Current transport in an ion-implanted diode

In a Schottky diode, the diode saturation current is controlled by the barrier height at the metal and semi-conductor contact, assuming that the dominant current is due to thermionic emission. When ion implantation is used to increase the barrier height, both thermionic emission and drift-diffusion of carriers become important in calculating the current. Numerical methods are used in solving Poisson's equation and the current continuity equations for an ion implanted doping profile. The electron and hole current in the surface region are calculated as a function of the total implantation dosage. The results show that the decrease of saturation current and the increase of effective barrier height in an ion implanted diode is mainly due to the suppression of the thermionic emission current by the implanted impurity atoms, rendering the diode to act like a pn junction.     

新型GaAs/AlGaAs量子阱红外探测器暗电流特性

对基于GaAs/AlGaAs系子带间吸收的一种新型量子阱红外探测器,采用Poisson方程和Schrodinger方程,计算了新器件结构的能带结构、电子分布特性,在此基础上采用热离子发射、热辅助遂穿模型对器件的暗电流特性进行了模拟,计算结果与器件实测的暗电流特性吻合得很好,说明热离子发射、热辅助遂穿机制是形成器件暗电流的主要构成机制,增加垒高、降低阱中掺杂浓度及降低工作温度是抑制器件暗电流的主要途径,计算结果对进一步优化器件的设计将起到重要的理论指导作用．     

Very high temperature operation of diamond Schottky diode

For the first time, the operating temperature of a Schottky diode structure has been pushed to 1000°C. The diode structure consists of a Si-based Schottky material deposited onto a homoepitaxial boron doped diamond surface. At high temperatures, the forward I-V characteristics are dominated by the thermionic emission (n≈1.01) across a barrier of 1.9 eV height. The reverse characteristics are still dominated by thermally activated defects. The series resistance shows thermal activation associated with the boron doping     

Effect of p-n junction overheating on degradation of silicon high-power pulsed IMPATT diodes

The thermal limits of the two-drift impact avalanche and transit-time (IMPATT) diode operating in the pulsed mode in the 8-mm wavelength region with a microwave power as high as 30–35 W have been estimated. It is shown that p-n junction overheat at an operating pulse length of 300 ns and a supply current amplitude of 11.3–15 A amounts to 270–430°C relative to an ambient medium. The temperature limit of junction overheating, above which IMPATT diodes rapidly degrade, was determined as 350°C. The presented results of X-ray phase analysis and depth profiles of Au-Pt-Ti-Pd-Si ohmic contact components confirm thermal limits of the IMPATT diode operating in the pulsed mode.

     

IMPATT diode quasi-static large-signal model

A quasi-static large-signal model of an IMPATT diode with general doping profile is derived. The numerical solution of this model has been implemented in a Fortran IV program which executes economically. This model has been used to analyze large-and small-signal admittances of GaAs double-drift and quasi-Read IMPATT diodes. The small-signal results are in good agreement with calculations done using a linearized small-signal model. The large-signal calculations exhibit power and efficiency saturation when reasonable values of parasitic resistance are included and are in good agreement with experimental GaAs diode performance. The generalized quasi-static formulation simplifies analysis of IMPATT structures with arbitrary doping profiles, specifically those with distributed avalanche zones, by providing the correspondence between these devices and the Read diode model.     

IMPATT oscillators with enhanced leakage current

The behavior of IMPATT oscillators with enhanced leakage current has been experimentally evaluated by irradiating operating diodes with transient ionizing radiation. Leakage current was induced in diffused junction GaAs and silicon X-band IMPATT diodes by irradiation with 100 nsec pulses of 10 MeV electrons. With increasing leakage current, the oscillator RF power decreases and the frequency of oscillation increases. A large signal circuit model of the IMPATT diode is developed which correlates well with experimental measurements.     

Electrical characteristics of rectifying polycrystalline silicon/silicon carbide heterojunctions

This paper presents a study of the rectifying properties of heavily doped polycrystalline silicon (polysilicon) on 4H silicon carbide (4H-SiC). Current properties and barrier heights were found using analysis of the heterojunction. This revealed that Schottky analysis would be valid for the large barrier height devices. Isotype and an-isotype devices were fabricated on both p-type and n-type SiC and the electrical characteristics were investigated using capacitance vs voltage measurements, current vs voltage measurements (I-V), and temperature I-V measurements. Extraction of the barrier height, built-in potential, and Richardson constant were made and then compared to theoretical values for the heterojunction. Temperature I-V measurements demonstrated that the current transport mechanism is thermionic emission, confirming the validity of the Schottky diode model. The I-V characteristics show near ideal diode rectifying behavior and the capacitance-voltage characteristics show ideal junction space charge modulation for all polysilicon/SiC combinations. These experimental results match well with heterojunction band-offset estimated barrier heights and demonstrate that the barrier height of the polysilicon/4H SiC interface may be controlled by varying the polysilicon doping type.     

Investigation of Al Schottky junction on n-type CdS film deposited on polymer substrate

A systematic study has been made on the behavior of Al/n-CdS thin film junction on flexible polymer substrate (polyethylene terephthalate, PET) grown using thermal evaporation method. Temperature dependence of I-V measurements for this junction has been done which closely follow the equations of Schottky barrier junction dominated by thermionic emission mechanism. Intrinsic and contact properties such as barrier height, ideality factor and series resistance have been calculated from I-V characteristics. The barrier height of Al/n-CdS junction is found to increase with increase in temperature whereas ideality factor and series resistance decrease with increase in temperature.     

High frequency noise properties of a double avalanche region (DAR) IMPATT diode

The high frequency noise properties of a double avalanche region (DAR) IMPATT diode consisting of two avalanche layers interspaced by a drift layer have been studied. In view of the fact that SDR IMPATT diode shows a high value of noise figure, one may think that the presence of two avalanche layers in DAR IMPATT diode may lead to a noise figure of the order of 2 or 3 times larger than that of the SDR (or SAR) IMPATT. However, from the study, it has been observed that the DAR IMPATT has the same order of noise as that of SDR IMPATT under operating condition. Since the DAR IMPATT diode with unequal avalanche layer width can be used as microwave oscillator with minimum coupling between the harmonically related frequencies [1], the device may be very useful in the microwave frequency range.     

Modes of avalanche oscillations in silicon diodes

A computer program which includes both electronic and thermal processes has been used to study avalanche oscillations in a diode which is punched through only well above breakdown. IMPATT, relaxing avalanche, and MULTIPATT oscillations have been studied. The MULTIPATT mode is shown to be a superpesition of transit-time oscillations upon a relaxation oscillation. It is postulated that the TRAPATT mode is initiated by the IMPATI mode via the MULTIPATI mode. The frequency of the IMPATT oscillations was found to vary with the square root of the current over a factor of 100 in current. For parallel operation of TRAPATT diodes, it is shown that nonpunched-through diodes should be used.     

Reliability and micro-structural properties of GaAs Schottky diodes for submillimeter-wave applications

Whisker contacted GaAs Schottky barrier diodes are the standard devices for mixing and multiplier applications in the THz frequency range. This is mainly due to their minimum parasitics and mature technology. But with the decreasing size of the anode contact, which is required for operation at high frequencies (up to approx. 3 THz), the reliability and the micro-structural understanding of the Schottky barrier becomes increasingly important. This contribution presents new results concerning the reliability of Schottky diodes and the physical properties of small-area Schottky junctions, especially at low current densities. For these purposes a number of different Schottky diodes have been fabricated with different epilayer doping concentrations and anode diameters. Measured I/V characteristics show that the diode current deviates considerably from the ideal thermionic current behavior with decreasing diode diameter. This deviation shows an exponential dependence on the diode voltage and is a function of the doping concentration of the active layer. For a given doping concentration in the epi-layer and decreasing anode diameter, this phenomenon shifts the minimum of the ideality factor towards higher current densities. An explanation is given in terms of a difference of the cyrstallinity of the polycrystalline platinum films on the GaAs for decreasing SiO₂ aperture size in connection with a reduced Pt mobility in the electrolyte. The reliability of Schottky barrier diodes under thermal and electrical stress has been investigated on different THz Schottky diode structures. The results show that the barrier height and the ideality factor of the fabricated structures are not affected by thermal stress. Electrical stress induced by large forward currents up to a current density of 10 kA/mm² even leads to a slight increase of the barrier height and a reduction of the series resistance.     

Calculation of photogenerated carrier escape rates from GaAs/AlGa1-xAs quantum wells

We present a new theory for photogenerated carrier escape rates from single quantum wells, as a function of an applied electric field, that includes thermionic emission, direct tunneling, and tunneling via thermal occupation of upper subbands, and compare the results for GaAs/Al_xGa_1-xAs quantum wells with recent experiments. We account for the two dimensional (2D) density of states below the barrier, assume thermal equilibrium of carriers within the well, allow for the possibility of strain in the well and/or barrier, and include the contribution to electron thermionic emission from indirect conduction band minima. Our expressions for thermionic emission reduce, in the limit of large well width, to those derived by assuming a three-dimensional (3D) density of states. The results for electron emission from GaAs/Al_xGa_1-xAs quantum wells with x=0.2 and x=0.4 barriers at room temperature agree well with experiment. For wells with x=0.2 barriers, thermally assisted tunneling overtakes thermionic emission around 40 kV/cm, while for wells with x=0.4 barriers thermionic emission from the L valley conduction band minima dominates for fields less than 70 kV/cm. For holes we show that the escape rates are very sensitive to the in-plane effective masses, and results using simple expressions for the in-plane masses that do not include light/heavy-hole mixing agree poorly with experiment. The agreement with experiment is improved using in-plane masses that include light/heavy-hole mixing, particularly for wells with high barriers. We suggest that agreement with experiment would be improved by using more accurate in-plane hole masses for all of the subbands     

Effect of Temperature on Device Admittance of GaAs and Si IMPATT Diodes

The effect of temperature on the small-signal admittance of IMPATT diodes with uniformly doped and high-low doped (Read) structures is investigated experimentafly and theoretically. Small-signal admittance characteristics of X-band Si p+-n-n+, GaAs M-n-n+ (Schottky-uniform), and GaAs M-n+-n-n+ (Schottky-Read) IMPATT diodes are measured at various junction temperatures for different dc current levels. Small-signal analysis is performed on GaAs IMPATT diodes of uniformly doped and high-low doped structures, and the calculated results on temperature dependence of the device admittance are compared with the experimental results. Reasonable agreement is found between theory and experiment. It is shown that GaAs IMPATT diodes are superior to Si diodes in admittance temperature characteristics and that the uniformly doped structure has a small admittance temperature coefficient in magnitude, compared to the high-low doped structure. It is also shown by calculation that the admittance temperature coefficient of a punch-through diode is small in magnitude, compared to that of a non-punch-through diode.     

A simplified field analysis of a distributed IMPATT diode usingmultiple uniform layer approximation

A small-signal field analysis of a distributed IMPATT diode is presented. The active region of the diode is assumed to consist of a uniform avalanche layer and avalanche-free drift layers. The propagation constant and field distributions are obtained without numerical solution of differential equations. The effects of losses caused by the presence of inactive layers are included in the analysis. Numerical examples of GaAs double-Read distributed IMPATT diodes are given which show the dependence of the amplification characteristics on the thicknesses of the avalanche and drift layers     

Heat transport and current crowding in IMPATT diodes

The current crowding due to the temperature dependence of avalanche breakdown is analyzed for the steady-state operation of an IMPATT diode mounted on a semi-infinite heat sink. The solution depends on a single new nondimensional number, the "Lambda" number, which we define here. This number can be determined from experimental data, and the current density and temperature distribution can then be determined from the results given in this paper. Experimental measurements of the I-V plot for a laboratory diode are shown to agree with the theoretical predictions. The theoretical model, which is based upon the assumption that the heat is entirely produced at the interface between the diode and the heat sink, is shown to agree with numerical results from a finite-difference program in which heat is introduced into a GaAs diode in a plane 0.5 µm above the interface. It is shown that electrical measurements of thermal resistance are related to an "effective" temperature which is about 15 percent below the temperature on the diode axis.     

A mechanism for catastrophic failure of avalanche diodes

A simple computer model of the dc electrical-thermal behavior of a Schottky-barrier GaAs IMPATT diode has been modified to include the effects of temperature-dependent thermal resistance. This has made possible the computation of dc V-I characteristics for various IMPATT diode designs and parameters. Computed terminal V-I characteristics, as well as E-J characteristics for points within the depletion layer invariably have shown successive regions of increasing, then decreasing, positive differential resistance, culminating in a region of negative differential resistance. According to an analysis of differential negative resistance appearing in the literature [5], it is a natural consequence of operation in a negative resistance region for high-current filaments to form. Furthermore, a phenomenological argument is cited to justify high-current filamentation in a region of decreasing positive resistance. Experimental evidence is advanced to support the contention that IMPATT shortouts are the natural consequence of diode operation beyond a differential resistance maximum, where the resistance, although positive, is decreasing and the formation of destructive high-current filaments is inevitable.     

Theoretical and Experimental Investigation of Barrier-Energy-Dependent Charge Injection Mechanisms in Organic Photodetectors

Charge injection is known as the major source of dark current under an applied reverse bias, which directly influences the performance of organic photodetectors with diode architecture. However, it is unclear which of various contributions, such as electron flow through the junction, shunt leakage, thermionic emission, and tunnelling, are dominant. This study investigates the thermionic emission and tunneling models to describe the origin of experimentally measured dark current generated in an organic photodetector. To elucidate the dominant mechanism, the barrier energies at anodic contacts are set from 0.6 to 1.0 eV using photosensitive layers composed of different acceptors. A linear relation is found between the natural logarithm of the dark current density under reverse bias and the square root of the barrier height, which strongly suggests direct tunneling as dominant mechanism for dark current injection. This conclusion is strengthened by temperature dependent dark current analysis. Further knowledge of the dominant mechanism by charge injection can help devise an effective strategy to suppress dark current for effective organic photodetector device implementation.     

Junction model and transport mechanism in hybrid PEDOT:PSS/n-GaAs solar cells

This study investigates the junction formation and interface properties of PEDOT:PSS/n-GaAs hybrid solar cells on planar GaAs substrates. Barrier height, photocurrent, dark saturation current and build-in potential at this hybrid interface are measured by varying n-GaAs doping concentrations. The work function and valence band edge of the polymer are extracted from ultraviolet photoelectron spectroscopy to construct the band diagram of the hybrid n-GaAs/PEDOT:PSS junction. The current-voltage characteristics were analyzed by using abrupt (p⁺n) junction and Schottky junction models. Contrary to the earlier results from the PEDOT:PSS/n-Si solar cells, the experimental evidence clearly suggested that the interface between n-GaAs and PEDOT:PSS more likely exhibited a Schottky type instead of a p⁺n junction. The current transport is governed by the thermionic emission of majority carriers over a barrier and not by diffusion. The dark saturation current density increases markedly owing to the increasing surface recombination rate in heavier n-doped GaAs substrates, leading to significant deterioration in solar cells performance.