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A JUNCTION FIELD EFFECT TRANSISTOR BASED ON HYDROGENATED AMORPHOUS SILICON D. Caputo, G. de Cesare, A. Nascetti, V. Kellezi, F. Palma Department of Electronic Engineering, University of Rome "La Sapienza" via Eudossiana 18, 00184, Rome, Italy ABSTRACT An hydrogenated amorphous silicon junction field effect transistor suitable for analog and digital applications is presented. The device is constituted by a p+ - i - n- junction, with the drain and source contacts patterned on the n-doped layer and the gate electrode patterned on the p+ doped layer. As in the crystalline case, the device is a voltage-controlled resistor, and its drain-source resistance can be varied, with a voltage applied to the gate electrode, by modulating the width of the depletion layer extending into the n-type channel. The doping value of this layer has been chosen as a trade-off between high value of channel conductivity and a relatively low defect density in the material. The manufactured device, with W/L=5000/200 µm, shows the typical current-voltage curves of a JFET. In particular, at low VDS, the current presents the linear behavior of the triode zone, where the JFET operates as a linear resistance whose value is controlled by the gate voltage. At higher VDS the JFET works in the pinch-off region as a dependent current generator. First results are very encouraging, since we have achieved transconductance values of 10-6 V/A, which are comparable to those of state of the art TFT. INTRODUCTION The potential advantages of hydrogenated amorphous silicon (a-Si:H) and their application in photosensing arrays has been discussed extensively in the literature [1, 2, 3]. The development of the semiconductor industry has made possible the realization of large size active matrix displays, where both the switching device integrated in the pixel and the photosensor are based on hydrogenated amorphous silicon technology. Switching devices utilized in photosensing arrays can be two or three terminal devices [4, 5] . In particular, since the very small minority carrier diffusion length of amorphous silicon [6] does not permit the use of this material in bipolar transistor, the fabrication of the three terminal devices is restricted to the Metal Oxide Semiconductor structure [7]. The channel is an undoped a-Si:H layer, used in N-channel accumulation mode operation, deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) on a dielectric layer, tipically silicon dioxide or silicon nitride films [8, 9].

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Although this thin film transistor (TFT) has been successfully used in large area active matrix arrays, application in linear circuit is limited by the low channel conductance, due the trapping at the interface between the dielectric and silicon films. In this work we present a junction field effect transistor (JFET) based on a-Si:H materials, which could offer the possibility to be used as switching device as well as analog amplifier. Its operation is similar to that of a crystalline JFET, where however the problems common to amorphous materia