Photocapacitance of Hydrogenated Amorphous Silicon Phototransistors
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PHOTOCAPACITANCE OF HYDROGENATED AMORPHOUS SILICON PHOTOTRANSISTORS D. Caputo1, G. de Cesare1, F. Lemmi1, A. Nascetti1*, F. Palma1, F. Roca2, M. Tucci2 1 University of Rome “La Sapienza”, Department of Electronic Engineering, Rome, 00184 Italy 2 ENEA Research Center Località Granatello Portici (NA) 80055 Italy * Present address: Philips Research Laboratories Aachen, Aachen, Germany.
ABSTRACT Amorphous silicon-based phototransistors are studied as an alternative solution to replace pixellevel amplifiers simplifying large-area imaging systems. We report electrical characterization by means of current-voltage and capacitance measurements. The measured capacitance increases with decreasing frequency of the probe signal and values largely exceeding the geometrical one at low frequencies have been achieved both in the dark and under illumination. In particular, values in excess of 200 µF/cm2 are measured under 220 µW/cm2 illumination at 600 nm at 100 mHz. The capacitance dependence on frequency is interpreted in terms of trapping and release kinetics processes in the base and of the gain of the device.
INTRODUCTION The optical gain of the amorphous silicon bulk barrier phototransistors is appealing for largearea arrays of photodetectors [1]. Where a very large dynamic range is required, the large gain that can be achieved by phototransistors can satisfy the increasing demand for pixel-level amplifiers [2]. The current-voltage characteristics have been extensively studied in the past both in the dark and under illumination and optical gains non-linearly decreasing with increasing incident power have been measured [3, 4, 5]. However, in a typical charge storage operation mode in imaging arrays, sensors integrate charge on their capacitance [6] and therefore to understand this operation mode in the case of phototransistor pixels, a detailed study of capacitance behavior is needed, both in the dark and under illumination. We found that the photo-capacitance is dependent on frequency, bias voltage and illumination intensity. The physical mechanisms for these dependences are discussed.
DEVICE STRUCTURE AND OPERATION The hydrogenated amorphous silicon phototransistors is a n-i-δp-i-n stacked structure of amorphous silicon layers grown on a glass substrate coated by Transparent Conductive Oxide (TCO). An aluminum evaporation on top of the structure is realized for the emitter contact. A sketch of the band diagram of the device structure is reported in Fig. 1.
A26.3.1
300Å
4200Å
1.E+3
50Å 500Å 300Å
1V 2V 3V 4V
1.E+2
Gain
base
1.E+1 1.E+0
Incident radiation
emitter
1.E-1 collector
1.E-2 1.E-7
1.E-6
1.E-5
1.E-4
Pin (W)
Figure 1. Band diagram of an a-Si:H phototransistor in dark conditions (solid line) and under illumination (dashed line).
Figure 2. Measured optical gain versus power radiation at different bias voltages.
The floating-base electrode is the δp layer, whose thickness and doping values ensure a complete depletion of the layer even in thermal equilibrium in the absence of a bias voltage. In fact,
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