Operation of Lateral SOI PIN Photodiodes with Back-Gate Bias and Intrinsic Length Variation

This paper presents an analysis of the operation of lateral thin-film SOI PIN photodiodes for the detection of short wavelengths. Experimental measurements were done varying the back-gate bias in order to point out the behaviour of the device. In addition, by using two-dimensional numerical simulations, the intrinsic length (L I) was changed, with the purpose of predicting the performance of this photodetector in more advanced technologies. Introduction: Optical detection at short wavelengths close to blue and UV (�<480nm) have many applications in biomedical and environmental fields (1) and in data storage applications (2). These applications require the use of efficient photodetectors such as PIN Photodiodes, which consist of a PN junction separated by an intrinsic region with length Li (3), which in practice corresponds to a weakly doped P or N region. In such devices, carriers generated by light radiation can be collected more efficiently; because the depletion region is formed from the surface to the end of silicon film, and silicon devices absorb light as a function of their thickness (3). Furthermore, the hole-electron pairs must not recombine, and have to be quickly separated by the action of the electric field present in the depletion region that corresponds approximately to the intrinsic length. Therefore, the intrinsic length must be large enough to allow for the absorption of a significant number of photons, but sufficiently short to reduce the transit time for drift of photogenerated carriers (4). The characterized devices follow the 2�m technology from UCL described in (5). They have 8 and 9�m of Li, consisting of a doping profile of P+P-N+. Figure 1 shows a schematic cross-section of the studied device. Experimental Measurements: The normalized photocurrent as a function of the applied voltage (VD) is presented in Figure 2 under the incidence of three different wavelengths for both devices. It is clearly seen that the higher the wavelength, the lower the photogenerated current due to the decreased photon energy (3). Besides, the current has shown to be larger in the diode with L I=9�m since it has larger photosensitive area (6). The variation of back-gate voltage (VBG) modifies the operation mode (inversion, accumulation or depletion) of the silicon film (7). In Figure 3 is presented the photodiode current as a function of the V BG when illuminated with 397nm. When the film is biased towards accumulation regime (V BG<-5V), there is a decrease of the current, related to the decrease of electron mobility and the increase of recombination, due to higher holes concentration. Once the intrinsic area assumes lateral depletion (increasing V BG), the current abruptly raises, meaning that carriers recombination is smaller. Moreover, for VBG higher than about 0.5V, biasing the film towards inversion regime, an effective P + N - + regions becomes more effective, as they are closer to each other. This behaviour is similar to the short-channel effects in MOS transistors (10), if we make an analogy taking the backgate of the PIN diode as the MOS gate. Figure 6 presents the normalized photocurrent as a function of V BG. For higher LI, the observed curve is similar to that of experimental data, however, as LI decreases, the accumulation photocurrent becomes smaller than the inversion one, thanks to the higher recombination rate in accumulation and the N D change is more pronounced. Conclusions: In this work the influence of back-gate bias and the intrinsic length variation on the performance of lateral SOI PIN photodiodes was presented. Experimental results demonstrated that the operation mode of the photodiodes was affected by backgate bias, modifying the photogenerated current, which presents its maximum value when the silicon film is laterally depleted, indicating minimal carriers recombination. Two-dimensional numerical simulations were used to reproduce the experimental data and showed that the choice of the intrinsic length