| In recent years, GaN-based p-i-n ultraviolet detectors attract wild attention with its natural solar-blind property and quick response speed. GaN-based material has the advantages of small permittivity, high electron saturation velocity and wide energy band gap, and has been widely applied in the fields of microwave power devices, photo electronic devices. p-i-n structure detectors play an important role in the field of photo electronic devices duo to its advantages of low dark current, quick response speed and working well at the bias of zero voltage. In this paper, GaN-based p-i-n ultraviolet detector is researched systematically, and the author’s major contributions are outlined at follows:1. Based on the semiconductor classic physics and the limited-difference method, the numerical calculation of energy band diagram and carriers distribution of the GaN-based homostructure and heterostructure is realized by Matlab program. For the GaN-based homostructure, the energy band diagram and carriers distribution are calculated by self-consistent solving of carriers statistics equation and Poisson equation. For the AlGaN/GaN heterostructure, the energy band diagram and carriers distribution are calculated by self-consistent solving of Schrodinger-Poisson equations. Based on that, the influence of polarization effect on the energy band diagram and carriers distribution is researched systematically. Finally, compared with the results simulated by the Silvaco software, which is based on the physical models, and calculated by others, the correctness of numerical calculation built in this paper is confirmed. Therefore, the calculation model in this paper is not only based on the physical model, but also can jump the limitation of the existing simulation software based on the physical model, so as to add the new physical mechanism to the numerical calculation for deep research.2. Based on the physical mechanism of photo-generated carriers screening effect, the numerical models of the photo-generated screening effect and the photo-generated parasitical capacitance are presented by self-consistent solving the Poisson equation and carriers’statistic equation. According to the two models, the effect of the photo irradiation on the response time and cut-off frequency of the detector is calculated and discussed. It is concluded that, the photo-generated screening effect and photo-generated parasitical capacitance have some perturbation on the response time and cut-off frequency. The perturbation degree depends on the electric field in depletion region, carriers lifetime and photo irradiation intensity. The detail calculation results show that, when the electric field in depletion region is less than the critical electric field of the mobility model, the photo-generated screening effect makes the response time increase and the cut-off frequency decrease, however, it is just the opposite when the electric field is lager than the critical electric field; the larger the carriers lifetime, the more serious the influence of the photo-generated screening effect on the response time and cut-off frequency, until the carriers lifetime increases to200ps, the influence reaches saturation; the larger the photo irradiation intensity, the more serious the influence of the photo-generated screening effect on the response time and cut-off frequency, however, if the photo irradiation intensity is less than1W/cm2, the influence is very small. Therefore, in application, the influence of the photo-generated screening effect on the response time and cut-off frequency can be reduced by adjusting the bias voltage so as to the electric field in depletion region is near to the critical electric field or large the critical electric field much more. At the same time, a small capacitance at the order of fF degree series to the detector can speed up the response and enhance the cut-off frequency of the detector, and also reduce the influence of the photo-generated parasitical capacitance on the response time and cut-off frequency of the detector.3. Based on the influence of the polarization effect on the energy band diagram of the AlGaN/GaN heterostructure, AlGaN/GaN heterostructure p-i-n ultraviolet detector with dual band response is designed, and the influence of the doping concentration in AlGaN, bias voltage and the degree of polarization relaxation on the response characteristic of the detector is simulated by the above numerical method. The calculation results show that, in the case of fixed doping concentration and fixed degree of polarization relaxation, the transform from single band response to dual band response for the AlGaN/GaN heterostructure p-i-n ultraviolet detector can be realized by adjusting the bias voltage, furthermore, the threshold voltage corresponding to the response transform will be larger if the Al mole fraction of the AlGaN is large or the doping concentration is shallow or the degree of the polarization relaxation is low. Of course, the analytical expression of the threshold voltage is deduced according to the charge distribution in the depletion region. In order to shorten the response time of the detector, it is shown that by the calculation results, the depth of the AlGaN layer should be shorten or the bias voltage should be enlarged so as to avoid long diffusion length for the photo carriers transporting to electrode.4. Based on the influence of the polarization effect on the energy band diagram of the AlGaN/GaN heterostructure, AlGaN/AlGaN/GaN double heterostructure p-p-i-n ultraviolet detector with three band response is designed, and the influence of the doping concentration in AlGaN, bias voltage and the Al mole fraction of the AlGaN on the response characteristic of the detector is simulated by the above numerical method. The calculation results show that, in the case of fixed doping concentration and fixed Al mole fraction of AlGaN, influenced by the polarization charges, the three band response can be realized for the AlGaN/AlGaN/GaN double heterostructure p-p-i-n ultraviolet detector by adjusting the bias voltage. Of course, the analytical expression of the threshold voltage corresponding to the three band response is deduced according to the charge distribution in the depletion region. It is conclude that from the calculation results, at the same time, the Al more fraction of the top AlGaN layer should be larger than that of the bottom AlGaN layer, the doping concentration in AlGaN should be much more heavy with the increasing of the Al mole fraction of the AlGaN layer, and the detector with arbitrary three band response in the range from200nm to400nm can be realized by adjusting the Al mole fraction of each AlGaN layer.Besides, when the photo-generated screening effect considered, it is shown from the calculation results, under the condition of three band response, the photo-generated screening effect has a perturbation effect on the cut-off frequency of the detector, and the perturbation effect decreases with the the increasing of the incident wavelength. |
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