New “semi-floating” gate makes for fast, low-power circuitry
As well as a very compact image sensor.
by John Timmer - Aug 10 2013, 2:42am +0800
PHYSICAL SCIENCES
After a long period during which the emphasis had been on building ever-faster computer circuits, things have shifted toward making them more energy-efficient. Some efficiency gains have come through small tweaks to the layout of the individual components, but most of the efficiency was a product of changes to the manufacturing process: new materials and ever-smaller features. Unfortunately, we're getting close to the point where shrinking the features of circuits any further will cause the inherent noise of quantum mechanics to start interfering with the chip's operations.
But that doesn't mean an end to potential improvements. A team of Chinese researchers have now described a new structure for the individual gates that control the flow of electrons within chips. Their design, which they're calling a
semi-floating gate,
switches states in as little as a nanosecond
, and it
requires very little power to operate.
The gates in electronics share a common design. They have a source of electrons and a drain for them connected by something that can be switched between two states: one that allows the current to flow between the source and drain and one where the current is blocked. Typically, the switch material has been a semiconductor that directly connects the source and the drain. A neighboring bit of material can switch the semiconductor between insulating and conducting, controlling the flow of electrons through the gate.
Flash memory uses a distinctive variant on this called a floating gate. In these structures, the material that bridges the source and sink is electrically isolated from them by a thin layer of insulator—in other words, it floats. This forces the electrons to transit through the gate by tunneling, with the rate of tunneling set by the control wire. A floating gate can stably trap charges, letting it be set in a semi-permanent on or off state, which is why flash can work as a long-term storage solution.
The semi-floating gate is like a hybrid of the two. On the source side, the gate is electrically isolated, forcing electrons to tunnel into the semiconductor that can transfer them to the drain. On the drain side, the semi-floating gate directly contacts the drain, allowing electrons to flow through. The control wiring that sets the state of the semiconductor is also slightly different. In addition to sitting above the semiconductor, it curves around to flank the junction between the semiconductor and the drain, forming a structure called a tunneling field-effect transistor. This provides finer control of the flow of electrons through the gate.
The end result is a device that can store its state, much like flash, but switches much more quickly. Changing between the on and off states took only 1.3 nanoseconds. All the switching also took place within a range of 3V and required very little current: less than one micro-Amp. The device was also very stable. Even after 1012 cycles of writing and erasing, it retained about 90 percent of its original performance. The researchers estimate it would still work out to 1015 cycles, which handily beats floating gate performance.
In effect, the authors say that
the device has the speed of SRAM, but it only requires a single transistor to provide the equivalent behavior.
It's also
remarkably flexible. The authors replaced the control gate with a photosensitive material that built up charge in response to light. The amount of current that flowed through the gate ended up being proportional to the amount of light the device was exposed to,
meaning that each one of these gates could act as an incredibly compact photosensor.
The device doesn't hold its state without power, so it's not currently a replacement for flash. But the authors suggest it could eventually stand in for current forms of RAM (both SRAM and DRAM)—and there are obvious applications in digital imaging. The authors also note that by lowering the speed of operations, it's possible to use even less power, which could make the technology useful for mobile applications.
Science, 2013. DOI: 10.1126/science.1240961 (About DOIs).
New “semi-floating” gate makes for fast, low-power circuitry | Ars Technica
