A discrete element small-signal equivalent circuit for forward-biased p+n diodes containing deep levels

A discrete element small-signal equivalent circuit model for p-n diodes containing deep defect levels is developed, by extending an existing model for undamaged devices. With the aid of a simple analytical expression which accurately describes the forward bias d.c. current, the enhanced small-signal conductance due to carrier recombination in the depletion region is included in the model. The influence of trapped charge on the space charge capacitance is incorporated using a simplified version of the analysis of Beguwala and Crowell. The predictions of the model are verified by experimental data from silicon p⁺n diodes, in which deep levels have been induced by electron irradiation. It is shown that the deep level activation energies may be estimated from the forward bias capacitance-voltage characteristics, yielding values which agree well with those obtained by established techniques.     

Effect of double reflection on the forward current-voltage characteristics of a silicon p+-s-n+ epitaxial diode

The behaviour of both majority and minority carriers in a p⁺-s-n⁺ epitaxial diode (where s may be p or n) has been investigated in this paper. Forward current-voltage characteristics of the diodes are obtained by exact numerical analysis, taking into account the effects of energy-gap narrowing, Auger recombination and carrier-carrier scattering. As with previous authors, it is found that the forward current increases with increasing middle layer thickness. The present analysis shows that such increase occurs only upto a specific applied bias, after which the forward current decreases with increasing middle layer thickness. This behaviour is attributed to double reflection, i.e. the reflection of both majority and minority carriers by the p-n junction and high-low junction respectively. Beyond the specific bias so determined, junction reflection loses its effectiveness. Distribution of carrier concentration and junction voltages for several device configurations are given to illustrate these features. The majority carrier reflection by the p-n junction is found to have a dominating effect in all cases.     

Analysis of the soft reverse characteristics of n+p drain diodes

Soft leakage current characteristics of n⁺p drain diodes of a p-well C-MOS process have been measured and analyzed. A field dependent component of the leakage current could be determined after subtraction of the diffusion current. The values of the field-dependent generation current found were compared with calculated values owing to thermal generation according to the Poole-Frenkel effect and phonon-assisted tunneling. The measured values for different diode structures were found to agree well with the theoretical prediction of phonon-assisted tunneling through a barrier lowered by the presence of an electric field.     

A dynamic model of the p-n-n+ step recovery diodes for the numerical analysis of pulse circuits

A new p-n-n⁺ diode model for circuit transient analysis is developed. In contrast to existing circuit models, this model reflects all step-recovery diode (SRD) effects during switching on and off, including “ramp” of slow recovery phase. It is accomplished by taking into account the dynamic physical phenomena in the p-n-n⁺ diodes when switched. A non-linear dynamic diffusion capacitance of the diode model is determined by the dependence of the instantaneous base charge on the instantaneous diode voltage.The accuracy of the presented model is verified by comparison of the calculated and measured wave forms of some pulse circuits.The present model has been proved to be more accurate than SRD models previously published.     

<Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-Si-<Emphasis Type="Italic">n</Emphasis>-CdF<Subscript>2</Subscript> heterojunctions

Boron diffusion and the vapor-phase deposition of silicon layers are used to prepare ultrashallow p⁺-n junctions and p⁺-Si-n-CdF₂ heterostructures on an n-CdF₂ crystal surface. Forward portions of the I–V characteristics of the p⁺-n junctions and p⁺-Si-n-CdF₂ heterojunctions reveal the CdF₂ band gap (7.8 eV), as well as allow the identification of the valence-band structure of cadmium fluoride crystals. Under conditions in which forward bias is applied to the p⁺-Si-n-CdF₂ heterojunctions, electroluminescence spectra are measured for the first time in the visible spectral region.     

Influence of the electron-hole equilibrium shift on transition-metal diffusion in GaAs

Diffusion of impurities of transition metals Fe, Cu, and Cr in heavily doped p ⁺-, n ⁺-, and intrinsic (at diffusion temperature) GaAs is studied. A technique in which impurity diffuses into GaAs-based structures with heavily doped layers (p ⁺-n or n ⁺-n) was used. It is shown that the impurity diffusivity values in p ⁺-GaAs and n ⁺-GaAs are significantly higher and lower, respectively, than for i-GaAs. The results obtained are discussed taking into account the effect of the electron-hole equilibrium shift in semiconductors on the diffusion of impurities migrating according to the dissociative mechanism. The interstitial-component concentration for Fe, Cu, and Cr impurities in GaAs was determined at the diffusion temperature.     

Generation of microwave oscillations in a no-base diode

The GHz-frequency microwave oscillations of voltage in a no-base p ⁺-p-n ⁺ silicon diode driven by reverse current with a pulse duration of ～300 ns and a current density of several kA/cm² were experimentally observed for the first time. The mechanism of initiation of these oscillations was theoretically considered. The frequency and the modulation percentage of the microwave oscillations were shown to depend on the current density and dopant-concentration gradient in the p-n-junction plane.     

C–V index of hyperabrupt p-n junctions

C–V index n for hyperabrupt p⁺-n junctions with exponentially retrograded n-region has been computed numerically for different values of parameters characterizing the impurity profile and the results have been plotted graphically. Although n is found to vary with the bias across the junction for any given impurity distribution, the maximum value nmax of n is determined only by the ratio of the background concentration to the crossover concentration in the retrograded region. By making this ratio R₀ smaller and smaller, values of n substantially larger than unity can be obtained. Practical considerations, however, limit the maximum value of n to about 10. An empirical relation expressing nmax as a function of the ratio R₀ has been obtained. Calculated results are compared with the values of n measured on hyperabrupt junctions fabricated by a double diffusion process.     

Subnanosecond 4<Emphasis Type="Italic">H</Emphasis>-SiC diode current breakers

Mesa epitaxial 4H-SiC-based p ⁺-p-n ₀-n ⁺ diodes have been fabricated and their reverse recovery characteristics have been measured in modes typical of fast semiconductor current breakers, drift step recovery diodes, and SOS diodes. It has been found that, after the short (～10 ns) pulsed injection of nonequilibrium carriers by a forward current with a density of 200–400 A cm^?2 and the subsequent application of a reverse voltage pulse (with a rise time of 2 ns), diodes can break a reverse current with a density of 5–40 kA cm^?2 in a time of about (or less than) 0.3 ns. A possible mechanism for ultrafast current breaking is discussed.     

Profiling the boron distribution in asymmetric n +-p silicon photodiodes and a new concept for creating selectively sensitive photocells for megapixel color-image receivers

The spectral photosensitivity of n ⁺-p silicon photodiodes with a p ⁺ layer implanted in the substrate is studied experimentally. It is demonstrated that such p ⁺ doping effectively shifts the long-wavelength edge of the photosensitivity in the optical spectral range and the shift depends on the depth of the p ⁺ layer. A new concept for creating selectively sensitive photocells for megapixel color-image receivers is proposed. The receivers are based on n ⁺-p photodiode structures containing a few layers that are implanted at different depths and form desired color-separating potential barriers and lateral diffusion channels for collection of the minority carriers generated by photons of different colors.     

Minority carrier lifetimes in silicon solar cells determined from spectral and transient measurements

Measurements of the spectral collection efficiency and short circuit current decay rate following an X-ray pulse have been made on three types of single crystal silicon solar cells. The cell types were n⁺ - p, p⁺ - n, and p⁺ - n - n⁺ with base resistivities of 0.3, 10 and 10 Ω-cm, respectively. Minority carrier lifetimes were determined from both experiments using analytical or device code calculations, as required. For the n⁺ - p and p⁺ - n cells, nominal lifetimes of 2 and 5 ωsec, respectively, were obtained. A lifetime greater than 100 ωsec was inferred for the p⁺ - n - n⁺ device. This value represents a minimum estimate since our analysis is inaccurate when the diffusion length exceeds the cell thickness, as is the case here. The difference in base lifetime for the p⁺ - n and p⁺ - n - n⁺ structures is attributed to gettering during the phosphorus diffusion to form the back surface field layer.     

Numerical analysis on abnormal thyristor forward voltage increase due to heavy doping in gated p-base layer

An abnormal forward voltage increase was observed for a p-base gated double diffused n⁺pn^?p⁺ high power thyristor with high impurity concentration at the n⁺-p emitter-base junction. Accurate numerical analysis shows that heavy doping effects are the most responsible mechanism for the abnormality and that depletion layer formation at the center junction accompanies it.It will be shown that appropriate control of the impurity concentration at the emitter-base junction is necessary to avoid this abnormality by realizing the common base transistor current gain of greater than 0.73 for n⁺n^?-portion.     

Electron-beam-induced conductivity in self-organized silicon quantum wells

Electron-beam diagnostics are used to study self-organized quantum wells which form within ultrashallow silicon p ⁺-n junctions under the conditions of nonequilibrium boron diffusion. The energy dependence and current-voltage characteristics of the electron-beam-induced conductivity are investigated with relative dominance of both longitudinal and transverse quantum wells, which are oriented parallel and perpendicularly to the p-n junction plane, respectively. Current-voltage characteristics of the electron-beam-induced conductivity are exhibited for the first time with both reverse and forward biasing of the silicon p ⁺-n junction. This became possible because of the presence of self-organized transverse quantum wells within the ultrashallow p ⁺ diffusion profile, while self-organized longitudinal quantum wells promote the appearance of electron-beam-induced conductivity only when the p ⁺-n junction is reverse-biased. The distribution of the probability for the separation of electron-hole pairs across the thickness of the crystal derived from the energy dependences of the electron-beam-induced conductivity reveals effects of the avalanche multiplication of the nonequilibrium carriers as a result of the spatial separation of electrons and holes in the field of a p ⁺-n junction that contains self-organized transverse quantum wells. Fiz. Tekh. Poluprovodn. 33, 851–857 (July 1999)     

Ultrashallow p +-n junctions in Si(111): Electron-beam diagnostics of the surface region

Ultrashallow p ⁺-n junctions fabricated in Si(111) are investigated by low-and intermediateenergy electron-beam probing of the surface region in order to determine how the crystallographic orientation of the silicon films affects the mechanisms for nonequilibrium diffusion of boron. A comparative study is made of p ⁺-n junctions made on both (111) and (100) silicon with regard to how the irradiation-induced conductivity depends on the energy of the primary electron beam, and also its distribution with area. Using this method, it is possible to determine how the probability of an electron-hole pair being separated by the electric field of the Si(111) and Si(100) p ⁺-n junctions varies with depth into the crystal, which experiments show is different, depending on whether diffusive motion of impurities is dominated by the kick-out or dissociative-vacancy mechanisms. It was found that for boron in silicon the kick-out type of diffusion mechanism is strongly enhanced in the [111] crystallographic direction, whereas diffusion in the [100] direction is primarily driven by vacancy mechanisms. It is shown that collection of nonequilibrium carriers in the p ⁺-n junction field is strongly enhanced when the diffusion profile consists of certain combinations of longitudinal and transverse quantum wells. Fiz. Tekh. Poluprovodn. 33, 58–63 (January 1999)     

Characteristics of metal/tunnel-oxide/n/p+ silicon switching devices—II: Two-dimensional effects in oxide-isolated structures

Effects of oxide isolation on the two-terminal D.C. characteristics of metal/tunnel-oxide/n/p⁺ silicon switching devices have been studied.Recent experimental results have shown that the switching characteristics are strongly dependent on area, and area-to-perimeter ratio of the device. To carry out a systematic investigation of this phenomenon, the devices in this study were isolated using V-grooves of various areas. For a given tunnel-oxide thickness and area, it was found that the magnitude of the switching voltage and holding current of the device increased with isolation area, whereas the switching current remained essentially constant. Furthermore, it is shown that the switching current is almost completely determined by the characteristics of the tunnel-oxide; in particular, the minority carrier concentration at the SiSiO₂ interface. Physical arguments are presented which adequately explain the observed trends. It is also experimentally shown that both switching current and holding current decrease as the tunnel-oxide thickness is increased.A simple two-dimensional model for the oxide-isolated MISS device is derived which effectively explains the above area-related phenomena. In agreement with experimental results, the model predicts that for a given tunnel-oxide thickness and area, an increase in switching voltage magnitude and holding current will result as the isolated p⁺-n junction area is increased. Calculations based on this model are shown to be in good agreement with experimental data.     

A computer-aided simulation model for the I–V characteristic of M-n-p silicon Schottky-barrier diodes produced by use of low-energy arsenic-ion implantation

Current-transport properties of Al-n-p silicon Schottky-barrier diodes have been studied both experimentally and theoretically. An analytical model for the I-V characteristic of a metal-n-p Schottky barrier diode has been developed by using an interfacial layer-thermionic-diffusion model. Assuming a Gaussian distribution for the implanted profile, the barrier-height enhancement and ideality factor have been derived analytically. Using low energy (25 KeV) arsenic implantation with the dose ranged form 8 × 10¹⁰/cm² to 10¹²/cm², Al-n-p silicon Schottky barrier diodes have been fabricated and characterized. Comparisons between the experimental measurements and the results of computer simulations have been performed and satisfactory agreements between these comparisons have been obtained. The reverse I–V characteristics of the fabricated Al-n-p silicon Schottky barrier diodes can also be well simulated by the developed model.     

Generation-recombination and diffusion currents in HgMnTe n +-p junctions

Special features of the distributions of the space charge, the electric-field strength, and potential in the p-n junction in the narrow-gap HgMnTe semiconductor were considered using the Poisson equation. It is shown that, as the band gap narrows, the effect of free charge carriers induces the coordinate dependence of electric-field strength to deviate from the linear dependence and that of the potential to deviate from the quadratic dependence. As a result of this, and also because of an appreciable increase in the diffusion potential in the n ⁺-p junction with the degenerate n ⁺ region, the mechanisms of the charge transport become peculiar: the voltage dependence of the recombination current deviates from those following from the conventionally used analytical expressions, whereas for higher bias voltages, the diffusion current of holes from the lightly doped p region into the n ⁺ region is prevalent.     

‘Modulation effect by intense hole injection’ in epitaxial silicon Schottky-barrier-diodes

The forward characteristics of different epitazial n-n⁺ silicon Schottky barrier diodes have been studied up to high current densities. Modulation has been observed in these experiments, that means an increase in charge carrier density in the series resistance region, i.e. in the epitaxial n-layer. By appropriate analysis of the forward characteristics the carrier density can be determined as a function of the current density. In accordance with the predictions of Scharfetter we find that the modulation increases with the barrier height but decreases with rising donor density. Furthermore it is shown how modulation is affected by recombination centres. The modulation effect is attenuated by irradiation with 1·5 MeV electrons and in a similar manner by doping the epitaxial layer with gold.     

Planar,ion-implanted bipolar devices in GaAs

Selected-area ion implantation using heavy metal masks to define the device geometry has been used to fabricate doubly implanted npn bipolar transistors and planar, isolated pn junction devices in GaAs. The bipolar transistors exhibited common-emitter current gains as high as 25. Collector-base breakdown voltages of 45 V were observed. The junction diodes (～200 um dia.) exhibited sub-nanoampere leakage currents at 15 V of reverse bias. Surface leakage appears to be the dominant mechanism responsible for the observed leakage currents. The diode forward current is limited by recomination in the space charge region.     

Study of the potential distribution in a forward-biased silicon diode using electrostatic force microscopy

Electrostatic force microscopy was used to study the potential distribution in a forward-biased epitaxial-diffused n ⁺-n-p-p ⁺ silicon diode. Distributions of potential and capacitance were determined across the cleaved surface, which intersected the layers in the diode structure. Variations in the surface potential and capacitance were preliminarily measured with a submicrometer spatial resolution and were used to determine the position and width of the n-p junction; the distribution of applied forward bias in the diode was also assessed. It is shown that an additional potential barrier for injected charge carriers may exist in the vicinity of the n ⁺-n junction in the diode under consideration. For an injection-current density exceeding 100 mA/cm², the voltage drop across this barrier becomes comparable with the voltage variations across the operating n-p junction.