Silicon Carbide Bipolar Junction Transistors for High Temperature Sensing Applications


An integrated sensing module capable of operating at high temperatures would be beneficial to a number of industrial applications, such as automotive industries, aerospace systems, industrial turbines and deep-well drilling telemetric systems. Consider industrial turbines as an example. It is important to monitor a variety of physical parameters within the hot sections of the turbines in order to increase turbine efficiency, reliability and to reduce pollution. In addition, real-time monitoring can help to detect and predict the failures of critical components in a timely fashion to reduce the maintenance costs of the systems. A high temperature integrated circuit is an important part of such systems, because it provides power management function for the energy scavenger, builds the electrical interface with MEMS-based harsh environment sensors, and amplifies the sensing signals. Therefore, it is essential to have transistors, the building blocks of integrated circuits, which can operate at high temperatures.

Silicon carbide (SiC) is a promising semiconductor for high temperature applications due to its excellent electrical and physical properties. The wide bandgap energy (3.2 eV for 4H-SiC) and low intrinsic carrier concentration allow SiC semiconductor devices to function at much higher temperatures. Moreover, high breakdown electric field (3-5 MV/cm), high-saturated electron velocity (2×10cm/s) combined with high thermal conductivity (3-5 W/cm·°C) enable SiC devices to work under extreme conditions. There are growing interests on developing high temperature integrated circuits using SiC bipolar junction transistors (BJTs) because SiC BJTs are not as strongly affected by oxide quality as SiC metal-oxide-semiconductor field effect transistors (MOSFETs). In addition, SiC BJTs are normally-off devices and have higher transconductance compared with SiC junction field effect transistors (JFETs).

This dissertation presents comprehensive analytical and experimental results on 4H-SiC NPN BJTs capable of operating at high temperatures up to 400 °C. Comprehensive characterization including current gain, early voltage, output resistance, and intrinsic voltage gain was performed. At room temperature, the device has a current gain of 14.5 and an intrinsic voltage gain of 3300. At elevated temperatures, the intrinsic voltage gain increases to 5900 at 400 °C, although the current gain of the device is reduced to 6.7. 

This suggests that 4H-SiC BJT has the potential to be used as a voltage amplifier at extremely high temperatures. High temperature effects of 4H-SiC are theoretically studied. The incomplete ionization effect and the temperature dependence of minority carrier lifetime are the two main competing mechanisms for the change of device performance with rising temperature. To further enhance the current gain, fabrication process can be improved for reduction of interface traps residing between SiC and the passivation oxide layer.

This dissertation also presents the design, fabrication and characterization of a high- performance temperature sensor based on 4H-SiC pn diode. The device shows stable operation from room temperature up to 600 °C. Under forward bias condition, the temperature sensitivity of the sensor changes from 2.3 mV/°C at a forward current density of 0.44 A/cm2, to 3.5 mV/°C at a forward current of 0.44 mA/cm2. Higher sensitivity can be achieved at a lower forward current level. The experimental results indicate a good agreement with theoretical analysis. These results show that the device has the potential to be integrated with supporting circuitries to build a sensing module for high temperature applications.

Publication date: 
May 31, 2014
Publication type: 
Ph.D. Dissertation
Zhang, N. (2014). Silicon Carbide Bipolar Junction Transistors for High Temperature Sensing Applications. United States: University of California, Berkeley.

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