Papers by Author: Robert S. Okojie

Abstract: We report on the initial demonstration of a tungsten-nickel (75:25 at. %) ohmic contact to silicon carbide (SiC) that performed for up to fifteen hours of heat treatment in argon at 1000 °C. The transfer length method (TLM) test structure was used to evaluate the contacts. Samples showed consistent ohmic behavior with specific contact resistance values averaging 5 x 10-4 Ω-cm². The development of this contact metallization should allow silicon carbide devices to operate more reliably at the present maximum operating temperature of 600 °C while potentially extending operations to 1000 °C.

Abstract: This paper updates the long-term 500 °C electrical testing results from 6H-SiC junction field effect transistors (JFETs) and small integrated circuits that were introduced at ICSCRM-2007. Two packaged JFETs have now been operated in excess of 7000 hours at 500 °C with less than 10% degradation in linear I-V characteristics. Several simple digital and analog demonstration integrated circuits successfully operated for 2000-6500 hours at 500 °C before failure.

Abstract: This paper reports on the fabrication and testing of 6H-SiC junction field effect transistors (JFETs) and a simple differential amplifier integrated circuit that have demonstrated 2000 hours of electrical operation at 500 °C without degradation. The high-temperature ohmic contacts, dielectric passivation, and packaging technology that enabled such 500 °C durability are briefly described. Key JFET parameters of threshold voltage, on-state resistance, transconductance, and on-state current, as well as the gain of the differential amplifier integrated circuit, exhibited less than 7% change over the first 2000 hours of 500 °C operational testing.

Abstract: While there have been numerous reports of short-term transistor operation at 500 °C or above, these devices have previously not demonstrated sufficient long-term operational durability at 500 °C to be considered viable for most envisioned applications. This paper reports the development of SiC field effect transistors capable of long-term electrical operation at 500 °C. A 6H-SiC MESFET was packaged and subjected to continuous electrical operation while residing in a 500 °C oven in oxidizing air atmosphere for over 2400 hours. The transistor gain, saturation current (IDSS), and on-resistance (RDS) changed by less than 20% from initial values throughout the duration of the biased 500 °C test. Another high-temperature packaged 6H-SiC MESFET was employed to form a simple one-stage high-temperature low-frequency voltage amplifier. This single-stage common-source amplifier demonstrated stable continuous electrical operation (negligible changes to gain and operating biases) for over 600 hours while residing in a 500 °C air ambient oven. In both cases, increased leakage from annealing of the Schottky gate-to-channel diode was the dominant transistor degradation mechanism that limited the duration of 500 °C electrical operation.