HBT空间电荷区复合电流的解析模型

建立了异质结双极晶体管ＥＢ结空间电荷区复合电流的解析模型,基于该模型计算出了不同基区掺杂浓度下空间电荷区的复合率,获得了空间电荷区复合电流随外加电压的变化关系。     

高温Al_(0.3)Ga_(0.22)In_(0.48)P/GaAsHBT电流增益的计算分析

建立了Ａｌ０．３Ｇａ０．２２Ｉｎ０．４８Ｐ／ＧａＡｓ异质结双极型晶极管（ＨＢＴ）中电流输运过程的模型,利用实验得到的材料特性参数进行了ＨＢＴ电流增益随温度变化的模拟．随着温度上升,小电流时电流增益下降较多,而大电流时电流增益基本保持不变．模拟表明,小电流下电流增益的下降主要是由ｅｂ结空间电荷区的复合电流随温度增加而造成的;而大电流下电流增益直至７２３Ｋ下降仍小于１０％．最高工作温度可达８４８Ｋ．由于采用的计算方法充分考虑了空间电荷区复合电流的影响,模拟结果较为符合实际情况,可为研制高性能ＨＢＴ器件所需材料     

The effect of grain boundary shunt currents on mis solar cell performance

The degradation of solar cells by grain boundaries can take any of three forms: recombination of minority carriers, forward current due to recombination in the space charge region of the junction or Schottky barrier, or forward currents due to shunting. There is no doubt that minority carrier recombination occurs and degrades the short circuit current. There seems little doubt that grain boundaries also degrade the solar cell open circuit voltage, but whether the degradation is due to recombination in the space charge region or due to shunting is not clear. To date most attention has been paid to space charge recombination. There is data, however, that shunt currents can flow, especially in doped regions of the grain boundaries with the conductivity along the grain boundary estimated from that data to be 10⁻¹² to 10⁻⁵ mhos/square. We will present an analysis assuming such grain boundary conductivities and show that the predictions of such a shunt model are in agreement with experiment. Specifically the shunt current is predicted to increase exponentially as qV/2kT where V is the forward bias, the shunt current significantly lowers the open circuit voltage, and it has negligible effect on the short circuit current.     

Thermionic emission-diffusion theory of isotype heterojunctions

A closed-form expression for the current-voltage characteristics of isotype heterojunctions is developed. Unlike previous treatments of this problem which considered only thermionic emission, this theory, in analogy to metal-semiconductor theory, includes both transport by thermionic emission across the interface and by drift-diffusion in the space charge region. A representative isotype heterojunction is analyzed and it is found that both transport mechanisms must be considered when calculating the current-voltage characteristics.     

Effects of displaced p-n junction of heterojunction bipolartransistors

Two-dimensional simulations that demonstrate the effects of displacements of the p-n junctions from the heterojunctions of symmetrical Al_0.28Ga_0.72/GaAs double-heterojunction bipolar transistors (DHBTs) are reported. When the emitter and/or collector p-n junctions do not coincide with the AlGaAs/GaAs heterojunctions, the electrical characteristics are shown to be drastically altered due to changes in the potential profiles and to changes in recombination rates both in the neutral base and in the space-charge region of the emitter. The effects of a small displacement of the p-n junction from the emitter-base or the base-collector heterojunctions are examined and results for current gain β and cutoff frequency f_T are given that demonstrate enhanced performance for DHBTs with p-n junctions that are not coincident with the heterojunctions     

Charge transport mechanisms in anisotype n-TiO2/p-Si heterostructures

Anisotype n-TiO₂/p-Si heterojunctions are fabricated by the deposition of a TiO₂ film on a polished poly-Si substrate using magnetron sputtering. The electrical properties of the heterojunctions are investigated and the dominant charge transport mechanisms are established; these are multi-step tunneling recombination via surface states at the metallurgical TiO₂/Si interface at low forward biases V and tunneling at V > 0.6 V. The reverse current through the heterojunctions under study is analyzed within the tunneling mechanism.     

Bidirectional rectifier with gate voltage control based on Bi2O2Se/WSe2 heterojunction

Two-dimensional (2D) WSe₂ has received increasing attention due to its unique optical properties and bipolar behavior. Several WSe₂-based heterojunctions exhibit bidirectional rectification characteristics, but most devices have a lower rectification ratio. In this work, the Bi₂O₂Se/WSe₂ heterojunction prepared by us has a type Ⅱ band alignment, which can vastly suppress the channel current through the interface barrier so that the Bi₂O₂Se/WSe₂ heterojunction device has a large rectification ratio of about 10⁵. Meanwhile, under different gate voltage modulation, the current on/off ratio of the device changes by nearly five orders of magnitude, and the maximum current on/off ratio is expected to be achieved 10⁶. The photocurrent measurement reveals the behavior of recombination and space charge confinement, further verifying the bidirectional rectification behavior of heterojunctions, and it also exhibits excellent performance in light response. In the future, Bi₂O₂Se/WSe₂ heterojunction field-effect transistors have great potential to reduce the volume of integrated circuits as a bidirectional controlled switching device.     

Time‐of‐Flight Studies of Electron‐Collection Kinetics in Polymer:Fullerene Bulk‐Heterojunction Solar Cells

The charge‐collection dynamics in poly(3‐hexylthiophene:[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) bulk heterojunctions are studied in thick (>1 μm) devices using time‐of‐flight measurements and external quantum‐efficiency measurements. The devices show Schottky‐diode behavior with a large field‐free region in the device. Consequently, electron transport occurs by diffusion in the bulk of the active layer. At high applied biases where the depletion region spans the entire active layer, normal time‐of‐flight transients are observed from which the electron mobility can be determined. Here, the electron mobility follows Poole–Frenkel behavior as a function of field. At lower applied biases, where the depletion region only spans a small portion of the active layer, due to a high density of dark holes, the recombination kinetics follow a first‐order rate law with a rate constant about two orders of magnitude lower than that predicted by Langevin recombination.     

Recombination at surfaces with statistically distributed energy levels

One of the major problems in the measurement of the recombination lifetime in semiconductor wafers is how to distinguish the bulk, surface and interface contributions to the total recombination process. Distinguishing of these two effects requires a good understanding of the bulk, surface and interface recombination processes. The contribution of surfaces and interfaces to the total recombination process is usually expressed by two linear boundary conditions and taking a constant (S) as the surface recombination velocity. It is shown that this model is valid only under special conditions. A new model for the contribution of surfaces and interfaces to the total recombination process is presented. This model proves that the contribution of surfaces and interfaces are not only determined by the parameters concerning the recombination at the surfaces but also by the parameters concerning the transport of the free carriers through the space charge region towards surfaces and interfaces.     

Experimental comparison of base recombination currents in abrupt and graded AlGaAs/GaAs heterojunction bipolar transistors

The relative importance of the base bulk recombination current and the base-emitter junction space charge recombination current is examined for AlGaAs/GaAs HBTs with different grading schemes in the base-emitter junction. Experimental results demonstrate that, in abrupt HBTs, the base bulk recombination current is larger, and the base current increases with the base-emitter bias with an ideality factor of approximately 1. In contrast, in graded HBTs, the space charge recombination current dominates and the base current ideality factor is approximately 2. These experimental results agree well with a published theoretical calculation.<>     

Improved analysis of pulsed C-t measurements on silicon MOS capacitors

A modified analysis of the pulse response of silicon MOS capacitors is presented which takes into account the lateral spreading of the depletion region around the gate area. Two aspects of this lateral depletion region which have previously been ignored, namely the bulk generation and space charge in this region, are taken into account and used to explain the experimentally observed dependence of the capacitor relaxation time on the device aspect ratio. The analysis allows the bulk lifetime to be obtained more accurately and also enables the surface recombination velocity resulting from a particular device processing schedule to be estimated. The results obtained agree well with those determined from junction leakage current measurements and the relevance of the lateral edge effect to the characteristics of a more complex charge coupled device has been discussed.     

Impact of fluorine incorporation in the polysilicon emitter of NPNbipolar transistors

We demonstrate that fluorine incorporation in the polysilicon emitter of NPN bipolar transistors significantly reduces the current gain h_fe. The gain degradation can be related to a reduction of the barrier to hole transport at the poly-Si/mono-Si interface. In addition to a gain reduction, fluorinated-emitter transistors display lower base recombination currents (at low base-emitter biases) than nonfluorinated emitter devices, suggesting that fluorine passivates recombination centers in the emitter-base space charge region     

Analysis of leakage currents in 1.3-µm InGaAsP real-index-guided lasers

We describe leakage current calculation in several real-index-guided laser structures using an electrical equivalent circuit model. The structures analyzed are different types of buried heterostructures in which heterojunctions are used for lateral carrier confinements. The device types are: 1) the etched-mesa buried heterostructure (EMBH); 2) the channeled-substrate buried heterostructure (CSBH); 3) the double-channel planar buried heterostructure (DCPBH); 4) the planar buried heterostructure (PBH); and 5) the buried crescent (BC). Adequate current confinement is necessary in a laser structure for both linearity and low threshold operation. Thus leakage current, i.e., the difference between the injected current and the current through the active region, should be small. We have identified the main leakage paths in these structures and the parameters (the relevant layer thicknesses and doping levels) that determine the magnitude of the leakage current. The effect of nonradiative recombination sites on junction parameters and the consequent increase in leakage current is discussed.     

State‐of‐the‐Art Progress in Diverse Heterostructured Photocatalysts toward Promoting Photocatalytic Performance

Semiconductor photocatalysts have received much attention in recent years due to their great potentials for the development of renewable energy technologies, as well as for environmental protection and remediation. The effective harvesting of solar energy and suppression of charge carrier recombination are two key aspects in photocatalysis. The formation of heterostructured photocatalysts is a promising strategy to improve photocatalytic activity, which is superior to that of their single component photocatalysts. This Feature Article concisely summarizes and highlights the state‐of‐the‐art progress of semiconductor/semiconductor heterostructured photocatalysts with diverse models, including type‐I and type‐II heterojunctions, Z‐scheme system, p–n heterojunctions, and homojunction band alignments, which were explored for effective improvement of photocatalytic activity through increase of the visible‐light absorption, promotion of separation, and transportation of the photoinduced charge carries, and enhancement of the photocatalytic stability.     

Space charge region effects in bidirectional illuminated P3HT:PCBM bulk heterojunction photodetectors

Organic semiconductors are widely investigated for their application in photovoltaics and photodetectors. We show that the efficiency of these devices is strongly influenced by the position of the space charge region, due to unintentional doping, and wavelength-dependent absorption properties in bulk heterojunctions. Spray-coated P3HT:PCBM bulk heterojunction photodiodes with thicknesses up to 4.2 μm and semitransparent top contact enable the characterization of exciton generation and separation in both irradiation directions. A large difference in external quantum efficiency (EQE) is observed for top and bottom illuminated configurations and is explained by a bias dependent arrangement of the space charge region at the two contact electrodes. Numerical drift–diffusion simulations allow to get insight into first order mechanisms behind the spectral features of EQE data in highly-doped organic photodiodes.     

Temperature dependence of the threshold current density of aquantum dot laser

Detailed theoretical analysis of the temperature dependence of threshold current density of a semiconductor quantum dot (QD) laser is given. Temperature dependences of the threshold current density components associated with the radiative recombination in QDs and in the optical confinement layer (OCL) are calculated. Violation of the charge neutrality in QDs is shown to give rise to the slight temperature dependence of the current density component associated with the recombination in QD's. The temperature is calculated (as a function of the parameters of the structure) at which the components of threshold current density become equal to each other. Temperature dependences of the optimum surface density of QD's and the optimum thickness of the OCL, minimizing the threshold current density, are obtained. The characteristic temperature of QD laser T_o is calculated for the first time considering carrier recombination in the OCL (barrier regions) and violation of the charge neutrality in QDs. The inclusion of violation of the charge neutrality is shown to be critical for the correct calculation of T_o. The characteristic temperature is shown to fall off profoundly with increasing temperature. A drastic decrease in T_o is shown to occur in passing from temperature conditions wherein the threshold current density is controlled by radiative recombination in QD's to temperature conditions wherein the threshold current density is controlled by radiative recombination in the OCL. The dependences of T_o on the root mean square of relative QD size fluctuations, total losses, and surface density of QDs are obtained     

Mechanisms of charge transport in anisotype <Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-CdTe heterojunctions

Surface-barrier anisotype n-TiO₂/p-CdTe heterojunctions are fabricated by depositing thin titanium-dioxide films on a freshly cleaved surface of single-crystalline cadmium-telluride wafers by reactive magnetron sputtering. It is established that the electric current through the heterojunctions under investigation is formed by generation-recombination processes in the space-charge region via a deep energy level and tunneling through the potential barrier. The depth and nature of the impurity centers involved in the charge transport are determined.     

Space-charge region recombination in heterojunction bipolartransistors

A new comprehensive model for space-charge region (SCR) recombination current in abrupt and graded energy gap heterojunction bipolar transistors (HBTs) is derived. It is shown that if a spike is present in one of the bands at the heterojunction interface, the SCR recombination current becomes interrelated with the collector current. A previously proposed charge control model for the HBT is modified to include the SCR recombination current. The model is used to study SCR recombination characteristics in HBTs     

Low 1/f noise characteristics of AlGaAs/GaAs heterojunction bipolartransistor with electrically abrupt emitter-base junction

It is shown that the use of an electrically abrupt emitter-base junction considerably reduces the 1/f noise of self-aligned AlGaAs/GaAs heterojunction bipolar transistor (HBT). Although this device does not have depleted AlGaAs ledge passivation layer, the low-frequency noise spectra show a very low 1/f noise corner frequency of less than 10 kHz, which is much lower than previously reported value of about 100 kHz from conventional passivated or unpassivated AlGaAs/GaAs HBT's. Except for a residual generation-recombination (g-r) noise component, the noise power is comparable to that of Si BJT. It is also found that the low-frequency noise power of the AlGaAs/GaAs HBT is proportional to the extrinsic GaAs base surface recombination current square. Unlike the other HBT's reported, the noise sources associated with interface state and emitter-base (E-B) space charge region recombination are not significant for our device     

THE EFFECT OF DOPANT OUTDIFFUSION ON THE NEUTRAL BASE RECOMBINATION CURRENT IN Si/SiGe/Si HETEROJUNCTION BIPOLAR TRANSISTORS

A new analytical model for the base current of Si/SiGe/Si heterojunction bipolar transistors(HBTs) has been developed. This model includes the hole injection current from the base to the emitter, and the recombination components in the space charge region(SCR) and the neutral base. Distinctly different from other models, this model includes the following effects on each base current component by using the boundary condition of the excess minority carrier concentration at SCR boundaries: the first is the effect of the parasitic potential barrier which is formed at the Si/SiGe collector-base heterojunction due to the dopant outdlffusion from the SiGe base to the adjacent Si collector, and the second is the Ge composition grading effect. The effectiveness of this model is confirmed by comparing the calculated result with the measured plot of the base current vs. the collector-base bias voltage for the ungraded HBT. The decreasing base current with the increasing the collector-base reverse bias voltage is successfully explained by this model without assuming the short-lifetime region close to the SiGe/Si collector-base junction, where a complete absence of dislocations is confirmed by transmission electron microscopy (TEM)[1]. The recombination component in the neutral base region is shown to dominate other components even for HBTs with a thin base, due to the increased carrier storage in the vicinity of the parasitic potential barrier at collector-base heterojunction.