Contacts: Stephen Chou, Electrical Engineering Department, (612) 625-1316, [email protected] Deane Morrison, News Service, (612) 624-2346, [email protected]
One-Electron Transistor Developed at U of Minnesota
A transistor that stores a single electron to represent one bit of information and could revolutionize the way computers work has been designed and fabricated by University of Minnesota researchers. The ultra-tiny, silicon-based transistor, which works at room temperature, holds promise for increasing the number of transistors--and thus the memory storage potential--in a single computer chip by two or three orders of magnitude without increasing the chip's overall size, power consumption or fabrication cost. The work is described in the Jan. 31 issue of Science.
"The only way to make an integrated circuit (computer chip) powerful is to put more transistors in it and make each transistor operate faster," said electrical engineering professor Stephen Chou, the principal investigator for the study. "So the size of each transistor must be reduced. A smaller memory cell also leads to a faster speed and a lower power consumption." If the speed of each transistor is raised, then the time needed to access information from the chip can be kept constant, even though the chip has 100 or 1,000 times more transistors, Chou said. Similarly, more transistors operating at lower power could keep total power consumption constant. Further, the cost of fabricating a transistor goes down as its size goes down; therefore, having more, but smaller, transistors could allow an entire chip to be fabricated in the same time as is now done.
Chou's device differs from conventional transistors in the proportions and small size of the components, which allow one electron to represent one bit. Conventional transistors use about 10,000 electrons to store a bit of information, he said. They also require more power to store each bit and exhibit more noise than his device. The device works by applying a voltage to a polysilicon control gate, which functions as a switch control. Below the control gate is a narrow silicon channel that runs between two terminals. Between the control gate and the channel is a tiny "dot" of polysilicon, only seven nanometers (seven billionths of a meter) in diameter, which touches neither of the other two components and is termed a "floating gate."
Information is represented by storing charges on the floating gate. In binary code, a stored charge--in Chou's device, a single electron--could represent "1," in which case no stored electron would mean "0." If no electron is stored on the tiny floating gate, a certain voltage (the threshold voltage) must be applied to the control gate to cause current to flow in the channel, turning the transistor on. But if an electron is stored on the floating gate, it will screen the channel from the voltage in the control gate, and the threshold voltage needed to turn on the transistor will increase.
In conventional devices, thousands of electrons are stored on the dot and the increase occurs smoothly, in proportion to the charge on the dot. But in Chou's device, the increase occurs in a sharp, stepwise manner, making it easy to tell from the size of the threshold voltage whether an electron is stored or not. In order for the system to work, the size of the polysilicon dot and the width of the channel must be very small; otherwise, a single electron will not be able to screen the channel from the voltage applied to the control gate.
"This single-electron memory is orders of magnitude smaller than the conventional floating-gate design and is a major step forward in taking advantage of single-electron effects to build ultra-small and ultra-high-density transistor memories," said Chou.
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