Congratulations on your graduation, Doctor Arash Salemi!

Publicerad 2017-03-23

The topic of Doctor Arash Salemis PhD dissertation is "Silicon Carbide Technology for High- and Ultra-High-Voltage Bipolar Junction Transistors and PiN Diodes"

Where are you from and where did you study before coming to KTH?

I am from Iran, where I got my B.Sc. and M.Sc. in Physics. I came to KTH in 2009 as a student and became a Ph.D. candidate in Micro- and Nanoelectronics in 2011.

What is your topic and why did you choose it?

Climate change is today considered as one of the most important global issues. It is disrupting national economies and costing peoples lives in every country on every continent. In order to control the global warming trend, renewable energies are being considered as good candidates. Moreover, high-speed rail, hybrid cars, and electrical cars are expanding rapidly in the whole world. All these new applications need power electronic systems, and power semiconductor devices are recognized as a key component. It is estimated that about 50% of the electricity used in the world is controlled by these devices.

In order to achieve a more energy-efficient utilization of electric power, there is an urgent need for rectifying and switching devices with lower losses than silicon (Si) devices we are employing today. Silicon carbide (SiC), with its superior properties such as wide bandgap, high critical electric and high-temperature conductivity, is an interesting semiconductor material for high-voltage and high-temperature applications. 4H-SiC 1.7 kV-class unipolar devices are now commercially available. However, ultra-high-voltage (+10 kV) devices with much low-losses are needed in future electric power networks. Bipolar devices such as PiN diodes and bipolar junction transistors (BJTs) have the advantage of reduced ON-resistance by conductivity modulation which makes them a good candidate for future electric power networks.

Describe your topic in short.

Wide bandgap semiconductors such as SiC, gallium nitride (GaN), and diamond, permit devices to operate at much higher voltages, temperatures, and frequencies than traditional semiconductors like silicon and gallium arsenide (GaAs). GaN and diamond are candidates for future power semiconductor materials. Among different SiC polytypes, 4H-SiC exhibits a wider bandgap and higher electron mobility along the c-axis. As a result, 4H-SiC has been exclusively employed for power device applications. The n- and p- type 4H-SiC substrates up to 150 mm are now commercially available. In this thesis, design, fabrication, and characterization of high- and ultra-high voltage, 4H-SiC PiN diodes and BJTs were demonstrated.

Tell me something about your results.

In order to improve the breakdown voltage, two different terminations were utilized: an efficient and area optimized implantation-free junction termination extension (O-JTE), and multiple-shallow-trench junction termination extension (ST-JTE). They resulted in high termination efficiency of 92% and 93%, respectively. The simulation results also showed that utilizing this termination results in a uniform electric field distribution and a lower electric field peak at the edges. No breakdown voltage was recorded up to 10 kV (our measurement system limit at present) with a low leakage current of 0.2 µA for 10+ kV PiN diodes which is the highest blocking capability for 4H-SiC devices using on-axis to date.

In order to improve the current gain, all geometrical parameters were optimized. The sacrificial oxidation and surface passivation layer were also optimized in order to reduce the surface defects caused by the etching steps and minimize the interface charges at the SiO2/SiC interface, respectively. This comprehensive study resulted in a high record current gain of 139 for 15 kV-class BJTs. New cell geometries showed a lower ON-resistance and higher current density at a given current gain due to better utilization of the base area. Hexagon cell geometry has about 42% higher current density and about 21% lower ON-resistance than the interdigitated fingers. It was also observed that the square cell geometry has about 21% higher current density and about 23% lower ON-resistance than the interdigitated fingers.

In order to investigate any bipolar degradation, forward bias stress test was carried out at 100 A/cm2 for 100 hours and 200 A/cm2 for 10 hours. The results showed that the 10+ kV PiN diodes using On-axis 4H-SiC are degradation-free.

What will the future bring for your research topic?

SiC technology is in progress for high-voltage and high-temperature applications and will have a brilliant future. Today, all new results in this field are being used in several international companies in different countries such as USA, Japan, Sweden, South Korea, and China.

What are your future plans?

At the first step, I will join to the School of Electrical and Computer Engineering at Purdue University in the US as Post-Doctoral Research Assistant. And then, who knows?

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