New physical formulation of the thermionic emission current at theheterojunction interface

The thermionic emission current for electrons across the heterojunction interface is classically modeled as the difference between two opposing electron fluxes. A new, consistent, physical model, which includes the carrier degeneracy and nonideal behavior effects, is presented. It is shown that the emission current at the interface can be expressed in a simple closed-form formalism that gives the relationships among the average directional thermal velocity of electrons, the conduction band discontinuity, and the carrier activities at both sides of the interface. The conditions under which the thermionic emission occurs at the heterointerface are discussed     

Currents and current gain analysis of passivated heterojunctionbipolar transistors (HBT)

In this paper, we investigate the effect of the effective interface velocities on the currents, and the current gain of arbitrary base doping and different types of level injection in the base of a heterojunction bipolar transistors (HBT). It is shown that the space-charge region recombination current and the collector current are strong functions of the effective interface velocities and the thermionic flux. An expression of the effective interface velocities and the carrier boundary conditions of the HBT are derived. The effect of the thermionic emission flux on the quasi-Fermi level discontinuity is studied, and an expression of the quasi-Fermi level discontinuity is derived and shown to be function of the interface velocities and the space-charge recombination current. An expression of the current gain of an HBT is derived. The calculated current gain is in excellent agreement with other reported experimental results     

PLhenomenological model of grain-boundary trapping states in polycrystalline silicon under optical illumination

We have developed a new phenomenological model of grain boundary (GB) recombination, carrier transport and electrostatics under assumptions of Gaussian energy distributions of GB interface states, and unequal capture cross-sections of these GB states for electrons and holes in polycrystalline materials. Calculations have been performed of the recombination current density and the recombination velocity at grain boundaries in polycrystalline silicon for four different energy distributions. The results indicated that the recombination velocity at the grain boundary strongly depends on the location of the trap energy level. The assumption of GB interface states with a discrete (δ-function) distribution has been found to be inappropriate.     

Physical model of depletion and accumulation in quantum-wellinfrared photodetectors

Numerical work has shown that, at low operating temperatures or large incident photon fluxes, carriers deplete from the quantum wells near the emitter contact in a quantum-well infrared photodetector (QWIP). A physical model is developed in this work to describe, with closed-form analytical expressions, the accumulation and depletion of carriers in QWIPs. In QWIPs having the same growth sequence (layer widths and compositions) in each period, carrier depletion is found to occur only in one or two QWs near the emitter contact at the small applied biases for which the electron drift velocity is linear in the electric field. At intermediate applied biases for which the electron drift velocity is saturated, carrier depiction is found to be partial, uniform (throughout the depletion region), and abrupt, with the total charge in the depletion region fixed and with the depletion width increasing linearly with applied bias. At a large applied bias, carriers are found to be uniformly accumulated in the device. Carrier depletion or accumulation in QWIPs arises from the different dependences on the local electric field of the different physical mechanisms which are responsible for the carrier injection from the contacts (via thermionic emission or thermionic field assisted tunneling) and for the photoconduction process (via drift)     

Noise transport in the crossed-field diode

The present work treats the transport of noise, originating at a thermionic cathode, across a planar crossed-field diode. A multivelocity Monte Carlo analysis is used to simulate thermionic emission by using random numbers to generate electron emission times and velocities. Trajectories for many charges are followed through the diode space by means of a high-speed digital computer. Measurements performed on a planar triode model for the crossed-field diode show that the anode noise current increases rapidly near the critical magnetic field, at which the stream just grazes the anode. The theory agrees well with these experimental findings. The Monte Carlo study predicts the increase of both the normal anode velocity fluctuations and the spread of the velocity distribution with magnetic field. This is caused mainly by the "geometrical" spreading of the initial velocity distribution. The crossed-field potential minimum exhibits no instabilities in the planar model. The noise current smoothing at low frequencies in ordinary streams is predicted for moderate fields but disappears near the critical field.     

Effective recombination velocity of polysilicon contacts for bipolar transistors

The effective surface recombination velocity is determined analytically for a doped polysilicon contact to the emitter of a bipolar transistor in the presence of a thin interfacial oxide layer. Results are presented for various doping levels, oxide thicknesses and barrier heights. The analysis considers both tunnelling and thermionic emission through the interface.     

Carrier recombination at grain boundaries and the effective recombination velocity

The recombination of excess minority carriers at grain boundaries, or other interfaces with space-charge regions, is treated theoretically for general energy distributions of interface states (recombination centers). The distinction is made between minority carrier recombination velocity at the (grain-boundary) interface itself, and the effective recombination velocity for the collection of these carriers by the adjacent space-charge region. Calculations of the effective recombination velocity are made, as a function of the excess minority-carrier concentration at the edge of the space-charge region, since this is the quantity of most convenience for device modelling.     

Inclusion of carrier temperature effects in a thermionic-diffusion theory of the schottky barrier

Recent evidence indicates that thermionic emission is a more important process than conduction-diffusion for current limitation in many Schottky barriers. By choosing suitable momentum and energy conservation equations an expression for the thermionic-diffusion current is obtained where it is assumed that the carrier temperature in the space charge region of the metal-semiconductor interface does not differ substantially from the equilibrium temperature. When thermionic emission is the dominant process for current limitation it is shown that carrier temperature effects may still be important.A discussion is presented where the above solution is applied to a model Si-type diode. Carrier temperature effects reduce the current below its ‘thermal equilibrium’ value in all cases.     

The effects of grain boundary and interface recombination on the performance of thin-film solar cells

The effects of grain boundary and interface minority carrier recombination on polycrystalline thin-film photovoltaic heterojunctions are presented. The grain boundary is modeled accounting for interface states, due to low-, medium- and high-angle grain boundaries. The dark and illuminated diffusion potentials are calculated as functions of interface state densities and carrier concentrations. These are used to estimate the recombination velocities and minority carrier lifetimes. These parameters are, in turn, correlated with the short-circuit currents and open-circuit voltages. The dependence of V_oc upon grain diameter is predicted. Junction interface states are discussed in terms of a dislocation model. The dependence of V_oc, affected primarily through dark reverse current, on the recombination velocity is indicated. The combined effects of grain boundary and interface recombination mechanisms on J_sc is discussed. Data are presented to verify the model based upon the CdS/CuInSe₂ photovoltaic heterojunction.     

Polysilicon base contact in I2L transistors

The extrinsic base diode minority carrier distribution under poly in I2L transistors can be calculated (or computed) in terms of an effective surface recombination velocity Seff at the mono/poly silicon interface similar to the oxide and metal cases. An analytical expression for Seff as a function of polysilicon parameters is given and compared to numerical solutions. Calculated ring speeds show a slight improvement in propagation delay for metal contact compared to poly contact.     

A simulation model for cathodoluminescence in the scanning electronmicroscope

An improved three-dimensional model for simulating cathodoluminescence (CL) in a semiconductor under electron-beam irradiation is described. The Monte Carlo method is used to simulate electron-beam-semiconductor interaction while F. Berz and H.K. Kuiken's (1976) formulation is used to obtain the excess carrier distribution. Optical losses of photons both within the semiconductor and at the semiconductor-air interface are also accounted for in this model. This model has been used to simulate the CL intensity as a function of electron-beam voltage, beam incidence angle, surface recombination velocity, diffusion length, absorption coefficient, and surface dead-layer thickness. The radiation patterns over the top face of a specimen with flat geometry are also simulated     

Simple model for carrier densities in the depletion region of p-njunctions

By applying the concepts of carrier velocity saturation, and electron-hole pair generation and recombination, in concert with the standard textbook derivation of the p-n abrupt junction diode equations, an analytic expression is found for the magnitude of the carrier densities near and through the depletion region of abrupt p-n junctions. The analytic expression is useful for determining carrier-density or recombination-dependent processes within the depletion region. The derivation is based on the assumption that electrons and holes pass through the depletion region at the saturation velocity. The analytic expression is compared to S-PISCES 2B simulation in a specific silicon-p-n junction. The model is called VESAT to indicate its dependence on carrier velocity saturation     

Non uniform recombination in thin silicon-on-sapphire films

Recombination parameters of SOS films are deduced from the study of the magnetoconcentration effect in double-injecting structures. The method of measurement is based on an original theory succintly developed; it takes into account general SRH bulk and surface recombination laws; moreover inhomogeneous distributions of recombination centers are considered.Experimental results (current-voltage characteristics of such “magnetodiodes”), when analysed according to the proposed method, lead to more realistic values of the global recombination parameters (τ_v, S₁, S₂) of the SOS film. It is proved, for example, that the previous simplified analysis overestimates the carrier recombination velocity on the Si-Al₂O₃ surface. On the contrary our method gives both a moderate value for this recombination velocity and a lower carrier lifetime near the Sapphire interface, which well agrees with the continuity of recombination rates at the surface and in the underlying bulk; the higher recombination region is found to be 100–1000 Å thick.     

轰击区的非辐射复合对作用区注入载流子空间分布的影响

在质子轰击型GaAs-AlGaAs DH条形激光器中,轰击区的非辐射复合速度v_B高达(7±1)×10~5厘米/秒.这对深轰击(轰击前沿越过作用层)条形激光器作用区注入载流子的空间分布必将产生重要影响.然而,过去的理论分析并未考虑此点.本文报导我们在考虑了这一因素后所得到的结果.     

The gate-controlled diode s0 measurement and steady-state lateral current flow in deeply depleted MOS structures

A detailed analysis of steady state lateral current flow in the deeply depleted gate-controlled diode structure is performed primarily to ascertain the minority carrier quasi-Fermi level positioning and lateral variation near the oxide-semiconductor interface. From a consideration of limiting case solutions it is quantitatively established that, under typical conditions, the semiconductor surface will be weakly inverted over all but a small fraction of the surface channel for gate biases far below the voltage required to strongly invert the surface. It follows from this weak inversion positioning of the minority carrier quasi-Fermi level that the s₀-value deduced from the previously published interpretation of gate-controlled diode data will be less, typically much smaller, than the true depleted surface recombination velocity. A method for ascertaining the minimum error in the previously published s₀ determination procedure is presented and illustrated.     

The importance of bandgap narrowing distribution between theconduction and valence bands in abrupt HBTs

Abrupt heterojunction bipolar transistors (HBTs) show interfaces where discontinuities in the energy levels appear. Currents through these interfaces are controlled by tunneling and thermionic emission. The values of these currents depend on the form and height of the energy barriers, which are disturbed by the heavy doping effects on semiconductor energy band structure. In this work, the real bandgap narrowing is distributed between the conduction and valence bands according to Jain-Roulston model, and its effect on the base and collector currents of Si/SiGe and InP/InGaAs HBTs is analyzed. This analysis is carried out through a numerical model which combines the drift-diffusion transport in the bulk of transistor with the thermionic emission and tunneling at the base-emitter interface, and an empirically determined surface recombination current     

Multidimensional augmented current equation including velocityovershoot

The augmented drift-diffusion current equation, which includes velocity overshoot effects through the space derivatives of the electric field, cannot be directly extended beyond one dimension. A new formalism is developed which considers the carrier heating and the distribution relaxation effects to obtain a multidimensional augmented drift diffusion current equation. The equivalent mobility containing the velocity overshoot correction is derived from the perturbation analysis on the carrier temperature using the energy balance equation. The issues related to the numerical implementation of this generalized model and the validity of the assumptions are also discussed     

Voltage drop in n- and p-type Bragg reflectors for vertical-cavitysurface-emitting lasers

An analysis of carrier transport in n- and p-type distributed Bragg reflectors (DBR) of vertical-cavity surface-emitting lasers that consist of stacks of quarter-wave GaAs-AlAs layers is presented. The analysis is based on the diffusion-drift approximation with the thermionic boundary conditions at heterojunction interfaces. The spatial distribution of carrier effective masses and mobilities has been taken into account. While the voltage drop in n-type DBR is determined mostly by thermionic emission at the interfaces, the drift-diffusion component of the voltage drop is comparable with the thermionic emission in p-type DBR. We present the calculated resistance as a function of graded-region thicknesses and doping levels, which can be useful for low-resistive DBR design     

Minority carrier lifetimes and lasing thresholds of PbSnTe heterostructure lasers

A new calculation of radiative minority carrier lifetime in PbSnTe is presented which yields lifetimes an order of magnitude greater than previously accepted values. These results are used in combination with recent measurements of minority carrier lifetime and interface recombination velocity in PbSnTe double heterojunction laser structures to calculate DH laser thresholds. It is demonstrated that radiative recombination is a relatively inefficient process in present PbSnTe lasers, and that when this inefficiency is taken into account the magnitude of the experimentally observed threshold and its variation with active region width and temperature can be accurately predicted, even at low temperatures where all previous models have failed.     

PC1D方法对铝背场钝化技术的分析

降低单晶硅原材料成本,采用更薄的硅片作为太阳电池的原料是晶体硅太阳电池产业发展的趋势之一。对薄片化的太阳电池,铝背场的背表面钝化工艺显得愈加重要。采用PC1D太阳电池软件模拟的方法,对以商业用p型硅为衬底的单晶硅125×125太阳电池的铝背场的背表面钝化技术进行了模拟,分析得出,对一定厚度的电池片来说,尤其是当少数载流...