Study Of Lateral PIN Photodiode Gated By Transparent Electrode Based On SOI

Based on the analysis of principle for photodiode, especially for PIN photodiode and SOI Lateral PIN photodiode (LPIN PD), Lateral PIN Photodiode Gated by Transparent Electrode (LPIN PD-GTE) was reported in this paper combining the advantages of bipolar devices, MOS devices and SOI thin film devices. Subsequently, physical models, analytical models and equivalent circuit models of LPIN PD-GTE were established by solving the basic semiconductor physics equations. In the end, characteristics in LPIN PD-GTE were verified by SILVACO.In LPIN PD-GTE, the absorption by gate electrode is the main loss of incident light. Thus, in order to obtain good photoelectric properties and high quatumn efficiency, a kind of transparent electrode with high transmission should be adopted as the gate is LPIN PD-GTE. Thereby, electrical and optical properties of pure ZnS and La-doped ZnS were studied by first principles calculations and experiment.2×1×1and3×2×1ZnS supercell models were adopted to realize different concentration of doped La. Numerical calculations were carried out by CASTEP in Material Studio4.0based on first principles. Calculation results show that La-doping narrows the band gap of ZnS systems, and La doped ZnS system changes from semiconductor into metal through the Mott transition. Moreover, with La-doping increasing, the decrease of absorption coefficient and redshift of absorption spectra are obtained. Absorption spectra of pure ZnS and La-doped ZnS are in good agreement with the experimental results. Besides, optical transmittance increased as the effect of La doping, with a transmittance of more than80%for wavelength above360nm in La-doped ZnS thin films. Therefore, La-doped ZnS thin film could be adopted as gate electrode in LPIN PD-GTE at wavelength X.=400nm.In LPIN PD-GTE, the gate voltage VGK plays a very important role. The models of VGK in partially depleted SOI device and fully depleted SOI device were established by solving semiconductor physical equations and validated by ATLAS. Comparing to partially depleted SOI LPIN PD-GTE, fully depleted SOI LPIN PD-GTE (FD SOI LPIN PD-GTE) could obtain lower dark current and better frequency characteristics. Consequently, FD SOI LPIN PD-GTE is detailedly studied in this paper. The photoelectric characteristics of LPIN PD-GTE were studied by ATLAS and compared to that of LPIN PD with the same parameters. In LPIN PD-GTE, photocurrent increases along with the increasing VGK in the range of0-0.5V. In addition, the longer channel length, the larger increment of photocurrent in LPIN PD-GTE. At L=2μm, the photocurrent of LPIN PD-GTE under different VGK is almost the same with that of LPIN PD. With increasing channel length, comparing with LPIN PD, the influence of VGK to photocurrent becomes more obvious in LPIN PD-GTE. In LPIN PD-GTE, different from LPIN PD, dark current increases with the increasing channel length. Under the same parameters, dark current in LPIN PD-GTE is larger than thar of LPIN PD. However, the photo-to-dark-current contrast ratio is beyong106in LPIN PD-GTE. Furthmore, the responsibility of LPIN PD-GTE is larger than that of LPIN PD due to VGK. In addition, for different VGK, the Full-Widths at Half-Maximum (FWHM) is in the range of290nm-490nm, and peak value locates at λ=400nm.Models of Internal Quantumn Efficiency (QI) and Quantumn Efficiency (QE) in LPIN PD-GTE were established by solving Laplace equations and verified by ATLAS in SILVACO. When VGK=0, QI decreases with increasing channel length. While, QI increases along with VGK increasing from OV to0.5V, and the longer channel length, the larger increment of QI in LPIN PD-GTE. However, due to weak inversion with VGK exceeding0.5V, QI in LPIN PD-GTE keeps invariant. At VGK=0.5V, for QI in LPIN PD-GTE, calculation results of1-D modles are in good agreement with simulation results by ATLAS. In LPIN PD-GTE, for different channel length, the trends of variation for QE are consistent with that of QI. As transparent thin film with transmittance above80%at λ=400nm is adopted as gate electrode in LPIN PD-GTE, QE of LPIN PD-GTE is larger than that of LPIN PD under the same parameters due to VGK.As the channel length increases, the greater increasing magnitude of QE could be achieved in LPIN PD-GTE comparing to that in LPIN PD. With L=50μm and the other same parameters, comparing with QE in LPIN PD, the magnitude of increase for QE in LPIN PD-GTE is up to50%.In LPIN PD-GTE, the frequency characteristic is dominated by transit time of carriers and RC constant. The small-signal equivalent circuit models of LPIN PD-GTE were established based on transmission line equation. Compared to traditional MOSFET, lower output capacity of A electrode could be achieved in LPIN PD-GTE. In LPIN PD-GTE, the cut-off frequency fT increase with increasing VGK and decrease with increasing channel length. Different from LPIN PD, capacity of A electrode is dominated by gate capacity in LPIN PD-GTE. Thus, the output capacity of A electrode in LPIN PD-GTE is larger than that in LPIN PD. However, the frequency characteristics of LPIN PD-GTE is still domoniated by carrier transmit time due to minor absolue value of output capacity. Due to VGK, LPIN PD-GTE could get lower carrier transimit time accorresponding to higher cut-off frequency and bigger highest frequency comparing to LPIN PD with the same channel length. At L=10μm and VGK=0.5V, the cut-off frequency and highest frequency in LPIN PD-GTE are respectively30MHz and100GHz. Whereas, in LPIN PD, the the cut-off frequency and highest frequency are3MHz and1GHz, respectively. As the contribution of RC constant to frequency characteristics becomes more important with increasing channel length, the amptitude of increase for frequency in LPIN PD-GTE becomes smaller comparing to that in LPIN PD. At L=4μm, VGK=0.5V, the bandwidth of LPIN PD-GTE is up to250MHz, which meets the application requirements of Blu-ray DVD system.In a word, large photocurrent, high quantum efficiency and good frequency characteristics could be achieved in LPIN PD-GTE due to gate voltage VGK-Thereby, LPIN PD-GTE can be applied in optical storage or communication system requiring high sensitivity and frequency characteristics, such as Blu-ray DVD system. It is worth noting that the quantum efficiency and frequency characteristics vary different orientation according to channel length. Thus, choice should be carried out between quantum efficiency and frequency characteristics in LPIN PD-GTE for specific application.