Photodiodes operating in the spectral range with a wavelength of up to approximately 1.1 μm are usually produced from silicon. Photodiodes composed of germanium or InGaAs are sensitive in the infrared range. By using further compound semiconductor materials, for example II-VI semiconductors or else organic semiconductor materials, further spectral ranges can furthermore be covered.
The p-n junction in the semiconductor material is produced by using regions having differing doping which provide for a change of charge carrier type. In the region of the p-n junction, a depletion layer, also called space charge zone, depleted of charge carriers is formed on account of diffusion and recombination processes, in which depletion layer an electric field prevails which provides for a separation of the charge carriers in the space charge zone. On account of the continuous thermal generation of charge carriers in the space charge zone and in adjacent regions of the neutral zone from which the charge carriers migrate into the space charge zone as a result of diffusion, a small current, also called reverse current, flows permanently via the space charge zone.
If electromagnetic radiation impinges on the space charge zone or, respectively, the adjacent diffusion region, further electron-hole pairs are generated which, in addition to the permanent reverse current, provide for a photocurrent generated by the electromagnetic radiation. In this case, the photocurrent is proportional to the intensity of the electromagnetic radiation. The electromagnetic radiation has to have a minimum energy in order to generate the charge carriers, the known energy being dependent on the band gap of the semiconductor material used. A limit wavelength above which the sensitivity decreases greatly thus results for each semiconductor material. Furthermore, the penetration depth of the electromagnetic radiation is also greatly dependent on the wavelength, the penetration depth increasing with longer wavelengths. In the case of silicon, for example, electron-hole pairs are generated by using visible light up to a limit wavelength of 1.1 μm, approximately 90% of the incident light radiation being absorbed at a depth of 0.5 μm in the case of blue light, up to depth of 1.1 μm in the case of green light and up to a depth of 8.5 μm in the case of red light.
In order to detect the colour information of electromagnetic radiation over a relatively large wavelength range with the aid of photodiodes, as is carried out for example during the detection of colour images with the aid of photodiode arrays in digital cameras, various solutions are known. Thus, for colour resolution, a plurality of photodiodes each coordinated with the spectral range to be detected are interconnected, wherein the individual photodiodes are provided with spectral filters in order to transmit the desired spectral range to the associated photodiode. In this case, the spectral filter can be embodied as an absorption filter for masking out non-relevant radiation and can be used in a Bayer pattern. Furthermore, transmission gratings are known, which are applied over the photodiodes and act as spectral filters in order that only electromagnetic radiation having a specific wavelength is transmitted. Such a construction is described for example by P. B. Catrysser, B. A. Wandell in: Journal of Optical Society of America, Vol. 20, No. 12, December 2003.
Furthermore, methods are known wherein the electromagnetic radiation is spectrally split with the aid of prisms and fed separately to the assigned photodiodes. Furthermore, there is the possibility, in the case of photodiodes which are sensitive over a wide spectral range, of additionally using discrete filters which are fitted alternately above the photodiode structure in order that the individual spectral ranges are detected sequentially one after another.
Furthermore, in the case of photodiodes, the space charge zone can be expanded by applying a reverse voltage, in order, with the aid of the larger space charge zone, to improve the effectiveness of the charge carrier detection particularly for long-wave electromagnetic radiation having a large penetration depth. However, the range in which the expansion of the space charge zone can be varied under typical operating voltages for photodiodes is predetermined by the doping concentration and is generally not adjustable over the range of approximately 0.1 to 10 μm which is of interest for visible light.
Furthermore, individual photodiodes with spectral resolution are also known wherein no additional spectral filter is required. Thus, U.S. Pat. No. 5,965,875 describes a photodiode construction wherein a plurality of p-n junctions are formed at different depths in the semiconductor material, as a result of which a plurality of space charge zones are formed one above another and separately from one another, which can then be used to simultaneously detect different spectral ranges of the incident light radiation. In this case, each p-n junction has a dedicated electrical connection for reading out the photocurrent detected in the respective spectral range.
WO 2008/068616 furthermore describes a photodiode construction including a large space charge zone, in which, as a result of transverse electric fields being applied, the charge carriers generated at different absorption depths in the space charge zone are separately collected and read out, such that different spectral ranges of the incident light radiation can be resolved. A similar principle is employed in US 2006/0244089, wherein a large-area electrode is applied above a photoactive layer with a large space charge zone, which electrode is insulated from the photoactive layer and can be used to control the extraction region for the charge carriers in a manner dependent on the absorption depth, in order to obtain a spectral resolution of the incident light radiation.
The implementation of spectrally sensitive photodiodes, also referred to as colour sensors hereinafter, as clusters composed of a plurality of photodiodes specifically tailored to the spectrum that is respectively to be detected, or as broadband photodiode with additional colour filters is complex in production and can be integrated only with difficulty into standard processes for producing semiconductor circuits, in particular in CMOS circuits. Furthermore, a large area requirement is generally necessary, which limits the image resolution when such colour sensors are used in CMOS cameras, for example.
Moreover, generally only a discrete colour resolution is possible with the known colour sensors. This also applies, in particular, to photodiodes having multiple p-n junctions in order to provide different absorption depths and hence spectral sensitivities. Furthermore, each p-n junction has to be individually electrically connected in order that the charge carriers generated can be read out separately and the individual colours can thus be resolved.
The design of colour sensors as lateral photodiodes having a multiplicity of electrical connections in order, by using corresponding transverse fields, to read out the charge carriers from different absorption depths and thus to obtain a colour resolution demands a large area requirement and is furthermore complex in production. In the case of the photodiode structure proposed in US 2006/0244089, wherein control is obtained with the aid of an insulated surface electrode, there is the difficulty that the electrode material already absorbs electromagnetic radiation prior to penetration into the actual photoactive layer. As a result, particularly in the short-wave spectral range, less light intensity is available for electrical signal formation.
For these and other reasons there is a need for the present invention.