Diffusion Current Component Research Articles

Evaluation of Surface-Potential-Based Bulk-Charge Compact MOS Transistor Model

The existing surface-potential-based compact metal-oxide-semiconductor transistor models are based on the 1978 Brews delta-function charge-sheet approximation, which was derived empirically from the 1966 Pao-Sah drift-diffusion double integral formula. This paper provides a device physics-based derivation of a surface-potential-based compact model by analytical approximation of the double and single bulk-charge integrals of the four one-dimensional components of the six-component 1996 Sah two-dimensional formula. In this compact model development, the mobile carrier-space-charge-limited parabolic-drift and linear-diffusion current components are analytically represented by the surface potential without approximation, while the immobile-impurity bulk-space-charge-limited double-integral drift-current and single-integral diffusion-current components are evaluated analytically using three possible surface-potential compact model approximations. This paper calculates the accuracy of these approximate analytical bulk-charge-limited drift and diffusion current components in both the inversion and subthreshold ranges and discusses factors that affect the accuracy in the subthreshold range and near flatband.

Circuit models for quasi-3D spice simulation of turn-on transients in four-layer power bipolar devices

Improved transport models for quasi-3D circuit-based simulation (Q3DSim) of four-layer devices such as thyristors and transient over-voltage protectors (TOVPs) are presented. Q3DSim is an attractive alternative to full 3D transport equations based simulations (3D-TES), since it is much faster and requires less computer power. In Q3DSim, the thyristor is divided into four-layered square prisms, and a 1D PNP–NPN transistor pair model is associated to each of them in the anode to cathode direction. The PNP–NPN elements are interconnected through two transversal grid planes of circuit elements. The resulting equivalent circuit is simulated with Spice. Plasma-spreading velocity, used here as a benchmark, depends strongly on the current-dependent transport properties in the anode to cathode path given by the transistor gains, and on the transversal transport properties of both transistor bases.The new circuital models reported here, based on the quasi-static approximation, add drift and diffusion current components to Q3DSim transversal base planes. The circuit version of the base model was implemented with finite differences in Spice. The PNP transistor of the PNP–NPN basic model was complemented with a second PNP transistor that simulates the transport enhancement in the N-base due to the ohmic effect. All the required parameters are extracted from static TES. The power of the method was demonstrated by comparing 2D-TES plasma spreading velocity simulations with Q3DSim simulations. Both simulations were very close to each other in the 50–1200A/cm2 anode current density range. Moreover, the Q3DSim current wave front shapes were very close to 2D-TESs in the same current density range, showing the validity of the presented models.

The role of diffusion current in the formation of spherical striations

We have experimentally studied spherical dc discharges with opposite polarities of the applied voltage. Under otherwise identical conditions, the phenomenon of spherical stratification takes place only when the central electrode is the anode (positive discharge). An analysis of the experimental data shows that the diffusion current component plays a decisive role in the formation of striations in a spherical glow discharge.

Simulation of Temperature Dependence of Microwave Noise in Metal-Oxide-Semiconductor Field-Effect Transistors

We present results of a two-demensional (2D) numerical simulation of high-frequency noise in a short-channel metal-oxide-semiconductor field-effect transistors (MOSFET). Conventionally this noise in MOSFETs is treated as a thermal noise with a rather slow temperature dependence of the noise spectra. On contrary, the results obtained indicate that this dependence is more of an exponential kind, typical for a shot noise. Shot noise is conventionally related to a forward biased p-n junction where the current is mostly due to diffusion in the quasi-neutral region. We therefore speculate that most likely in short channel MOSFETs most of the high-frequency noise is produced not in the cut-off region of the channel but near the source side of the channel where diffusion current component is dominant.

Silicon substrate effects on the current–voltage characteristics of advanced p–n junction diodes

An in-depth analysis of the forward and reverse current–voltage characteristics allows determination of the different geometrical (area, perimeter and corner) and physical (diffusion and generation) current components. This is a powerful technique to assess the silicon substrate quality. In this paper it is shown that the diffusion current of a good quality silicon p–n junction is significantly lower for an epitaxial (Epi) wafer compared to a Czochralski (Cz) wafer. This can be explained by a correction factor, F, which depends on the parameters of the highly doped substrate. The impact of the substrate is less pronounced when the leakage current is dominated by the peripheral component. Furthermore, for low reverse bias, current transients occur for large area diodes in Cz substrates. These are related to the presence of generation centres, which are absent in epitaxial wafers.

Surface-barrier p-CdTe-based photodiodes

Diodes fabricated by electron-beam evaporation of Al on p-CdTe substrates followed by removal of the metal layer have been investigated. It is shown that the diode electrical characteristics are determined by generation-recombination processes in the surface barrier, whereas the photoelectric properties are governed by the drift and diffusion current components, which are strongly influenced by surface recombination of carriers. The measured data are used to calculate the photoelectric quantum yield and the x-ray detection efficiency.

Accurate extraction of the diffusion current in silicon p-n junction diodes

An accurate method for the extraction of the reverse diffusion current component in a silicon p-n junction diode is proposed. It combines capacitance–voltage and current–voltage measurements on an array of diodes with different geometry in order to separate the peripheral and the volume leakage current components. The corrected volume capacitance is then used to calculate the depletion width as a function of the reverse bias. Extrapolation of the reverse current to zero depletion width results in the diffusion current part, both for the volume and for the peripheral component. From the temperature dependence, a thermal activation energy of 1.12 eV is obtained. The volume diffusion current density of the p-type Czochralski wafers studied, shows a pronounced substrate dependence, while the peripheral diffusion current density is constant. Finally, the implications for the extraction of the effective bulk recombination lifetime are discussed.

A Comprehensive Study of High-Level Free-Carrier Injection in Bipolar Junction Transistors

This paper presents a comprehensive study on the effects of high-level free-carrier injection on the current transport of bipolar junction transistors (BJTs). Detailed information for the free-carrier concentration, electric field, and drift and diffusion current components in the quasi-neutral base (QNB) under high-level injection are calculated using the modified ambipolar transport equation and using several different approximations for the majority-carrier current in the QNB. It is shown that high-level injection can create a large retarding field which is in the opposite direction of the built-in field associated with the nonuniform doping concentration. High-level injection also enhances recombination in the QNB, thus resulting in a position-dependent minority-carrier current in the region even if the base is thin. Our results further suggest that the widely used zero majority current approximation gives rise to a larger error compared to other lesser known approximations.

Frequency and Time Response of Pin Photodiode

Frequency and time responses of the PIN photodiode have been derived for front-and back-surface illumination. The analysis is based on solvign the continuity equation in the depletion-region, subject to boundary conditions, and gives the drift current component which is transit-time dependent. To get the diffusion current component, the continuity equation is solved in the n and p regions, respectively. The frequency and time responses of the PIN photodiode with different depletion region widths has been calculated. It is also found that the bandwidth is dependent on the side of illumination. With lighter illumination from the n-side, the bandwidth of homo junction (HJ) photodiode of 1 µm i-region width is found to be 18 GHz for an n-region width of 0.5 µm, and 29GHz for an n-region width of 0.2 µm. However, the corresponding bandwidths are 5 GHz, and 21 GHz for the same p-region widths of 0.5µm and 0.2 m and light illumonation from the P-side. The bandwidth of single hetero-structure junction (SHJ) PIN photodiode with 1 µm i-region width, where the layer facing the light is made from a material with wider band gap, is 26 GHx for both sides of illuminations. The bandwidth of double hetreo-structure junctions (DHJ) PIN photodiode with 1 µm i-

Effects of Current Stress on the Characteristics of a Si Heterojunction Bipolar Transistor with a Hydrogenated Microcrystalline Si Emitter

Changes in the characteristics of a µc-Si:H-emitter Si-HBT induced by forward current stress imposed on the emitter-base junction are studied. The principal changes appear in the base current which consists of the diffusion and the band-gap-state-assisted tunneling currents. The diffusion current at first increases and later decreases as stressing proceeds; the overall result due to long-term stressing is reduction. Consequently, the maximum hFE, governed by the diffusion current component, is enhanced as a result of stressing. The tunneling current increases monotonically with increasing stress time, resulting in degradation of hFE at low output currents. Changes in both diffusion and tunneling currents can be explained by the simultaneous occurrence in µc-Si:H of increases in the active dopant and the defect densities.

Numerical and charge sheet models for thin-film SOI MOSFETs

Numerical charge sheet models applicable for all bias conditions are presented for the channel currents of long-channel SOI MOSFETs. From a comparison of the two models it is shown that the charge sheet analytic model accurately predicts the channel currents from weak to strong inversion regions. The results include analytic expressions for the drift and diffusion current components of individual channel currents, the front-gate and back-gate interaction parameter, and an analytic correlation between the surface potentials of the front and back channels when there is coupling between the two gates under nonthermal equilibrium conditions. The effect of SOI (silicon on insulator) film thickness on the drain current was investigated under different bias conditions for the back gate, and it was found that thin films are beneficial from the point of increased drain currents if the back channel is in depletion or inversion. It is also shown that, in addition to the charge coupling effects, dynamic interaction between the channels exists if the static current in one of the channels saturates.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Two-dimensional charge-control model for MODFET's

A dc model for MODFET's accounting for two-dimensional effects is proposed. In this model, charge control is realized by solving the two-dimensional Poisson equation in the depleted AlGaAs region. The transport picture used for the two-dimensional electron gas (2-DEG) in the AlGaAs/GaAs heterojunction relies on the quasi-Fermi level together with a field-dependent mobility and therefore includes 2-DEG diffusion effects. Our approach reduces the analysis to a single integral equation. I-V curves, which provide a good fitting to the reported experimental data, are obtained using a smooth velocity-field curve. The channel voltage, 2-DEG concentration, parallel electric-field, and drift velocity along the channel are given in this study and provide a clear picture of current saturation. The model is consistent with the approximate two-region saturation picture but provides a smoother transition. We observe a large diffusion current component along the channel in addition to the drift current. However, the total saturation current obtained has nearly the same value as found from the two-region model. This new model with two-dimensional charge control provides much insight into the current saturation mechanism of the MODFET.

Be-implanted In0.53Ga0.47As diodes with ideal forward current-voltage characteristics

The temperature dependence of the forward current-voltage characteristics of Be-implanted p+-n junctions in In0.53Ga0.47As is presented. The results are interpreted on the basis of a diffusion current component and a recombination current component. The best diodes obey a pure diffusion current equation, J=J0(T)exp(qV/kT) for a current range of five decades and for temperatures above 200 K.

A one-dimensional theory on the effects of diffusion current and carrier velocity saturation on E-type IGFET current-voltage characteristics

Current-voltage characteristics of an enhancement-type insulated gate field-effect transistor (E-type IGFET) are analyzed based on a one-dimensional model, taking account also of the diffusion current component. Explicit formulae for the entire I-V characteristic curve are given. The solution for the triode characteristic shows considerable deviation from “drift current theory” in terms of turn-on voltage (or threshold voltage) and drain voltage at just saturation. The solution for the pentode characteristic taking account of carrier's saturation velocity, shows that the increase in drain current per unit drain voltage is larger in short-channel devices than in long-channel devices. Agreement with experiment is very good.

Characteristics of p-i-n Junctions Produced by Ion-Drift Techniques in Silicon

p-i-n junctions were fabricated by ion-drift techniques using lithium in p-type silicon. Highly compensated intrinsic regions from 0.1 to 5.0 mm were obtained. At temperatures between 120° and 200°C, the space charge of thermally generated carriers in the intrinsic region distorted the lithium ion distribution, producing linear graded junctions whose gradients were between 1011 and 1014 atoms/cm3/cm. Room-temperature capacitance measurements were used to determine the impurity distribution. Agreement was found between calculated and measured values of the impurity gradient. Precipitation of lithium ions in the intrinsic region was not observed. The increase of the direct current under reverse junction potential as a function of the increased width of the depletion region permitted measurement of the diffusion and depletion-region-generated current components. The value of the diffusion current component at any temperature was proportional to the resistivity of the p-type silicon. The depletion-region-generated component was proportional to the width of the depletion region.