Record-Setting Phototransistor is Flexible and Sensitive

Inspired by mammals' eyes, a record-setting phototransistor could improve the performance of myriad products — from digital cameras, night-vision goggles and smoke detectors to surveillance systems and satellites — that rely on electronic light sensors.

Developed at the University of Wisconsin-Madison, the phototransistor can be shaped to fit different optical systems, unlike similar devices that are fabricated on rigid surfaces. The device has a back-gate configuration based on transferrable single-crystalline Si nanomembrane.

A unique phototransistor developed by UW-Madison electrical engineers is flexible, yet faster and more responsive similar phototransistors. Courtesy of Jung-Hun Seo.

Due to the mechanical flexibility of Si nanomembranes with the assistance of a polymer layer to enhance light absorption, the device exhibits stable responsivity with less than 5 percent of variation under bending at small radii of curvatures (up to 15 mm).

Another distinguishing feature of the phototransistors is the "flip-transfer" fabrication method, in which the final step is to invert the finished phototransistor onto a plastic substrate. At that point, a reflective metal layer is on the bottom.

"In this structure — unlike other photodetectors — light absorption in an ultrathin silicon layer can be much more efficient because light is not blocked by any metal layers or other materials," said professor Zhenqiang Ma.

The researchers also placed electrodes under the phototransistor's ultrathin silicon nanomembrane layer; the metal layer and electrodes each act as reflectors, improving light absorption without the need for an external amplifier.

Flexible phototransistors operate in two modes: first, a high-light detection mode that exhibits a photo-to-dark current ratio of 10⁵ at voltage bias of V_GS < 0.5 V and V_DS = 50 mV; and second, a high-responsivity mode with maximum responsivity of 52 AW^-1 under blue illumination at voltage bias of V_GS = 1 V and V_DS = 3 V.

The work was supported by the U.S. Air Force and was published in Advanced Optical Materials (doi: 10.1002/adom.201500402).