Thin-Film Phototransistor with nc-Si:H/a-Si:H Bilayer Channel
- PDF / 227,668 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 69 Downloads / 195 Views
Thin-Film Phototransistor with nc-Si:H/a-Si:H Bilayer Channel Y. Vygranenko1,2, A. Sazonov3, M. Fernandes1,2, M. Vieira1,2, A. Nathan4 1
Electronics, Telecommunications and Computer Engineering, ISEL, 1949-014 Lisbon, Portugal CTS-UNINOVA, 2829-516 Caparica, Portugal 3 Electrical and Computer Engineering, University of Waterloo, Waterloo, N2L 3G1, Canada 4 Engineering Department, Cambridge University, Cambridge, CB2 1PZ, United Kingdom 2
ABSTRACT There is significant interest in optical sensors whose fabrication process is fully compatible with existing flat panel display thin film transistor (TFT) technology. Here, we report a field-effect phototransistor with a channel comprising a thin nanocrystalline silicon (nc-Si:H) transport layer and a thicker hydrogenated amorphous silicon (a-Si:H) absorption layer. The implementation of nc-Si:H layer improves device stability in comparison with a-Si:H phototransistors, resulting in reduced threshold voltage shift. Semiconductor and dielectric layers were deposited by radio-frequency plasma enhanced chemical vapor deposition at 280°C. The device characterization included the dark and light transfer characteristics, spectral-response and dynamic measurements. The external quantum efficiency was measured as a function of incident photon flux at different biasing conditions. The phototransistor with channel length of 24 microns and photosensitive area of 1.4 mm2 shows an off-current of about 1 pA, and photoconductive gain up to 200 at low incident intensities. Thus, the results demonstrate the feasibility of the phototransistor for low light level detection. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) phototransistors can be used for a variety of applications such as contact image sensors [1], liquid-crystal displays (LCDs) to control the backlight brightness [2], and active matrix organic light-emitting diode (AMOLED) displays to control the pixel brightness [3]. The most challenging application lies in non-direct conversion xray image sensor, for which low light level detection is required. The advantage of thin-film phototransistors over conventional a-Si:H p-i-n photodiodes or metal-insulator-semiconductor (MIS) photosensors potentially lies in photoconductive gain. In the conventional a-Si:H transistor, however, the channel is too thin for efficient light absorption, and the light-to-dark current ratio is appreciable only in a narrow range of gate biases. Two device modifications such as a source-gated phototransistor [4,5], and a thick-layered etchedcontact phototransistor [6] have been proposed to increase the dynamic range and optical gain. In spite of a low mobility-lifetime product in a-Si:H (PW typically about 10-7 cm2/V), a high photo-conductive gain can be achieved by reducing the channel length, while a large channel width limits pixel scaling and fill factor. The threshold voltage instability of a-Si:H transistors is also a limiting factor. Both these issues could be resolved with a superior electron transport channel material, such as nanocrystalline
Data Loading...
