CMOS based imagers are beginning to compete against CCDs in many areas of the consumer market because of their system-on-a-chip capability. Sensitivity, however, is a main weakness of CMOS imagers and enhancements and deviations from standard CMOS processes are necessary to keep up sensitivity with downscaled process generations. In the introductory section several definitions for the sensitivity of image sensors are reviewed with regard to their potential to allow meaningful comparison of different detector structures. In the main section, the standard CMOS sensor architecture is compared to detector structures designed to improve the sensitivity, namely the photogate (PG), the pinned photodiode (PPD) and the thin film on ASIC (TFA) approach. The latter uses a vertical integration of the photodiode on top of the pixel transistors. A careful analysis of the relevant electrical, optical and technological parameters and many previously published experimental data for different imagers reveals that only the PPD and the TFA enhancements provide satisfactory sensitivity and withstand scaling down to 0.18 /spl mu/ processes. Due to the higher fill factor and the higher quantum efficiency TFA provides significantly better values than PPD. The radiometric sensitivity of a 5 /spl mu/m/spl times/5 /spl mu/m TFA pixel is found to amount to 11.9 V/(/spl mu//cm/sup 2/) for a 0.25 /spl mu/m process and 27.5 V/(/spl mu/J/cm/sup 2/) for a 0.18 /spl mu/m process.

Sensitivity of CMOS based imagers and scaling perspectives

We describe the progress in the science and technology of stabilized a-Se from its early use in xerography and xeroradiography to its present use in commercial modern flat panel X-ray imagers and ultrasensitive video tubes which utilize impact ionization of drifting holes. Both electrons and holes can drift in stabilized a-Se, which is a distinct advantage since X-ray photogeneration of charge carriers occurs throughout the bulk of the photoconductive layer. An a-Se photoconductor has to be operated at high fields to ensure that the photogeneration efficiency is sufficiently large to provide reasonable X-ray sensitivity. However, at high fields, the dark current is unacceptably large in simple metal/a-Se/metal devices, and special multilayer device structures need to be designed. The dark current decays with time and increases with the nominal applied field. The reduction of the dark current to a tolerable level was one of the key factors that lead to the commercialization of a-Se X-ray detectors. We discuss the origin of the dark current, and highlight some of the current challenges in the design of next generation detectors. We also discuss the origin of impact ionization in a-Se, and its fruitful utilization in ultrasensitive imaging devices, including the Harpicon, which are likely to lead to new high detective quantum efficiency detectors.

Amorphous selenium and its alloys from early xeroradiography to high resolution X‐ray image detectors and ultrasensitive imaging tubes

Experimental results on the constant current stressing in hydrogenated amorphous silicon (a-Si:H) Cr–p+–V thin film devices are presented. With increasing injection of charge via either increasing bias or time, the current-voltage characteristics of devices exhibit instability, as shown by a decrease in the reverse current. This is interpreted in terms of the creation of defects in the a-Si:H. The defect generation rate. as measured by the voltage shift ΔV at a constant reverse current in the J-V curve, is found to follow a square-root time dependent law. In addition, a decrease in device conductance after stressing is also observed, which is described by a mechanism of dopant equilibrium during and after stressing.

Current-induced instability in Cr–p+ a-Si:H–V thin film devices

Measurements are reported of the time-varying reverse-bias current in p─i─n devices made from hydrogenated amorphous silicon. Two mechanisms of current flow are identified, corresponding to the thermal generation of charge from states near the middle of the gap, and the injection of charge from the contacts. The temperature and time dependence of the thermal generation current allows the deep depletion quasi-Fermi energy and the depletion charge to be determined. Measurements of the slow transient currents from junction-recovery experiments give information about the density of states distribution near mid-gap. The contact injection current exhibits a bias-induced metastability which is annealed at elevated temperatures.

Long-time transient conduction in a-Si:H p─i─n devices

Hydrogenated microcrystalline silicon (μc-Si:H) layers with thickness from 100 to 540 nm were prepared in situ by plasma enhanced chemical vapor deposition. The growth of μc-Si:H on various substrates [NiCr, device quality, and laser annealed amorphous silicon (a-Si:H)] was studied in ultrahigh vacuum by atomic force microscope using a conductive cantilever which enabled simultaneous measurement of morphology and local current with lateral resolution below 5 nm. The effect of barriers, voltage, and time on contrast in local current map is discussed in detail. Coexistent amorphous and microcrystalline regions are clearly identified due to their different conductivity. Laser annealing of the a-Si:H substrate significantly increases the crystalline fraction at the same layer thickness. Grains as small as 10–30 nm separated by less conductive grain boundaries were revealed in microcrystalline regions.

Microcrystalline silicon thin films studied by atomic force microscopy with electrical current detection

Dark conductivity in amorphous silicon p‐i‐n devices arising from thermal generation through bulk defect states is explored. The current decays slowly after a voltage is applied, due to depletion of charge from the undoped layer, and is voltage dependent due to a field‐enhanced generation rate. Creation of metastable bulk defects by light soaking reversibly increases the current. The steady‐state generation current is dervied from the measured relaxation time and depletion charge.

R. A. Street

Papers

Thermal generation currents in hydrogenated amorphous silicon p - i - n structures