Field and carrier current density profiles in linearly graded silicon avalanche diodes

A detailed study for the d.c. field and carrier current density profiles of linearly graded double drift avalanche diodes is presented taking into account the effect of impurity and mobile charge density and the realistic field dependance of the ionization rates and drift velocities for the charge carriers. The study involves finding the location and magnitude of the electric field maximum by an iterative method. A small shift in the position of the electric field maximum towards the p-side of the metallurgical junction is observed which increases with increasing current density and decreasing doping gradient. The maximum field and the depletion layer width change sharply with doping gradient but very slightly with d.c. current density. Over a larger fraction of the depletion layer, hole current density exceeds electron current density and hole dominance increases with decreasing doping gradient. The avalanche centre where J_p = J_n is found to be always on the n-side of the junction.     

Small signal equivalent circuit of unsymmetrical junction diodes at high current densities

Finite difference equation methods and lumped models are applied to the solution of the differential equations for the carrier flow through a quasi-neutral semiconductor bulk region at low, moderate and high injection densities. An unsymmetrically doped diode is approximated by a single section lumped model. The static I-V characteristics for this model at low and high current densities are derived. The small signal equivalent circuit of an unsymmetrically doped diode at high injection densities is presented. Approximations for the elements (R, L, C) of this equivalent circuit are calculated from the single section lumped model. The static characteristic and the small signal equivalent circuit elements are in satisfactory agreements with experimental results for diodes in which the base length is not too long compared to the diffusion length.     

Shifting of the peak generation rate in double-drift silicon IMPATT diodes

In double-drift (DD) silicon IMPATT diodes, it is observed that the peak generation rates of both carriers (electrons and holes) lie within the n side. The shifting is due to the unequal ionization rates for electrons and holes in silicon. By neglecting the reverse saturation current, a simple analytical expression for the location where the peak generation occurs is derived. This simple result may be useful for the design of double-drift as well as complementary single-drift IMPATT diodes.     

Theory and experiment for silicon Schottky barrier diodes at high current density

Metal-silicon Schottky barrier diodes exhibit n values which theoretically vary as a function of doping and applied voltage. The expected variation depends on which theoretical model is used to describe the current transport.Titanium n-type silicon barriers were prepared. At a doping level of 3 × 10¹⁵ cm^?3 the barrier height and n-value measured at 100 mV were 0.485±0.005 V and 1.02±0.01 whereas for a doping level of 2 × 10¹⁴ cm^?3 the corresponding values were 0.500±0.005 V and 1.18±0.05.The experimental variation of the diode n value as a function of semiconductor band bending showed good agreement with the thermionic-diffusion model of Crowell and Beguwala: n values increased rapidly as the band bending β → 2, and n values were highest at a given β for diodes with the lowest doping concentration. Similar results were obtained by measurements on magnesium and aluminium barriers on n-type silicon.An analysis of the results has shown that the variation of the diode saturation current I_s follows the predictions of the thermionic-diffusion theory, although there were some anomalies at high current densities. The anomalies did not result from variation of the width of the undepleted region of the epitaxial silicon layer or from diode self-heating effects.     

Optical properties of blue light-emitting diodes in the InGaN/GaN system at high current densities

The current-voltage and brightness-voltage characteristics and the electroluminescence spectra of blue InGaN/GaN-based light-emitting diodes are studied to clarify the cause of the decrease in the emission efficiency at high current densities and high temperatures. It is found that the linear increase in the emission intensity with increasing injection current changes into a sublinear increase, resulting in a decrease in efficiency as the observed photon energy shifts from the mobility edge. The emission intensity decreases with increasing temperature when the photon energy approaches the mobility edge; this results in the reduction in efficiency on overheating. With increasing temperature, the peak of the electroluminescence spectrum shifts to lower photon energies because of the narrowing of the band gap. The results are interpreted taking into account the fact that the density-of-states tails in InGaN are filled not only via trapping of free charge carriers, but also via tunneling transitions into the tail states. The decrease in the emission efficiency at high currents is attributed to the suppression of tunneling injection and the enhancement of losses via the nonradiative recombination channel “under” the quantum well.     

Effect of finite current multiplication factor in the avalanche zone on the high-frequency noise properties of read diodes

The high-frequency noise properties of a Read diode, whose current multiplication factor in the avalanche zone can be controlled and is finite, has been analyzed. The analysis indicates that the open-circuit noise voltage and the noise figure of the diode are reduced with the lowering of the current multiplication factor. It is also found that the open-circuit noise voltage exhibits a finite peak at the avalanche frequency for finite but large values of M which, however, disappears at lower values of M(≤100).     

Generation efficiency of single-photon current pulses in the Geiger mode of silicon avalanche photodiodes

Statistical fluctuations of the avalanche’s multiplication efficiency were studied as applied to the single-photon (Geiger) mode of avalanche photodiodes. The distribution function of partial multiplication factors with an anomalously wide (of the order of the average) dispersion was obtained. Expressions for partial feedback factors were derived in terms of the average gain and the corresponding dependences on the diode’s overvoltage were calculated. Final expressions for the photon-electric pulse’s conversion were derived by averaging corresponding formulas over the coordinate of initiating photoelectron generation using the functions of optical photon absorption in silicon.     

Recent advances in device processing and packaging of high-power pulsed GaAs double-drift IMPATT's at X-band

High-power multimesa GaAs hybrid double-drift IMPATT's have been developed for pulsed operation at X-band. The diodes are fabricated from GaAs material grown by a novel "infinite" solution liquid phase epitaxial process. The use of specialized rapid thermal processing and packaging techniques has enabled the fabrication of high-power IMPATT oscillators that have delivered peak output powers of over 40 W with 20-percent efficiency under pulsed RF operation at X-band frequencies. The diodes are constructed with an integral heat sink and bounded with a Au-Sn eutectic solder in a microwave package.     

The influences of traps on the generation-recombination current in silicon diodes

The recombination current in the space charge region of an abrupt Si P-N junction is calculated under the assumption that the electrochemical potentials or quasi-Fermi levels for the majority carriers are constant throughout the bulk and space-charge regions, and that the Hall-Shockley-Read centers are distributed uniformly with respect to position in the space-charge region, but can be either a single, discrete level or a continuous and uniform distribution with respect to energy. The results for the single level show that the slope can have constant value over a wide range of applied forward bias. These values are mainly dependent on the trap level position and are nearly independent of the minority carrier lifetime at a given doping concentration. They are also dependent on the lower doping concentration. Furthermore, very good linearity in the characteristic curve can be obtained if we assume that the recombination centers are uniformly distributed in energy throughout the energy gap.     

Effect of junction temperature on the large-signal properties of a 94 GHz silicon based double-drift region impact avalanche transit time device

The authors have developed a large-signal simulation technique extending an in-house small-signal simulation code for analyzing a 94 GHz double-drift region impact avalanche transit time device based on silicon with a non-sinusoidal voltage excitation and studied the effect of junction temperature between 300 and 550 K on the large-signal characteristics of the device for both continuous wave(CW) and pulsed modes of operation.Results show that the large-signal RF power output of the device in both CW and pulsed modes increases with the increase of voltage modulation factor up to 60%,but decreases sharply with further increase of voltage modulation factor for a particular junction temperature;while the same parameter increases with the increase of junction temperature for a particular voltage modulation factor.Heat sinks made of copper and type-IIA diamond are designed to carry out the steady-state and transient thermal analysis of the device operating in CW and pulsed modes respectively. Authors have adopted Olson’s method to carry out the transient analysis of the device,which clearly establishes the superiority of type-IIA diamond over copper as the heat sink material of the device from the standpoint of the undesirable effect of frequency chirping due to thermal transients in the pulsed mode.     

Dependence of the read diode characteristics on the current multiplication factor in the avalanche zone

The effect of the finite current multiplication factor in the avalanche zone on the impedance of a Read diode has been investigated. The magnitude of the negative resistance has been found to decrease with the lowering of the multiplication factor M while the reactance does not appreciably change. The avalanche and resonant frequencies become different when M becomes finite and are markedly dependent on it.     

Study of the relaxation of the excess current in silicon Schottky diodes

This study is devoted to investigation of the relaxation of excess current in silicon α-NiTi-n-Si Schottky diodes subjected to either γ-ray radiation or local disturbance of the interface structure using a diamond indenter. A decrease in the excess diode current is attained using both thermal annealing and ultra-sound irradiation. Simultaneously, the parameters of solar cells manufactured from the above-mentioned Schottky diodes subjected to irradiation with γ-ray photons and to single or double irradiation with ultra-sound are studied. It is shown that, after the effect of the diamond indenter, the excess current decreases as a result of thermal annealing; however, a decrease in the excess current to the initial value is not attained. The photoelectric parameters of the studied solar cells before irradiation and after irradiation with γ-ray photons and after single or double irradiation with ultrasound show that ultrasonic treatment is more efficient than thermal annealing.     

Design of white tandem organic light-emitting diodes for full-color microdisplay with high current efficiency and high color gamut

Microdisplays based on organic light-emitting diodes (OLEDs) have a small form factor, and this can be a great advantage when applied to augmented reality and virtual reality devices. In addition, a high-resolution microdisplay of 3000 ppi or more can be achieved when applying a white OLED structure and a color filter. However, low luminance is the weakness of an OLED-based microdisplay as compared with other microdisplay technologies. By applying a tandem structure consisting of two separate emission layers, the efficiency of the OLED device is increased, and higher luminance can be achieved. The efficiency and white spectrum of the OLED device are affected by the position of the emitting layer in the tandem structure and calculated via optical simulation. Each white OLED device with optimized efficiency is fabricated according to the position of the emitting layer, and red, green, and blue spectrum and efficiency are confirmed after passing through color filters. The optimized white OLED device with color filters reaches 97.8% of the National Television Standards Committee standard.     

Correlation of the leakage current and charge pumping in silicon oninsulator gate-controlled diodes

The leakage and charge pumping currents were measured in gate-controlled MOS p-i-n diodes fabricated on thin SIMOX substrates. The efficiencies of the techniques as well as their complementary features are analyzed for various experimental conditions. The interface properties of device-grade SIMOX wafers are characterized and shown to be compatible with VLSI requirements. Special interface coupling effects, which occur only in fully depleted SOI devices and modify the conventional signature of charge pumping and leakage current, are thoroughly investigated     

Verification of the integral charge-control relation for high-speed bipolar transistors at high current densities

Gummel's integral charge-control relation (ICCR) IC= (const/Qp).exp (VBE/VT) is an important basis for developing self-consistent compact transistor models for the high-current region (including quasi-saturation). Such models are required for the simulation of future high-speed IC's with a high integration level. Unfortunately, the simplifying assumptions on which the ICCR is based seem to be doubtful especially for very fast transistors. Therefore, in this paper, the ICCR and its assumptions are checked via numerical simulation of such transistors (fT≈ 7-8 GHz). It is found that the one-dimensional ICCR is a fairly good approximation far into the high-current region. This satisfactory result is mainly due to the partial compensation of the influences of the spatially dependent doping concentration on both the electron mobility µnand the effective intrinsic density niewithin the product µnn²ie. Only in the emitter and in the emitter-base space-charge region there is a strong increase of this product which, in conjunction with the increasing contribution of the hole charge in these regions, was proved to be responsible for the errors observed at high current levels. The ICCR can also be applied to a two-dimensional transistor by additionally taking into account the excess hole charge stored outside the internal transistor for the determination of Qp. Thus the contribution of the minority charges can still be determined experimentally by measuring τf(IC).     

Simulation study of high injection effects and parasitic barrier formation in SiGe HBTs operating at high current densities

The effects of high carrier densities near the base–collector (B–C) heterojunction in npn SiGe heterojunction bipolar transistors during device operation at high current densities has been investigated using a commercial numerical device simulator. Due to electron velocity saturation in the B–C space charge region and the presence of the valence band discontinuity at the B–C junction, hole accumulation occurs at the collector end of the base at the onset of base pushout at high current densities. Formation of a parasitic barrier to electron flow in the conduction band occurs at the collector junction which increases recombination in the base and the base current and produces saturation in the collector current. Together, these effects produce an abrupt degradation in the transistor’s current gain and cutoff frequency with increasing emitter junction bias. In this paper, we investigate the onset and relationship of parasitic barrier formation and base pushout, and their dependence on device structure and biasing.