Infrared Detection at Room Temperature with Single-Electron Resolution
Lauren I. Rugani
Infrared signals with wavelengths longer than 1.2 μm often are excluded by the selectivity of silicon-based sensors. Sensors that use electron-hole pair generation require photons to have energy greater than the 1.1-eV bandgap energy of silicon, which only corresponds to wavelengths in the visible region. Other methods require low-temperature operation.
A scanning electron microscopy image shows (from right to left) the electron reservoir, an energy barrier formed by the lower gate and a storage node, where injected electrons are read out as current. The schematics show that no electrons pass into the storage node without infrared radiation. More electrons are allowed over the energy barrier, and the detection current decreases as the wavelength of the infrared radiation decreases.
A team from NTT Corp. in Atsugi, from Shizuoka University in Hamamatsu and from Hokkaido University in Sapporo, all in Japan, has developed nanoscale silicon metal-oxide semiconductor field-effect transistors to detect infrared signals at room temperature. By exciting electrons over an electrically induced energy barrier, both the range of detectable wavelengths and the sensitivity of the device can be controlled.
The sensor works when an infrared signal excites conduction-band electrons in a 25-nm-deep electron reservoir. A silicon-on-insulator channel measuring 40 × 400 nm is placed next to the reservoir to increase the number of excited electrons. A poly-silicon lower gate then turns off the transistor and electrically forms an energy barrier, creating a storage node on the other side. Electrons with energy greater than the height of the barrier are injected into the storage node, where they are read as changes in current flowing through the transistor.
Short-wave pass filter
When the lower gate is set to its threshold voltage of –3.5 V, the injection of one electron into the storage node is read as one decreasing step in current. Higher voltages create shorter energy barriers and allow electrons with less energy — those excited by longer wavelengths — to pass into the storage node. This effectively allows the device to function as a short-wave pass filter controlled by lower gate voltage.
The upper gate voltage, on the other hand, can control the sensitivity of the device. The upper gate serves as a source for the transistor and can control the number of detected electrons; a higher voltage increases the electron density in the electron reservoir and, thus, the number of excited electrons, regardless of the lower gate voltage.
Because a detected infrared signal can be stored and later read out nondestructively, the researchers note the potential of the device in large-scale silicon integrations or as part of an array. Adjusting the device to pick up desired wavelengths will prove useful in material analysis and in remote and thermal sensing applications.
Applied Physics Letters, May 28, 2007, 223108.
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