To make cellular phones smaller, electrical engineers integrated coils on a chip. However, the constant, high frequency changes in the electromagnetic field in the coils cause unwanted electric currents on the silicon parts of the chip.
Dr. Behzad Rejaei, from Iran, helped to minimise those side effects with his new computer model.
“Engineers looked at spiral inductors, coils that are used to build up an electromagnetic field, as simple devices. However, because they have side effects when integrated on a silicon chip, it’s necessary to study them in more detail than was previously required,” says Dr. Behzad Rejaei, assistant professor of Electronic component technology and materials at TU Delft.
The electromagnetic field in a cellular phone’s inductor changes with frequencies of two giga Hertz. When inductors are very close to a silicon wafer, these changes influence electrons in the wafer, resulting in new, unwanted currents. These currents heat up the system, thereby causing energy loss and thus consuming energy from the batteries. Moreover, by interconnecting two parts of the system that shouldn’t be interconnected, these currents produce noise.
“There were existing computer models for analysing those currents when I started my project two and half years ago,” says Rejaei. “But they were very slow. I improved the model by making it simpler. Instead of making calculations with a spiral, I estimated the spiral by using circles, which are interconnected with a straight line. With this approach you can solve the mathematical equations almost exactly.”
To minimise the currents, it’s necessary to optimise the coil length and the amount of coil windings by calculating the effect of all possible combinations. The best combination is different for different frequencies. Rejaei: “Calculating the effect of one combination takes one second with my program; the old programs needed twenty to thirty minutes.”
Rejaei now wants to focus on making inductors smaller, because smaller inductors have less influence on other components on the chip and therefore produce less noise. Inductors can be made smaller by adding a ferromagnetic layer; however, the high frequencies used in cellular phones present problems. The magnet consists of microscopic magnets (spins) that start twisting at high frequencies. While twisting, they encounter friction, which causes energy loss.
“Over the past seventy years, only five papers about this friction were published; but to improve the inductor’s performance we need to know more about fundamental aspects of this process. Currently, I’m working in a borderline area, between applied and fundamental research, where very few people are working.”
Visa
Rejaei, who has been in the Netherlands for about fifteen years, is not interested in returning to Iran, because Iranian universities conduct little fundamental research. “In Iran, the emphasis is more on education,” he says. “Although I like teaching, I wouldn’t want to miss doing research myself.”
Rejaei’s choice of the Netherlands was purely coincidental. “I wanted to study in Canada, because I wasn’t accepted by Iranian universities, although I wasn’t really politically active. Canada didn’t have an embassy in Iran, so I needed a visa from a European country, just for passing through. Because there was a long waiting line for the Italian embassy, I travelled via Holland instead. On arrival, Canada didn’t seem to want to give me a visa. In contrast, getting a Dutch visa was quite easy. I only had to sign up for a Dutch course and, after taking physics and mathematics tests, I was accepted by TU Delft’s Electrical Engineering MSc program.”
Rejaei had never heard of TU Delft prior to arriving here; but he now regrets that more Iranian students aren’t interested in coming to Delft. “They all want to go to the USA,” he laments. Rejaei believes an exchange program would be fruitful for Iranian students and TU Delft: “Many very good Iranian students want to study abroad.However, there are hardly any contacts between Dutch and Iranian professors that could stimulate student exchanges.”
To make cellular phones smaller, electrical engineers integrated coils on a chip. However, the constant, high frequency changes in the electromagnetic field in the coils cause unwanted electric currents on the silicon parts of the chip. Dr. Behzad Rejaei, from Iran, helped to minimise those side effects with his new computer model.
“Engineers looked at spiral inductors, coils that are used to build up an electromagnetic field, as simple devices. However, because they have side effects when integrated on a silicon chip, it’s necessary to study them in more detail than was previously required,” says Dr. Behzad Rejaei, assistant professor of Electronic component technology and materials at TU Delft.
The electromagnetic field in a cellular phone’s inductor changes with frequencies of two giga Hertz. When inductors are very close to a silicon wafer, these changes influence electrons in the wafer, resulting in new, unwanted currents. These currents heat up the system, thereby causing energy loss and thus consuming energy from the batteries. Moreover, by interconnecting two parts of the system that shouldn’t be interconnected, these currents produce noise.
“There were existing computer models for analysing those currents when I started my project two and half years ago,” says Rejaei. “But they were very slow. I improved the model by making it simpler. Instead of making calculations with a spiral, I estimated the spiral by using circles, which are interconnected with a straight line. With this approach you can solve the mathematical equations almost exactly.”
To minimise the currents, it’s necessary to optimise the coil length and the amount of coil windings by calculating the effect of all possible combinations. The best combination is different for different frequencies. Rejaei: “Calculating the effect of one combination takes one second with my program; the old programs needed twenty to thirty minutes.”
Rejaei now wants to focus on making inductors smaller, because smaller inductors have less influence on other components on the chip and therefore produce less noise. Inductors can be made smaller by adding a ferromagnetic layer; however, the high frequencies used in cellular phones present problems. The magnet consists of microscopic magnets (spins) that start twisting at high frequencies. While twisting, they encounter friction, which causes energy loss.
“Over the past seventy years, only five papers about this friction were published; but to improve the inductor’s performance we need to know more about fundamental aspects of this process. Currently, I’m working in a borderline area, between applied and fundamental research, where very few people are working.”
Visa
Rejaei, who has been in the Netherlands for about fifteen years, is not interested in returning to Iran, because Iranian universities conduct little fundamental research. “In Iran, the emphasis is more on education,” he says. “Although I like teaching, I wouldn’t want to miss doing research myself.”
Rejaei’s choice of the Netherlands was purely coincidental. “I wanted to study in Canada, because I wasn’t accepted by Iranian universities, although I wasn’t really politically active. Canada didn’t have an embassy in Iran, so I needed a visa from a European country, just for passing through. Because there was a long waiting line for the Italian embassy, I travelled via Holland instead. On arrival, Canada didn’t seem to want to give me a visa. In contrast, getting a Dutch visa was quite easy. I only had to sign up for a Dutch course and, after taking physics and mathematics tests, I was accepted by TU Delft’s Electrical Engineering MSc program.”
Rejaei had never heard of TU Delft prior to arriving here; but he now regrets that more Iranian students aren’t interested in coming to Delft. “They all want to go to the USA,” he laments. Rejaei believes an exchange program would be fruitful for Iranian students and TU Delft: “Many very good Iranian students want to study abroad.However, there are hardly any contacts between Dutch and Iranian professors that could stimulate student exchanges.”
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