An extension of the ideal diode analysis for the heavily doped p-n diode

In this paper, an extension of the ideal-diode analysis for the heavily-doped p-n junction diode is proposed. The heavy doping effects such as carrier degeneracy and band gap narrowing are accounted for by using a tractable empirical approximation for the reduced Fermi-energy given by[12] and employing effective intrinsic density. Under the assumption of low-level injection, it is found that the injected minority-carrier current, and the charge storage in the quasi-neutral regions should depend exponentially on values of F(Y), where F(Y) is a function of dopant dentisy at the depletion edge of the quasi-neutral emitter (or) base region of the p-n junction. Results of our calculations of excess hole current for the short base and the long-base diode show significant change from the values predictged by the conventional diode theory.     

Carrier densities and emitter efficiency in degenerate materials with position-dependent band structure

We consider the electron and hole densities in degenerate and nondegenerate materials with a band structure that is position-dependent and/or having a nonparabolic density of states function. For nondegenerate materials it is shown that the pn-product deviates from its classical value n_i² for two reasons which generally occur conjunctively. First a modifying factor exp (ΔE_g/kT) occurs due to the real change in bandgap. Secondly, a factor exp [(Γ_n+Γ_p)/kT] is found stemming from the modified density of states or apparent change in band gap. These effects can be separated by studing the temperature dependence. Next we calculate the minority carrier current and the emitter efficiency and current gain. It is shown that β_ψ will be degraded by the increased pn-product and will be further affected by the degeneracy of the emitter material, the direction of the change depending on the doping profiles.     

On the recombination of electrons and holes at traps with finite relaxation time

Shockley and Read[1] and Hall[2] (SRH) theory for electron hole recombination at traps is modified to include the effect of a finite time of relaxation before the captured electron or hole settles to the ground state. The modified expression for the recombination rate retains all the essential features of SRH theory and includes two additional effects: at low levels of injection lifetime shows a temperature dependence of the type τ = τ_n0+C_ne^?ΔE_n/kT and at high injection the recombination rate saturates at an upper value N_t/δt where δt is relaxation time and N_t is density of traps.     

一种双发射极沟槽栅超结IGBT

本文对传统沟槽栅超结IGBT进行了改进,得到一种沟槽栅双发射极超结IGBT,本结构第一个发射极区域和传统IGBT结构一样能够发射电子、接收空穴,在p型柱顶部的第二个发射极区域能够起到空穴分流的作用,在有效地提高器件抑制闩锁的能力的同时,保持了超结IGBT器件的高击穿电压(BVoff)和低关断损耗(Eoss)。仿真显示在VGE=10V的条件下,改进结构的闩锁电流从15000A/cm2 提升至 28300A/cm2,器件的击穿电压为810V,在导通压降为1.2V的条件下,关断损耗为6.5 mJ/cm2。     

Emitter space charge layer transit time in bipolar junction transistors

The charge defined emitter space charge layer transit times of double diffused transistors have been calculated using a regional approach, and compared with the corresponding base transit times. The results obtained for emitter space-charge layer transit times have been discussed with reference to Morgan and Smits' capacitance analysis for graded p?n junctions [8].     

An analytical model for the epitaxial bipolar transistor

In the n⁺pn^?n⁺ transistor, high-current effects in the base and collector regions are linked within the current ranges of practical interest. To describe such effects, we have derived an analytical model that is based primarily on five assumptions: (1) the structure is approximately one-dimensional; (2) recombination is negligible in the base and collector quasi-neutral regions, and in the three space-charge regions; (3) high-current effects are negligible in the emitter and n⁺-substrate regions; (4) the Fletcher boundary conditions (or the Misawa boundary conditions) can be used for the three space-charge regions; and (5) the ambipolar approach can be used for the base and collector quasi-neutral regions. The primary findings predicted by the n⁺pn^?n⁺ transistor model are: In current ranges of practical interest (usable current gain), the electron concentration profile has a significant “vertical step” located at the collector-base metallurgical junction for all values of collector current. In the limit of extremely-high-current operation, this step tends to vanish. In the current range where the current gain begins to decline rapidly with increasing collector current, the electron concentration at the base boundary of the collector-base space-charge region goes approximately as the square of the hole concentration at the collector boundary of the same region. Because of this relationship, a charge-control calculation is more difficult than a straightforward calculation of carrier concentration for a given degree of accuracy. The n⁺pn^?n⁺ transistor model (which consists of twelve algebraic equations) is particularly useful for the practically important case of an epitaxial bipolar transistor having a very thin, heavily-doped base region.     

Comparative investigation of InP/InGaAs heterostructure-emitter tunneling and superlattice bipolar transistors

In this article, the characteristics of InP/InGaAs heterostructure-emitter bipolar transistors with 30 n-InP layer tunneling layers and a five-period InP/InGaAs superlattice are demonstrated and comparatively investigated by experimentally results and analysis. In the three devices, a 200 Å n-In_0.53Ga_0.47As layer together with an n-InP tunneling emitter layer (or n-InP/n-InGaAs superlattice) forms heterostructure emitter to decrease collector-emitter offset voltage. The results exhibits that the largest collector current and current gain are obtained for the tunneling transistor with a 30 Å n-InP tunneling emitter layer. On the other hand, some of holes injecting from base to emitter will be blocked at n-InP/n-InGaAs heterojunction due to the relatively small hole transmission coefficient in superlattice device, which will result in a considerable base recombination current in the n-InGaAs layer. Therefore, the collector current and current gain of the superlattice device are the smallest values among of the devices.     

Highly efficient green phosphorescent organic light emitting diodes with simple structure

A series of simple structures is investigated for realization of the highly efficient green phosphorescent organic light emitting diodes with relatively low voltage operation. All the devices were fabricated with mixed host system by using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) which were known to be hole and electron type host materials due to their great hole and electron mobilities [μ_h(TAPC): 1 × 10^?2 cm²/V s and μ_e(TpPyPB): 7.9 × 10^?3 cm²/V s] [1]. The optimized device with thin TAPC (5–10 nm) as an anode buffer layer showed relatively high current and power efficiency with low roll-off characteristic up to 10,000 cd/m². The performances of the devices; with buffer layer were compared to those of simple devices with single layer and three layers. Very interestingly, the double layer device with TAPC buffer layer showed better current and power efficiency behavior compared to that of three layer device with both hole and electron buffer layers (TAPC, TpPyPB, respectively).     

Tunneling hot electron transfer amplifiers (theta): Amplifiers operating up to the infrared

A family of novel three-terminal devices which relies on the transfer of a quasi-monoenergetic hot electron beam through a thin base is described. The devices are similar in principle to the proposed tunneling amplifier by Mead in the early sixties (“Cold Cathode” or “Metal Base” amplifiers). Results are reviewed and the probable reasons for the poor performances are pointed out. It is predicted that, with a proper choice of parameters, metal-base amplifiers can operate as switches, negative resistance devices and continuous amplifiers in the subpicosecond range.Two subclasses are described: The tunneling emitter (THETA), in the major part of the work, and the nontunneling emitter (BHETA) amplifiers. In the THETA family the metal-oxide-metal-oxide-metal (MOMOM), the MOM-semiconductor (MOMS), and the heterojunctions devices are described. Members of the BHETA family generate quasi-monoenergetic electron beams by injecting electrons by an n⁺n^? or a metal-n^? junctions, and include a variety of metals and semiconductor combinations.Very thin films are required in these devices (oxides ～15 Å, metals ～100 Å, semiconductors ～100 Å). The molecular beam epitaxy technique and lattice matching considerations are required for pinhole free semiconductors and metal films with minimum interface states. Sputter-oxidation methods are needed for thin oxide growth. Systems which combine these features with availability of microfabrication make these devices feasible today.     

Analysis of the causes of the decrease in the electroluminescence efficiency of AlGaInN light-emitting-diode heterostructures at high pumping density

The study is devoted to theoretical explanation of a decrease in the electroluminescence efficiency as the pump current increases, which is characteristic of light-emitting-diode (LED) heterostructures based on AlInGaN. Numerical simulation shows that the increase in the external quantum efficiency at low current densities J ≈ 1 A/cm² is caused by the competition between radiative and nonradiative recombination. The decrease in the quantum efficiency at current densities J > 1 A/cm² is caused by a decrease in the efficiency of hole injection into the active region. It is shown that the depth of the acceptor energy level in the AlGaN emitter, as well as low electron and hole mobilities in the p-type region, plays an important role in this effect. A modified LED heterostructure is suggested in which the efficiency decrease with the pump current should not occur.     

Two-carrier conduction in MOS tunnel oxides—1 Experimental results

A novel device structure incorporating a p-channel MOSFET with a metal/tunnel-oxide/n-silicon device is proposed as a tool for separating electron and hole tunneling currents in ultra-thin silicon dioxide films. With this structure, the electron and hole tunneling currents can be independently measured at the substrate and source terminals, respectively. Furthermore, the injected minority carrier (hole) current which is supplied by the p-MOSFET can be varied independently of the tunnel-oxide bias. As expected, the injected hole current modulates the electron current and “current multiplication” was observed. By correlating experimental results for 22.5 Å SiO₂ films with theoretical calculations, the electron and hole barrier heights were determined to be 3.2 and 3.6 eV, respectively, where a trapezoidal tunneling barrier was assumed and a carrier effective mass of 0.5 m₀ was used.The tunnel-oxide quality and uniformity was evaluated by measuring I–V and C-V curves on two-terminal MOS capacitors of various areas. The results suggest that the oxide films are extremely uniform in thickness, and the measured interface trap density was determined to be less than 10¹¹ cm^?2eV^?1. For the reverse-biased tunnel-oxide, the electron/hole current ratio was found to be less than unity except for the condition when the injected hole current was very small compared to the electron current without any hole injection. In addition, this ratio was found to decrease rapidly with increasing oxide thickness and/or increasing hole injection level.     

Effects of S,Si, or Fe dopants on the diffusion of Zn in InP during MOCVD

Diffusion of Zn in InP during growth of InP epitaxial layers has been investigated in layer structures consisting of Zn-InP epilayers grown on S-InP and Fe-InP substrates, and on undoped InP epilayers. The layers were grown by metalorganic chemical vapour deposition (MOCVD) atT = 625° C andP = 75 Torr. Dopant diffusion profiles were measured by secondary ion mass spectrometry (SIMS). At sufficiently high Zn doping levels ([Zn] ≥8 × 10¹⁷ cm⁻³) diffusion into S-InP substrates took place, with accumulation of Zn in the substrate at a concentration similar to [S]. Diffusion into undoped InP epilayers produced a diffusion tail at low [Zn] levels, probably associated with interstitial Zn diffusion. For diffusion into Fe-InP, this low level diffusion produced a region of constant Zn concentration at [Zn] ≈ 3 × 10¹⁶ cm⁻³, due to kick-out of the original Fe species from substitutional sites. We also investigated diffusion out of (Zn, Si) codoped InP epilayers grown on Fe-InP substates. The SIMS profiles were characterised by a sharp decrease in [Zn] at the epilayer-substrate interface; the magnitude of this decrease corresponded to that of the Si donor level in the epilayer. For [Si] ≫ [Zn] in the epilayer no Zn diffusion was observed; Hall measurements indicated that the donor and acceptor species in those samples were electrically active. All these results are consistent with the presence of donor-acceptor interactions in InP, resulting in the formation of ionised donor-acceptor pairs which are immobile, and do not contribute to the diffusion process.     

Some aspects of LEC transistor behaviour

Some properties of bipolar transistors with low emitter concentrations are investigated both theoretically and experimentally. It turns out that the base current of LEC transistors at medium and high injection levels is the same as in double diffused transistors and can be explained by Auger recombination in the emitter n⁺ region. The cut-off frequency f_T is rather low, due to extra charge storage in the lightly doped emitter region. Small n⁺ emitter areas, surrounded by a p-ring may introduce anomalies such as kinks in the (_cb, V_be) characteristics and negative resistances.     

Thermal emission rates and capture cross sections of majority carriers at vanadium centers in silicon

The thermal emission rates and capture cross sections of majority carriers on the vandium associated centers in the depletion region of reverse biased silicon p-n junctions have been measured by the dark capacitance transient method. The three vanduim associated levels observed, two donor levels and a deep acceptor level, belong to the same vandium center. Least square fits of the emission data give the following emission rates; e_nl^t = 1.047 × 10⁶T² exp [?0.179±0.004 eV/kT], e_n0^t = 3.55 × 10⁷T² exp [?0.426±0.004 eV/kT] and e_p-2^t = 1.514 × 10⁶T² exp [?0.450±0.003 eV/kT]. The activation energy of the hole emission rate at the lower donor level is about 0.1 eV larger than the equilibrium thermal activation energy. The capture cross sections are σ_n0 = 3 × 10^?17cm² and σ_p0 = 8 × 10^?16cm² for the electron capture process at the deep acceptor level and the hole capture process at the upper donor level, respectively. The hole capture cross section on the lower donor level (σ_p-1) depends significantly on temperature. The large temperature dependence of the hole capture cross section can be expected due to the nonradiative multiphonon emission process.     

Ambipolar Charge Transport in Films of Methanofullerene and Poly(phenylenevinylene)/Methanofullerene Blends

Herein, we report experimental studies of electron and hole transport in thin films of [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) and in blends of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV) with PCBM. The low‐field hole mobility in pristine MDMO‐PPV is of the order of 10^–7 cm² V^–1 s^–1, in agreement with previous studies, whereas the electron mobility in pristine PCBM was found by current‐density–voltage (J–V) measurements to be of the order of 10^–2 cm² V^–1 s^–1, which is about one order of magnitude greater than previously reported. Adding PCBM to the blend increases both electron and hole mobilities, compared to the pristine polymer, and results in less dispersive hole transport. The hole mobility in a blend containing 67 wt.‐% PCBM is at least two orders of magnitude greater than in the pristine polymer. This result is independent of measurement technique and film thickness, indicating a true bulk property of the material. We therefore propose that PCBM may assist hole transport in the blend, either by participating in hole transport or by changing the polymer‐chain packing to enhance hole mobility. Time‐of‐flight mobility measurements of PCBM dispersed in a polystyrene matrix yield electron and hole mobilities of similar magnitude and relatively non‐dispersive transport. To the best of our knowledge, this is the first report of hole transport in a methanofullerene. We discuss the conditions under which hole transport in the fullerene phase of a polymer/fullerene blend may be expected. The relevance to photovoltaic device function is also discussed.     

On the fast recovery of the blocking property of silicon carbide diodes

The influence of various factors controlling the recovery dynamics of the blocking property of silicon carbide diodes is comparatively analyzed. It is shown that the mechanism related to the large ratio of electron and hole mobilities in SiC is dominant. This mechanism results in the effect of ultrafast (subnanosecond) current break independently of the emitter efficiency asymmetry, which causes initial plasma inhomogeneity in the high-resistivity base. This effect can be observed during the recovery of silicon carbide diodes with a p-type base, while “soft” recovery is inherent only to diodes with an n-type base.     

Improved degradation stability of blue-green II-VI light-emitting diodes with excluded nitrogen-doped ZnSe-based layers

Degradation characteristics of p-i-n BeZnSe/Zn(Be)CdSe light-emitting diodes were investigated. Undoped short-period superlattices, which provide efficient hole transport from the p ⁺-BeTe:N near-contact region (hole injector) into the active region, were used instead of the p-doped BeZnSe:N emitter. It is demonstrated that this makes it possible to considerably lengthen the operating life of the light-emitting diodes at highest direct current densities (～4.5 kA/cm²) at room temperature.     

Bipolar Charge Transport in PCPDTBT‐PCBM Bulk‐Heterojunctions for Photovoltaic Applications

An experimental study of the transport properties of a low‐bandgap conjugated polymer giving high photovoltaic quantum efficiencies in the near infrared spectral region (E_g‐opt ～ 1.35 eV) is presented. Using a organic thin film transistor geometry, we demonstrate a relatively high in‐plane hole mobility, up to 1.5 · × 10^?2 cm² V^?1 s^?1 and quantify the electron mobility at 3 × · 10^?5 cm² V^?1 s^?1 on a SiO₂ dielectric. In addition, singular contact behavior results in bipolar quasi‐Ohmic injection both from low and high workfunction metals like LiF/Al and Au. X‐ray investigations revealed a degree of interchain π‐stacking that is probably embedded in a disordered matrix. Disorder also manifests itself in a strong positive field dependence of the hole mobility from the electric field. In blends made with the electron acceptor methanofullerene [6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM), the transistor characteristics suggest a relatively unfavorable intermixing of the two components for the application to photovoltaic devices. We attribute this to a too fine dispersion of [C60]‐PCBM in the polymer matrix, that is also confirmed by the quenching of the photoluminescence signal measured in PCPDTBT [C60]‐PCBM films with various composition. We show that a higher degree of phase separation can be induced during the film formation by using 1,8‐octanedithiol (ODT), which leads to a more efficient electron percolation in the [C60]‐PCBM. In addition, the experimental results, in combination with those of solar cells seem to support the correlation between the blend morphology and charge recombination. We tentatively propose that the drift length, and similarly the electrical fill factor, can be limited by the recombination of holes with electrons trapped on isolated [C60]‐PCBM clusters. Ionized and isolated [C60]‐PCBM molecules can modify the local electric field in the solar cell by build‐up of a space‐charge. The results also suggest that further improvements of the fill factor may also be limited by a strong electrical‐field dependence of the hole transport.     

Long‐Lived and Highly Efficient TADF‐PhOLED with “(A)n–D–(A)n” Structured Terpyridine Electron‐Transporting Material

The electron‐transporting material (ETM) is one of the key factors to determine the efficiency and stability of organic light‐emitting diodes (OLEDs). A novel ETM with a “(Acceptor)_n–Donor–(Acceptor)_n” (“(A)_n–D–(A)_n”) structure, 2,7‐di([2,2′:6′,2″‐terpyridin]‐4′‐yl)‐9,9′‐spirobifluorene (27‐TPSF), is synthesized by combining electron‐withdrawing terpyridine (TPY) moieties and rigid twisted spirobifluorene, in which the TPY moieties facilitate electron transport and injection while the spirobifluorene moiety ensures high triplet energy (T₁ = 2.5 eV) as well as enhances glass transition temperature (T_g = 195 °C) for better stability. By using tris[2‐(p‐tolyl)pyridine]iridium(III) (Ir(mppy)₃) as the emitter, the 27‐TPSF‐based device exhibits a maximum external quantum efficiency (η_{ext, max}) of 24.5%, and a half‐life (T₅₀) of 121, 6804, and 382 636 h at an initial luminance of 10 000, 1000, and 100 cd m^?2, respectively, which are much better than the commercialized ETM of 9,10‐bis(6‐phenylpyridin‐3‐yl)anthracene (DPPyA). Furthermore, a higher efficiency, a η_{ext, max} of 28.2% and a maximum power efficiency (η_PE, _max) of 129.3 lm W^?1, can be achieved by adopting bis(2‐phenylpyridine)iridium(III)(2,2,6,6‐tetramethylheptane‐3,5‐diketonate) (Ir(ppy)₂tmd) as the emitter and 27‐TPSF as the ETM. These results indicate that the derivative of TPY to form “(A)_n–D–(A)_n” structure is a promising way to design an ETM with good comprehensive properties for OLEDs.     

GaAs–AlGaAs heterostructure thyristors with completely optical transfer of emitter current

Several variants of thyristors based on GaAs-AlGaAs heterostructures with optical transfer of the emitter current are considered. The possibility that thyristors with fully optical transfer of the emitter current can be, in principle, created is demonstrated by means of the results of a study of n-p-n and p-n-p optoelectronic transistors in which the emitter current is converted into light, this light, in turn, being converted into a collector current. Structures of optoelectronic switches of this kind are presented. A switch comprising three constituent transistors has been suggested and fabricated taking into account specific features of the technique for growth of undoped GaAs layers and fabrication of high-voltage p ⁰-n ⁰ junctions with back-ground impurities from these layers, which makes the turn-on delay cardinally shorter and raises the working frequency.