1、Optoelectronics and PhotonicsPrinciples and PracticesYang JunPhotonics Research Center School of Science Harbin Engineering University2008.12专业词汇选编专业词汇选编Free electron hole pairs(EHPs):自由电子空穴对自由电子空穴对Photodiode:光探测器光探测器Pyroelectric pairoilectric detector:热探测器热探测器Acceptor：受主：受主 Donor:施主施主Antireflection
2、 coating:抗反射膜、增透膜抗反射膜、增透膜Depletion region:耗尽区耗尽区Space charge layer:空间电荷层空间电荷层Built-in voltage:内建电场内建电场Neutral regions:中性区中性区Photogenerate:光生光生Photocurrent:光电流光电流专业词汇选编专业词汇选编Drift velocity:漂移速度漂移速度Transit time:渡越时间渡越时间Upper cut-off wavelength:长波截至波长长波截至波长Absorption coefficient:吸收系数吸收系数Penetration dep
3、th:穿透深度穿透深度Direct bandgap:直接带隙直接带隙Indirect bandgap:间接带隙间接带隙Phonon Momentum:声子动量声子动量Lattice vibration:晶格振动晶格振动Quantum efficiency of the detector:探测器的量子效率探测器的量子效率External quantum efficiency:外量子效率外量子效率Responsivity:响应度响应度Spectral responsivity(radiant sensitivity)光谱响应度光谱响应度(辐射响应度辐射响应度)光电探测器(Photodetector
4、)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
5、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
6、.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 Photodetectors5.1 Principle of the pn junction photod
7、iodePhotodetectors 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 th
8、e 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 importa
9、nt application is in optical communications.Figure 5.1 (a)A schematic diagram of a reverse biased pn junction photodiode.(b)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.(c)The field in the depletion region.The figu
10、re5.1(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
11、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 c
12、harged 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 photodiodegEhv The photodiode is normally reverse biased,the applied reverse bias Vr drop
13、s 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 regi
14、on 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
15、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 region
16、s.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
17、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.
18、6 Avalanche Photodiode 5.7 Heterojunction Photodiodes 5.8 Phototransistors 5.9 Noise In Photodetectors5.2 Ramos theorem and external photocurrentConsider a semiconductor material with a negligible dark conductivity.The electrodes do not inject carriers but allow excess carriers in the sample to leav
19、e 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 elect
20、rode 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.eevlLthhvlt 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 elec
21、tron.Suppose that the external photocurrent due to the motion of this electron is ie(t).Work done =eEdx=Vie(t)dtUsing E=V/L and ve=dx/dt we find the electron photocurrent eeettLevti ;hhhttLevti ;The current continues to flow as long as the electron is drifting.It lasts for a duration te at the end o
22、f 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 photocurrentHole photocurrentThe total external current will be the sum of ie(t)and ih(t).Evaluate the collected c
23、harge Qcollected hethteedttidttiQ00collected transit ;ttLtqvtidFigure 5.2(d)This result can be verified by evaluating the area under the iph(t)curve in Figure 5.2(d).Ramos theoremPhotodetectors 5.1 Principle of the pn Junction Photodiode 5.2 Ramos theorem and external photocurrent 5.3Absorption Coef
24、ficient 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 Photodetectors5.3 Absorption coefficient and photodiode materialsThe photon absorption process for photogeneration
25、,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
26、 therefore determined by the bandgap energy Eg of the semiconductor so that ggEch)(eVEg24.1m orFor 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 avail
27、able for use at these wavelengths.D70.18InSbD3.50.35InAsI1.870.66GeD1.640.75In0.53Ga0.47AsD1.40.89In0.7Ga0.3As0.64P0.36I1.111.12SiD1.081.15GaAs0.88Sb0.12D0.911.35InPTypeg(m)Eg(eV)SemiconductorTABLE5.1 Band gap energy Eg at 300 K,cut off wavelength g and type of bandgap(D=Direct and I=Indirect)for so
28、me photodetector materials.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
29、the semiconductor surface is given byAbsorption coefficientWhere 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/an
30、d 1/called the penetration depth.xIxIoexpFigure 5.3 The absorption coefficient()vs.wavelength()for various semiconductorsIn 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 proces
31、s 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 co
32、nduction band This process corresponds to a vertical transition on the E-k diagram in Figure 5.4(a).0momentumphoton-VBCBkkECBVBkkDirect BandgapEgPhotonEcEv(a)GaAs(Direct bandgap)Ekk(b)Si(Indirect bandgap)VBCBEcEvIndirect Bandgap,EgPhotonPhonon(a)Photon absorption in a direct bandgap semiconductor.(b
33、)Photon absorptionin an indirect bandgap semiconductor(VB,valence band;CB,conduction band)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
34、(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-men
35、tum of the incident photon which is very small.Thus,the momentum difference must be balanced by a phonon momen-tum:KkkVBCBmomentumphonon 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
36、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 fr
37、equency of the lattice vibrations then the phonon energy is h.The photon energy is where is the photon frequency.Conservation of energy requires thatThus,the onset of absorption does not exactly coincide with Eg,but typically it is very close to Eg inasmuch as is small(EgW(a a)(b b)(c c)(d d)VrThe s
38、chematic structure of an idealized pin photodiode(b)The netspace charge density across the photodiode.(c)The built-in fieldacross the diode.(d)The pin photodiode in photodetection isreverse biased.VoutElectrode 1999 S.O.Kasap,Optoelectronics(Prentice Hall)Figure 5.6 The schematic structure of an ide
39、alized 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 t
40、hat in a parallel plate capacitor.The junction or depletion layer capacitance of the pin diode is given byWACrdep0 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
41、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 ac
42、ceptor 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 toWVWVorrEEorVV The pin structure is designed so that photon absorption occu
43、rs 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
44、 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
45、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 isddrifttVWWhere vd is its drift velocity.To reduce the drift time,that is increase the speed of response
46、,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.Drift velocity vs.electric field for holes and electrons in Si.102103104105107106105104Electric field(V m-1)ElectronHoleDrift velocity(m
47、 s-1)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.
48、6 Avalanche Photodiode 5.7 Heterojunction Photodiodes 5.8 Phototransistors 5.9 Noise In Photodetectors5.6 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-
49、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-type doped(almost intrinsic)-layer and the third is a heavily doped p+layer.The diode is reverse biased to increa
50、se the fields in the depletions regions.The net space charge distribution across the diode due to exposed dopant ions is shown in Figure 5.9(b).Under zero bias the depletion layer in the p-region does not normally extend across this layer to the-layer.The electric field is given by the integration o
