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In this paper, we present a detailed investigation of an amorphous silicon sensor for the detection of ultraviolet (UV) radiation. The device is an n-i-p stacked structure with a grid-patterned top metal contact through which the incident radiation reaches the active layers. The performances of the sensor have been enhanced by using a very thin chromium silicide (CrSi) film formed on top of the p-doped layer. In particular, this film enhances the surface conductivity, reducing the effect of the self-forward bias that occurs in the device due to the high resistivity of the p-doped layer. As a result, the sensitivity and the linearity of the response increase, reaching a responsivity above 60 mAAV at 254.3 nm. Furthermore, the CrSi layer leads to a stable device because it hides the effect of the p-doped layer resistivity variation under UV radiation. The comparison between two sets of devices with different grid geometries, one with and one without the CrSi film, demonstrates the effectiveness of the alloy film.

An amorphous silicon bulk barrier phototransistor is studied as a basic element for the pixel of high dynamic photosensor arrays. The device is an n-i-/spl delta/p-i-n structure whose optical gain shows a nonlinear dependence on the illumination intensity. For each applied bias voltage, a quasi-hyperbolic decrease of the optical gain as a function of the incident power is found. This behavior can be explained taking into account both the material characteristics (defect distribution, dopant concentration), and the structure properties. Our measurements lead to a minimum detectable signal of about 0.7 nW/cm/sup 2/ independently on the applied voltage, making the device suitable for low illumination conditions. On the other hand, by increasing the input power up to 35 mW/cm/sup 2/, we did not find saturation of output photocurrent leading to a dynamic range of at least of 120 dB. This value can be further increased by using as basic cell of the pixel the phototransistor and a resistor connected between the voltage supply and the collector electrode.

A detailed investigation of the near infrared detection process of an hydrogenated amorphous silicon sensor is presented. Single junction devices with micro-compensated absorber layer show photocurrent response to radiation up to 2 /spl mu/m at room temperature. This sensitivity is ascribed to both the high number of trap states existing in the absorber material and to the electric field distribution in the device. Results of transient and frequency responses under different bias voltage and illumination conditions confirm the existence of a conduction mechanism controlled by traps. In particular, the exponential increase of the signal current with forward applied voltage suggests the re-excitation of the injected carriers from shallow localized states into extended states. Finally, a preliminary optimization of the response of the device achieved by including it in a front-end circuit allows detection of modulated radiation up to 100 kHz.

We present the modeling and realization of a two terminal hydrogenated amorphous silicon device with bistable current-voltage (I-V) characteristics, potentially suitable to obtain a new generation of switch and memory in large area application. The structure is basically constituted by three stacked junctions: p/sup +/-i-n/sup -/, n/sup -/-i-p/sup -/, p/sup -/-i-n/sup +/. A numerical device model allows us to investigate in detail the device behavior pointing out the fundamental role of the two lightly-doped n/sup -/ and p/sup -/ layers. In the OFF condition the central junction is reverse biased and limits the current in the device. The transition OFF-ON starts when, increasing the applied voltage, one of the two lightly-doped layers becomes completely depleted. In the ON state high injection of both carriers from the external-doped layers completely hides the doping concentrations of the bases and the device behaves like a single forward biased p/sup +/-i-n/sup +/ structure. The transition ON-OFF occurs when, decreasing the applied voltage, the free carrier densities in the lightly-doped layers become lower than the dopant concentrations. This transition is thus mainly dependent on the recombination processes occurring in the central-doped layers. Devices with hysteresis around 2 V and turn-on voltages ranging from 12 to 15 V have been obtained.

In this paper, we extensively investigate a family of solar-blind thin-film photodetectors optimized for the ultraviolet spectrum (UV). The devices are p-i-n structures made of hydrogenated amorphous silicon (a-Si:H) and silicon carbide (a-SiC:H) on a glass substrate. At room temperature the photodetectors exhibit values of quantum efficiency of 20% at /spl lambda/=187 nm, and are transparent to visible radiation. The excellent sensitivity of the device at short wavelengths is explained within the framework of a diffusive model of transport, taking into account the effects of hot carrier relaxation. The rejection of visible light is obtained with an appropriate design of the energy gap and intrinsic layer thickness. The great advantage of this technology lies in the possibility to produce low-cost, large-area arrays of photodetectors.

In this work, a junction field effect transistor (JFET) based on a-Si:H is presented. The drain-source contacts are made on top of the n-layer of a glass/metal/p/sup +/-i-n structure. The channel conductivity can be modulated by a reverse bias applied to the p/sup +/-i-n junction, which varies the depth or the length of the depletion region. In amorphous silicon, the depletion of doped layers is limited by the high defect density induced by the doping process. Here, the electron concentration of the n-doped layer (the device channel) in a p-i-n amorphous silicon junction is studied by using a one-dimensional finite-difference simulator. The n-channel conductivity is then obtained by integrating the free electron concentration along the drain-source direction. Pinch-off regime is achieved when the n-layer is fully depleted. A JFET with W/L = 400 /spl mu/m/40 /spl mu/m was fabricated. Transistors with pinch-off voltages around -3.6 V and transconductance values of the order of 10/sup -7/ A/V were obtained. Comparison between experimental and modeled output characteristics suggests the presence of a defect-rich layer at the channel-air interface. This is related to the damage induced by the process steps during the device fabrication. The achieved experimental results make the device suitable for applications in linear circuits. In particular, unlike thin film transistors (TFTs), JFETs do not require high-temperature, high-quality dielectric layers, and appear particularly attractive for process on plastic substrates.