光电子学第5章_光电探测器.ppt

Optoelectronics and Photonics Principles and Practices,Yang Jun Photonics Research Center School of Science Harbin Engineering University 2008.12,专业词汇选编,Free electron hole pairs ( EHPs ):自由电子空穴对 Photodiode:光探测器 Pyroelectric pairoilectric detector:热探测器 Acceptor：受主 Donor:施主 Antireflection coating: 抗反射膜、增透膜 Depletion region:耗尽区 Space charge layer:空间电荷层 Built-in voltage:内建电场 Neutral regions:中性区 Photogenerate:光生 Photocurrent: 光电流,专业词汇选编,Drift velocity:漂移速度 Transit time:渡越时间 Upper cut-off wavelength:长波截至波长 Absorption coefficient:吸收系数 Penetration depth:穿透深度 Direct bandgap:直接带隙 Indirect bandgap:间接带隙 Phonon Momentum:声子动量 Lattice vibration:晶格振动 Quantum efficiency of the detector:探测器的量子效率 External quantum efficiency:外量子效率 Responsivity :响应度 Spectral responsivity ( radiant sensitivity )光谱响应度(辐射响应度),光电探测器(Photodetector),光纤技术应用中，不可避免的会遇到将光辐射转换成易于测量和处理的电学量的问题，亦即光辐射的探测问题。光辐射探测技术是光纤技术的一个重要组成部分，而用于探测光辐射的器件通常称之为光探测器。 在光纤技术的大多数应用中都需要将光辐射信号转换成电信号（或图像信息），光探测器是上述应用中实现光电转换的关键元件，光探测器性能的优劣将影响整个探测系统的性能。 此外，利用将光辐射信号转换成电信号以进行显示或控制的功能，光探测器不仅可以代替人眼，而且由于其光谱响应范围宽，更是人眼的延伸。,光纤技术中几种典型的光电探测器,InGaAs-PIN 光电二极管,PIN-TIA 接收组件,Si-PIN 光电二极管,光探测器之所以能探测光辐射就是因为光辐射（即光频电磁波）传输能量。入射到光探测器上的光辐射使之产生光生载流子（或发射光电子）或使其本身的特性（如电阻、温度等）发生变化。 根据上述光辐射响应方式或工作机理的不同，前者称之为光电效应，后者称之为光热效应，由此构成的光探测器分别称为光子探测器和热探测器。 而光子探测器又分为：光电子发射探测器、光电导探测器、光伏探测器、光子牵引探测器；热探测器又分为：热探测器：测辐射热电偶、测辐射热计、热释电探测器、气动探测器。,光电探测器(Photodetector),Photodetectors,5.1 Principle of the pn Junction Photodiode 5.2 Ramos theorem and external photocurrent 5.3Absorption Coefficient and Photodiode Materials 5.4 Quantum Efficiency and Responsivity 5.5 The PIN Photodiode 5.6 Avalanche Photodiode 5.7 Heterojunction Photodiodes 5.8 Phototransistors 5.9 Noise in Photodetectors,5.1 Principle of the pn junction photodiode,Photodetectors convert a light signal to an electrical signal such as a voltage or current. In photoconductors and photodiodes, this conversion is typically achieved by the creation of free electron hole pairs by the absorption of photons. In pyroelectric detectors the energy conversion involves the generation of heat which increases the temperature of the device which changes its polarization and hence its relative permittivity. pn junction based photodiode type devices only as these devices are small and have high speed and good sensitivity for use in various optoelectronics. The most important application is in optical communications.,Figure 5.1 A schematic diagram of a reverse biased pn junction photodiode. Net space charge across the diode in the depletion region. Nd and Na are the donor and acceptor concentrations in the p and n sides. The field in the depletion region.,The figure5.1 (a) shows the simplified structure of a typical pn junction photodiode that has a p+ n type of junction. The illuminated side has a window, defined by an annular electrode, to allow photons to enter the device. There is an antireflection coating, typically Si3N4, to reduce light reflections. The p side is generally very thin (less than a micron) and is usually formed by planar diffusion into an n-type epitaxial layer.,Figure5.2 (b) shows the net space charge distribution across the p+n junction. These charges are in the depletion region, or in the space charge layer, and represent the exposed negatively charged acceptors in the p+ side and exposed positively charged donors in the n-side. The depletion region extends almost entirely into the lightly doped n-side and, it is a few microns.,Principle of the pn junction photodiode,The photodiode is normally reverse biased, the applied reverse bias Vr drops across the highly resistive depletion layer width W and makes the voltage across W equal to Vo+Vr where Vo is the built-in voltage. The field is found by the integration of the net space charge density net across W subject to a voltage difference of Vo+Vr . The field only exists in the depletion region and is not uniform. It varies across penetrates into the n-side. The regions outside the depletion layer are the neutral regions in which there are majority carriers. It is sometimes convenient to treat these neutral regions simply as resistive extensions of electrodes to the depletion layer.,When a photon with an energy greater than the bandgap Eg is incident, it becomes absorbed to photogenerate a free EHP. Usually, the photogeneration takes place in the depletion layer. The field E in the depletion layer separates the EHP and drifts them in opposite directions until they reach the neutral regions. Drifting carriers generate a current, called photocurrent Iph, in the external circuit that provides the electrical signal. The photocurrent Iph depends on the number of EHPs photogenerated and the drift velocities of the carriers while they are transiting the depletion layer. The photocurrent in the external circuit is due to the flow of electrons, not to both electrons and holes.,Photodetectors,5.1 Principle of the pn Junction Photodiode 5.2 Ramos theorem and external photocurrent 5.3Absorption Coefficient and Photodiode Materials 5.4 Quantum Efficiency and Responsivity 5.5 The pin Photodiode 5.6 Avalanche Photodiode 5.7 Heterojunction Photodiodes 5.8 Phototransistors 5.9 Noise In Photodetectors,5.2 Ramos theorem and external photocurrent,Consider a semiconductor material with a negligible dark conductivity. The electrodes do not inject carriers but allow excess carriers in the sample to leave and become collected by the battery. The field E in the sample is uniform and it is V/L.,Figure 5.2 (a) An EHP is photogenerated at x = l. The electron and the hole drift in opposite directions with drift velocities vh and ve.,Suppose that a single photon is absorbed st a position x = l from the left electrode and instantly creates an electron hole pair. Transit time: is the time it takes for a carrier to drift from its generation point to the collecting electrode.,Figure 5.2 (b) The electron arrives at time te = (L l)/ve and the hole arrives at time th = l/vh.,Consider first only the drifting electron. Suppose that the external photocurrent due to the motion of this electron is ie(t).,Work done = eEdx = Vie(t)dt,Using E = V/L and ve = dx/dt we find the electron photocurrent,The current continues to flow as long as the electron is drifting. It lasts for a duration te at the end of which the electron reaches the battery. Thus although the electron has been photogenerated instantaneously, the external photocurrent is not instantaneous and has a time spread.,Electron photocurrent,Hole photocurrent,The total external current will be the sum of ie(t) and ih(t). Evaluate the collected charge Qcollected,Figure 5.2 (d),This result can be verified by evaluating the area under the iph(t) curve in Figure 5.2 (d). Ramos theorem,Photodetectors,5.1 Principle of the pn Junction Photodiode 5.2 Ramos theorem and external photocurrent 5.3Absorption Coefficient and Photodiode Materials 5.4 Quantum Efficiency and Responsivity 5.5 The pin Photodiode 5.6 Avalanche Photodiode 5.7 Heterojunction Photodiodes 5.8 Phototransistors 5.9 Noise In Photodetectors,5.3 Absorption coefficient and photodiode materials,The photon absorption process for photogeneration, that is the creation of EHPs, requires the photon energy to be at least equal to the bandgap energy Eg of the semiconductor material to excite an electron from the valence band (VB) to the conduction band (CB). The upper cut-off wavelength (or the threshold wavelength) g for phhotogenerative absorption is therefore determined by the bandgap energy Eg of the semiconductor so that,or,For example :,Si Eg = 1.12eV, g is 1.11m; Ge Eg = 0.66eV, g is 1.87m;,From above, it is clear tat Si photodiodes cannot be used in optical communications at 1.3 and 1.55m whereas Ge photodiodes are commercially available for use at these wavelengths.,Incident photons with wavelengths shorter than g become absorbed as they travel in the semiconductor and the light intensity, which is proportional to the number of photons, decays exponentially with distance into the semiconductor. The light intensity I at a distance x from the semiconductor surface is given by Absorption coefficient Where Io is the intensity of the incident radiation and is the absorption coefficient that depends on the photon energy or wavelength . Absorption coefficient is a material property. Most of the photon absorption (63%) occurs over a distance 1/ and 1/ called the penetration depth .,Figure 5.3 The absorption coefficient () vs. wavelength () for various semiconductors,In direct bandgap semiconductors such as III-V semiconductors (e.g. GaAs, InAs, InP, GaSb) and in many of their alloys (e.g. InGaAs, GaAsSb) the photon absorption process is a direct process that requires no assistance from lattice vibrations. The photon is absorbed and the electron is excited directly from the valance band to the conduction band without a change in its k-vector inasmuch as the photon momentum is very small. The change in the electron momentum from the valence to the conduction band This process corresponds to a vertical transition on the E-k diagram in Figure 5.4(a).,In indirect bandgap semiconductors such as Si and Ge, the photon absorption near Eg requires the absorption and emission of lattice vibrations, that is phonons, during the absorption process as shown in Figure 5.4 (b). If K is the wavevector of a lattice wave ( lattice vibrations travel in the crystal ), then K is a phonon momentum. When an electron in the valence band is excited to the condu-ction band there is a change in its momentum in the crystal and this change in the momentum cannot be supplied by the mo-mentum of the incident photon which is very small. Thus, the momentum difference must be balanced by a phonon momen-tum:,The absorption process is said to be indirect as it depends on lattice vibrations which in turn depend on the temperature. Since the interaction of a photon with a valence electron needs a third body, a lattice vibration, the probability of photon absorption is not as high as in a direct transition. Furthermore, the cut-off wavelength is not as sharp as for direct bandgap semiconductors. During the absorption process, a phonon may be absorbed or emitted. If is the frequency of the lattice vibrations then the phonon energy is h . The photon energy is where is the photon frequency. Conservation of energy requires that Thus, the onset of absorption does not exactly coincide with Eg, but typically it is very close to Eg inasmuch as is small (0.1eV). The absorption coefficient initially rise slowly with decreasing wavelength from about g as apparent in Figure 5.3 for Ge and Si.,Choice of material for photodiode,The choice of material for a photodiode must be such that the photon energies are greater than Eg. Further, at the wave-length of radiation, the absorption occurs over a depth covering the depletion layer so that the photogenerated EHPs can be separated by the field and collected at the electrodes. If the absorption coefficient is too large then absorption will occur very near the surface of the p+ layer which is outside the depletion layer. First, the absence of a field means that the photogenerated electron can only make it to the depletion layer to cross to the n-side by diffusion.,Choice of material for photodiode,Secondly, photogeneration near the surface invariably leads to rapid recombination due to surface defects that act as recombination centers. On the other hand, if the absorption coefficient is too small, only a small portion of the photons will be absorbed in the depletion layer and only a limited number of EHPs can be photogenerated.,Photodetectors,5.1 Principle of the pn Junction Photodiode 5.2 Ramos theorem and external photocurrent 5.3Absorption Coefficient and Photodiode Materials 5.4 Quantum Efficiency and Responsivity 5.5 The pin Photodiode 5.6 Avalanche Photodiode 5.7 Heterojunction Photodiodes 5.8 Phototransistors 5.9 Noise In Photodetectors,5.4 Quantum Efficiency and Responsivity,The quantum efficiency (QE) is the number of the photo-carrier pairs generated per incident photon of energy hv and is given by The measured photocurrent Iph in the external circuit is due to the flow of electrons per second to the terminals of the photodiode. Number of electrons collected per second is Iph/e. If Po is the incident optical power then the number of photons arriving per second is Po/ hv. Then the QE can also be defined by,QE can be increased by reducing the reflections at the semiconductor surface, increasing absorption within the depletion layer and preventing the recombination or trapping of carriers before they are collected. To achieve a high quantum efficiency, the depletion layer must be thicker. However, the thicker the depletion layer, the longer it takes for the photo-generated carriers to drift across the reverse-biased junction. Compromise has to be made between response speed and quantum efficiency.,The performance of a photodiode is often characterized by the spectral responsivity R. This is related to the quantum efficiency h by,Representative values are 0.65-A/W for Si at 900- nm and 0.45-A/W for Ge at 1.3-m. For InGaAs, typical values are 0.9-A/W at 1.3-m and 1.0- A/W at 1.55-m.,From the definition of QE, it is clear that,Figure 5.5 Responsivity (R) vs. wavelength ( ) for an ideal photodiode with QE = 100% ( = 1) and for a typical commercial Si photodiode.,Photodetectors,5.1 Principle of the pn Junction Photodiode 5.2 Ramos theorem and external photocurrent 5.3Absorption Coefficient and Photodiode Materials 5.4 Quantum Efficiency and Responsivity 5.5 The pin Photodiode 5.6 Avalanche Photodiode 5.7 Heterojunction Photodiodes 5.8 Phototransistors 5.9 Noise In Photodetectors,专业词汇选编,P-intrinsic-n type photodiode： pin光电二极管 depletion layer capacitance：耗尽层电容 Response time：响应时间 Avalanche photodiode ( APD )：雪崩二极管 Reach-through APD：通达型雪崩二极管 Impact-ionize：碰撞电离 Avalanche of impact ionization processes：碰撞电离的雪崩过程 internal gain mechanism：内增益机制 Excess noise：过剩噪声 Avalanche multiplication factor：雪崩倍乘因子 Primary (unmultipied) photocurrent：初级(非倍增)电流 Avalanche breakdown voltage：雪崩开启电压 Guard ring：地环,5.5 The pin Photodiode,The simple pn junction photodiode has two major drawbacks:,First: its junction or depletion layer capacitance is not sufficiently small to allow photodetection at high modulation frequencies. This is an RC time constant limitation.,Secondly: its depletion layer is at most a few microns. This means that at long wavelengths where the penetration depth is greater than the depletion layer width, the majority of photons are absorbed outside the depletion layer where there is no field to separate the EHPs and drift them.,The QE is correspondingly low at these long wavelengths. These problems are substantially reduced in the pin ( p- intrinsic- n-type) photodiode.,Figure 5.6 The schematic structure of an idealized pin photodiode (b) The net space charge density across the photodiode. (c) The built-in field across the diode. (d) The pin photodiode in photodetection is reverse biased.,The separation of two very thin layers of negative and positive charges by a fixed distance, width W of the i-Si, is the same as that in a parallel plate capacitor. The junction or depletion layer capacitance of the pin diode is given by,A is the cross sectional area, o r is the permittivity of the semiconductor (Si). Since W is fixed by the structure, the junction capacitance does not depend on the applied voltage.,Cdep is typically of the order of a picofarad in fast pin photodiodes so that with a 50 resistor, the R Cdep time constant is about 50 ps.,When a reverse bias voltage Vr is applied across the pin device, it drops almost entirely across the width of i-Si layer. The depletion layer widths of the thin sheets of acceptor and donor charges in the p+ and n+ sides are negligible compared with W.,The reverse bias Vr increases the built-in voltage to Vo + Vr as shown in Figure 5.6 (d).,The field E in the i-Si layer is still uniform and increases to,The pin structure is designed so that photon absorption occurs over the i-Si layer. The photogenerated EHPs in the i-Si layer are then separated by the field E and drifted towards the n+ and p+ sides respectively as illustrated in Figure 5.6 (d).,While the photogenerated carriers are drifting through the i-Si layer they give rise to an external photocurrent which is detected as a voltage across a small resistor R in Figure 5.6 (d),The response time of the pin photodiode is determined by the transit times of the photogenerated carriers across the width W of the i-Si layer.,Increasing W allows more photons to be absorbed which increases the QE but it slows down the speed of response as carrier transit times become longer.,For a charge carrier that is photogenerated at the edge on the i-Si layer, the transit time or drift time tdrift across the i-Si layer is,Where vd is its drift velocity.,To reduce the drift time, that is increase the speed of response, we have to increase vd and therefore increase the applied field E.,Figure 5.7 shows the variation of the drift velocity of electrons and holes with the field in Si.,Example 5.5.15.5.3,Example 5.5.1, P228; Example 5.5.2, P228; Example 5.5.3, P229;,Photodetectors,5.1 Principle of the pn Junction Photodiode 5.2 Ramos theorem and external photocurrent 5.3Absorption Coefficient and Photodiode Materials 5.4 Quantum Efficiency and Responsivity 5.5 The pin Photodiode 5.6 Avalanche Photodiode 5.7 Heterojunction Photodiodes 5.8 Phototransistors 5.9 Noise In Photodetectors,5.6 Avalanche Photodiode,A simplified schematic diagram of a Si reach-through APD is shown Figure 5.9 (a). The n+ side is thin and it is the side that is illuminated through a window. There are three p-type layers of different doping levels next to the n+ layer to suitably modify the field distribution across the diode. The first is a thin p-type layer and the second is a thick lightly p-ty