Flexible phototransistor is world’s fastest, most sensitive

May dramatically improve performance of cameras and other light-capturing devices
October 30, 2015

New phototransistor is flexible yet fastest and most responsive in the world, according to UW engineers (credit: Jung-Hun Seo)

University of Wisconsin-Madison (UW) electrical engineers have created the fastest, most responsive flexible silicon phototransistor ever made, inspired by mammals’ eyes.

Phototransistors (an advanced type of photodetector) convert light to electricity. They are widely used in products ranging from digital cameras, night-vision goggles, and smoke detectors to surveillance systems and satellites.

Developed by UW-Madison collaborators Zhenqiang “Jack” Ma, professor of electrical and computer engineering, and research scientist Jung-Hun Seo, the new phototransistor design uses thin-film single-crystalline silicon nanomembranes and has the highest-ever sensitivity and response time, the engineers say.

They suggest it could improve performance of products that rely on electronic light sensors. Integrated into a digital camera lens, for example, it could reduce bulkiness and boost the acquisition speed and quality of video or still photos.

Silicon nanomembrane phototransistor design. An anti-reflection coating (ARC) with a low refractive index increases light absorption by the silicon nanomembrane (Si NM) below, which is backed by transistor electrodes (source, gate, and drain), a reflective metal layer, and protective polyethylene terephthalate (PET). (credit: Jung-Hun Seo et al./Advanced Optical Materials)

While many phototransistors are fabricated on rigid surfaces, and therefore are flat, the new devices are flexible, meaning they more easily mimic the behavior of mammalian eyes. “We actually can make the curve any shape we like to fit the optical system,” Ma says. The new “flip-transfer” fabrication method deposits electrodes under the phototransistor’s ultrathin silicon nanomembrane layer and a reflective metal layer on the bottom. The metal layer and electrodes act as reflectors and improve light absorption sensitivity without the need for an external amplifier.

“Light absorption can be much more efficient because light is not blocked by any metal layers or other materials,” Ma says.

The researchers published details this week in the journal Advanced Optical Materials. The work was supported by the U.S. Air Force. The researchers are patenting the technology through the Wisconsin Alumni Research Foundation.

Abstract of Flexible Phototransistors Based on Single-Crystalline Silicon Nanomembranes

In this work, flexible phototransistors with a back gate configuration based on transferrable single-crystalline Si nanomembrane (Si NM) have been demonstrated. Having the Si NM as the top layer enables full exposure of the active region to an incident light and thus allows for effective light sensing. Flexible phototransistors are performed in two operation modes: 1) the high light detection mode that exhibits a photo-to-dark current ratio of 105 at voltage bias of VGS < 0.5 V, and VDS = 50 mV and 2) the high responsivity mode that shows a maximum responsivity of 52 A W−1 under blue illumination at voltage bias of VGS = 1 V, and VDS = 3 V. Due to the good mechanical flexibility of Si NMs with the assistance of a polymer layer to enhance light absorption, the device exhibits stable responsivity with less than 5% of variation under bending at small radii of curvatures (up to 15 mm). Overall, such flexible phototransistors with the capabilities of high sensitivity light detection and stable performance under the bending conditions offer great promises for high-performance flexible optical sensor applications, with easy integration for multifunctional applications.