Study of capacitance in hydrogenated amorphous silicon phototransistors for imaging arrays M. Tucci a, * , D. Caputo b a ENEA Research Center, Localit a Granatello, 80055 Portici, Napler (Na), Italy b Department of Electronic Engineering, University of Rome ‘La Sapienza’, via Eudossiana, 18, 00184 Rome, Italy Abstract In this paper we report on the study of capacitance in hydrogenated amorphous silicon phototransistors in order to determine their applicability in large area imaging systems. Measured capacitance values exceed the geometrical one at low frequencies of the probe signal, both in the dark and under illumination. In particular, capacitance values in excess of 60 lF/cm 2 are measured under 220 lW/cm 2 illumination at 600 nm. The experimental data have been reproduced by a numerical device simulator, which takes into account the distribution of defects in amorphous materials. We have found that capacitance is mainly determined by the trapping and release processes occurring in the base and at the interfaces between the intrinsic and the n-layers of the device. At these interfaces, the Fermi level lies in correspondence with the band tails, whose high number of defects causes a large variation of trapped charge in response to the a.c. applied voltage. Ó 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Gc; 81.40.)z 1. Introduction Image sensors are becoming very popular with applications ranging from commercial digital cameras to medical imaging devices. In the medical field, in partic- ular, large area systems are required for digital radiog- raphy, which currently represents one of the driving forces for the advancements of medical diagnostics. These systems are based on amorphous silicon (a-Si:H) devices (a simple p–i–n structure used as photodiode), which detects the light produced by a scintillator that is optically coupled to the array and a readout switch for the selection of the pixel. In a typical charge storage operation mode in imaging arrays, sensors integrate charge on their capacitance [1]. Data is subsequently read out by charge-sensitive amplifiers. In this paper we investigate amorphous silicon-based phototransistors [2,3] to determine if their large gain can match the increasing demand for pixel-level amplifiers [4,5]. To understand the implications of the charge storage operation mode in the case of phototransistor pixels, we have measured the device capacitance both in dark condition and under illumination and interpreted the experimental results with a numerical device simu- lator for amorphous silicon structures [6,7]. 2. Device structure and operation The investigated a-Si:H phototransistor is a n–i–dp– i–n stacked structure grown on a glass substrate coated by transparent conductive oxide (TCO) in a PECVD system. It is a two terminal device, since the base (dp layer) of the phototransistor is floating. A sketch of the band diagram at 0 V in dark condi- tion, as obtained from the simulation program, is re- ported in Fig. 1. Starting from the left, we have the collector (a 300  A thick n-layer), a 4200  A thick intrinsic layer, the base (60  A thick lightly p-doped layer), a 500  A thick intrinsic layer and finally the emitter contact (a 300  A thick n-layer). From the position of the Fermi level in the base (E F ¼ 0:6 eV above the valence band) we deduce that this thin layer is depleted even in thermal equilibrium * Corresponding author. Tel.: +39-081 772 3312; fax: +39-081 772 3344. E-mail address: tucci@portici.enea.it (M. Tucci). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.090 Journal of Non-Crystalline Solids 338–340 (2004) 780–783 www.elsevier.com/locate/jnoncrysol