recombination velocity were obtained by means of those considerations.

Abstract: This paper reports on a 400 watt boost converter using a SiC BJT and a SiC MOSFET as the switch and a 6 Amp and a 50 Amp SiC Schottky diode as the output rectifier. The converter was operated at 100 kHz with an input voltage of 200 volts DC and an output voltage of 400 volts DC. The efficiency was tested with an output loaded from 50 watts to 400 watts at baseplate temperatures of 25°C, 100°C, 150°C and 200°C. The results show the converter in all cases capable of operating at temperatures beyond the range possible with silicon power devices. While the converter efficiency was excellent in all cases, the SiC MOSFET and 6 Amp Schottky diode had the highest efficiency. Since the losses in a boost converter are dominated by the switching losses and the switching losses of the SiC devices are unaffected by temperature, the efficiency of the converter was effectively unchanged as a function of temperature.

Abstract: We compare the on-state characteristics of five 4H-SiC power devices designed to block 20 kV. At such a high blocking voltage, the on-state current density depends heavily on the degree of conductivity modulation in the drift region, making the IGBT and thyristor attractive devices for high blocking voltages.

Abstract: We demonstrate a novel power Si/4H-SiC heterojunction tunneling transistor (HETT) on the basis of theoretical analysis and experimental results. The HETT is an insulated gate drive device and has a unique switching mechanism. In the off-state, the heterojunction barrier prevents current flow between the Si source region and the 4H-SiC drift region. In the on-state, the width of the heterojunction barrier is controlled by the gate bias to allow tunneling current to flow. The HETT has a zero channel length structure that is more independent of channel mobility compared with a conventional 4H-SiC MOSFET. As a result, the HETT is expected to have low on-resistance. A HETT was fabricated with n+-type polycrystalline silicon on an n--type 4H-SiC epitaxial wafer for power devices. The fabricated HETT shows a low specific on-resistance of 6.8 m
cm2 (at Jd=500 A/cm2).

Abstract: Depletion-mode 4H-SiC FETs were fabricated for use as harsh environment gas sensors. To enable sensitivity to NOx, O2 and H2 gases, metal oxide catalysts such as InOx were integrated into the gate of the device. The FETs had a total area of approximately 1 mm2. Devices with various gate widths and lengths were fabricated and tested, with sensor performance of 5% or greater in current change from the baseline resulting from designs having a length to width ratio of around 50.

Abstract: A variety of silicon carbide (SiC) detectors have been developed to study their sensitivity, including Schottky photodiodes, p-i-n photodiodes, avalanche photodiodes (APDs), and single photon-counting APDs. Due to the very wide bandgap and thus extremely low leakage current, SiC photo-detectors show excellent sensitivity. The specific detectivity, D*, of SiC photodiodes are many orders of magnitude higher than the D* of other solid state detectors, and for the first time, comparable to that of photomultiplier tubes (PMTs). SiC APDs have also been fabricated to pursue the ultimate sensitivity. By operating the SiC APDs at a linear mode gain over 106, single photoncounting avalanche photodiodes (SPADs) in UV have been demonstrated.

Abstract: We have tested the radiation detection performance of Silicon Carbide (SiC) PIN diodes originally developed as high power diodes. These devices consist of 100 micron thick SiC grown epitaxially on SiC substrates. The size and thickness of the devices make them appropriate for a number of radiation detection applications. We tested 0.25 cm2 and 0.5 cm2 devices and obtained X-ray spectra under illumination with an Am-241 radioactive source. The spectra showed an energy resolution that was consistent with the resolution expected for the large capacitance of the device. Smaller devices with a diameter of 1 mm were also tested and produced spectra with a room temperature energy resolution of ~550 eV, which is consistent with the electronics limit for the capacitance of the small device. We measured the absolute charge generated by X-rays per KeV in SiC by comparing the charge generation with similar silicon devices and determined the energy required per electron hole pair in SiC to be 8.4 eV. We also performed radiation damage tests on these devices and found no significant loss in charge collection up to a photon dose of 100 MRad. Applications for these devices can be found in the fields of particle physics, nuclear physics, nuclear medicine, X-ray fluorescence, X-ray astronomy and X-ray navigation.