High-level Injection Conditions Research Articles

Analytical and numerical simulation of electron beam induced current profiles in p-n junctions

The electron beam induced current (EBIC) mode of a scanning electron microscope (SEM) is a widely used technique for the quantitative assessment of minority carrier diffusion length and surface recombination. Point source (one-dimensional) and extended source (two-dimensional) analytical models are two widely used approaches to assess this information in geometry where the electron beam (e-beam) is parallel to the p-n junction. In this article, a two-dimensional (2D) analytical model is evaluated and compared with 2D finite element numerical simulations, where the electron beam-solid interaction is modeled using a Monte Carlo simulation coupled with a drift-diffusion solver. The simulations are computed for both low and high level injection conditions. The effect of an e-beam injection level on the shape of EBIC profiles is analyzed to evaluate limitations of the analytical models.

Switching performance of quasi-vertical GaN-based p-i-n diodes on Si

The switching performance of GaN-based p-i-n diodes on Si was investigated for the first time. A double-pulse test circuit using an inductive load was utilized to evaluate the diode's switching characteristics. When the GaN diode was switched from an on-state with IF = 450 mA to an off-state with VR = −200 V (dIF/dt = 16 A/μs), the peak reverse recovery current (Irr) and time (trr) was measured to be 19.4 mA and 7.1 ns, respectively. The amount of charges stored in the drift region and the turn-off energy was extracted to be 0.14 nC and 0.054 μJ, respectively. The carrier lifetime under high-level injection conditions in GaN was estimated to be 0.6 ns. The reverse recovery characteristics of the GaN diodes showed little sensitivity to elevated temperatures up to 150 °C. During the forward recovery (from off-state with VR = −190 V to on-state with IF = 450 mA), the GaN diode exhibited a negligible voltage overshoot. In comparison to a commercial fast recovery-Si diode, the superior reverse and forward recovery performance of GaN-based p-i-n diodes indicates their practicality in fast switching applications and reliability at high temperatures.

Modeling Inductive Switching Characteristics of High-Speed Buffer Layer IGBT

In this study, a physics-based compact model for high-speed buffer layer insulated gate bipolar transistor (IGBT) is proposed. The model utilizes the 1-D Fourier-based solution of ambipolar diffusion equation (ADE) implemented in MATLAB and Simulink. Based on the improved understanding on the inductive switching behavior of a high-speed buffer layer IGBT, the ADE is solved for all injection levels instead of high-level injection only as usually done. Assuming high-level injection condition in the buffer layer, the excess carrier transport, redistribution and recombination in the buffer layer are redescribed. Moreover, some physical characteristics such as the low conductivity of N-base at turn-on transient and free holes appeared in the depletion layer during turn-off process are also considered in the model. Finally, the double-pulse switching tests for a commercial field stop IGBT and a light punch-through carrier-stored trench bipolar transistor are used to validate the proposed model. The simulation results are compared with experiment results and good agreement is obtained.

Analysis of the reverse recovery oscillation of superjunction MOSFET body diode

The voltage and current oscillations occasionally occur during the reverse recovery transient of the superjunction MOSFET body diode. This paper identifies the unique reverse recovery oscillation characteristics of the superjunction MOSFET body diode. Base on the experimental investigation and theoretical analysis, the various reverse recovery oscillation mechanisms are clarified. At first, with a discrete 650V/47A superjunction MOSFET utilized, the high-frequency and low-frequency oscillation characteristics during the reverse recovery transient are obtained by the double-pulse test. After the theoretical analysis, it is found that the superjunction MOSFET body diode has various oscillation mechanisms depending on its drift region injection level. Under high-level injection condition, the high-frequency oscillation occurs due to the plasma extraction transient-time (PETT) effect. Under low-level injection condition, the body diode’s snappy reverse recovery results in the low-frequency LC oscillation. In the end of the paper, the oscillation behaviors under both high and low level injection conditions are reproduced by the mixed-mode numerical simulation, the simulation results validate the proposed oscillation mechanisms.

Enhanced Performance of GaInN LEDs by Abrupt Mg Doped p-AlGaN Electron Blocking Layer

Performance improvements in GaInN blue light-emitting-diodes (LEDs) including decreased wavelength shift, enhanced quantum efficiency, and reduced operating voltage are obtained by an abruptly and heavily Mg-doped p-AlGaN electron blocking layer. Moreover, the commonly observed efficiency collapse at cryogenic temperatures for conventional LEDs is not found in abrupt doped LEDs. Under high level injection conditions, the conventional LEDs have much higher electric field in the p-type neutral region than the abruptly doped LEDs at cryogenic temperatures. The higher electric field leads to the greater electron leakage, as well as the cause of efficiency collapse.

Metal/InGaN Schottky junction solar cells: an analytical approach

The photovoltaic behaviour of metal/n-InGaN Schottky junction solar cells with low- and high-level injection conditions are explored by using voltage model. Four metals Ni–Au, Ni, Au and Pt are used as Schottky contact with n-InGaN and Schottky junction solar cell studied for open-circuit voltage (V oc) and short circuit current density (J sc) with a variation of Indium composition in n-InGaN. The cut-off value of Indium composition in n-InGaN is 36, 42, 55 and 70 % for Ni–Au, Ni, Au and Pt, respectively. The effects of doping density and surface recombination velocity on V oc and J sc are also explored. Model predict increment in V oc and J sc under high-level injection condition as compared to low-level injection condition with ϕ = 2 × 1017 cm−2 s−1 photon flux. The metal, Pt, is found to be a better choice in terms of making Schottky junction with n-InGaN. The concentration of donor atoms in n-InGaN plays an important role in both types of injection. The donor concentration should preferably be kept higher (>1016 cm−3) to get higher V oc.

Operation of lightly doped Si microwires under high-level injection conditions

The operation of lightly doped Si microwire arrays under high-level injection conditions was investigated by measurement of the current-potential behavior and carrier-collection efficiency of the wires in contact with non-aqueous electrolytes, and through complementary device physics simulations. The current-potential behavior of the lightly doped Si wire array photoelectrodes was dictated by both the radial contact and the carrier-selective back contact. For example, the Si microwire arrays exhibited n-type behavior when grown on a n+-doped substrate and placed in contact with the 1,1′-dimethylferrocene+/0–CH3OH redox system. The microwire arrays exhibited p-type behavior when grown on a p+-doped substrate and measured in contact with a redox system with a sufficiently negative Nernstian potential. The wire array photoelectrodes exhibited internal quantum yields of ∼0.8, deviating from unity for these radial devices. Device physics simulations of lightly doped n-Si wires in radial contact with the 1,1′-dimethylferrocene+/0–CH3OH redox system showed that the carrier-collection efficiency should be a strong function of the wire diameter and the carrier lifetime within the wire. Small diameter (d 400 nm) exhibited higher carrier collection efficiencies that were strongly dependent on the carrier lifetime in the wire, and wires with carrier lifetimes exceeding 5 μs were predicted to have near-unity quantum yields. The simulations and experimental measurements collectively indicated that the Si microwires possessed carrier lifetimes greater than 1 μs, and showed that radial structures with micron dimensions and high material quality can result in excellent device performance with lightly doped, structured semiconductors.

An Examination of High-Injection Physics of Silicon P-N Junctions With Applications in Photocurrent Modeling

Techniques for modeling radiation-induced junction photocurrents often rely on assumptions that require the device remain in low-level injection; however transient radiation events can induce high-level injection conditions. Due to a combination of physical accuracy and computational efficiency, analytical models are often preferred to empirical and numerical modeling techniques. A more thorough understanding of the physical response can help improve the efficacy of simple modeling techniques, e.g. equivalent circuit modeling. This work explores the fundamental physical response of a p-n junction as it approaches, and after it reaches high-level injection (HLI) conditions through analysis of TCAD simulations. From the TCAD simulations and experimentally obtained data, this physical response is presented as a simple equivalent circuit model that acts as a hybrid between numerical and empirical modeling methodologies.

Effect of Bandgap Narrowing on Performance of Modern Power Devices

The effect of the bandgap narrowing (BGN) on performance of power devices is investigated in detail in this paper. The analysis reveals that the change in the energy band structure caused by BGN can strongly affect the conductivity modulation of the bipolar devices resulting in a completely different performance. This is due to the modified injection efficiency under high-level injection conditions. Using a comprehensive analysis of the injection efficiency in a p-n junction, an analytical model for this phenomenon is developed. BGN model tuning has been proved to be essential in accurately predicting the performance of a lateral insulated-gate bipolar transistor (IGBT). Other devices such as p-i-n diodes or punch-through IGBTs are significantly affected by the BGN, while others, such as field-stop IGBTs or power MOSFETs, are only marginally affected.

Discrete-contact nanowire photovoltaics

A series of finite-element simulations have been performed to assess the operational characteristics of a new semiconductor nanowire solar cell design operating under high-level injection conditions. Specifically, the steady-state current-voltage behavior of a cylindrical silicon (Si) nanowire with a series of discrete, ohmic-selective contacts under intense sunlight illumination was investigated. The scope of the analysis was limited to only the factors that impact the net internal quantum yield for solar to electricity conversion. No evaluations were performed with regards to optical light trapping in the modeled structures. Several aspects in a discrete-contact nanowire device that could impact operation were explored, including the size and density of ohmic-selective contacts, the size of the nanowire, the electronic quality and conductivity of the nanowire, the surface defect density of the nanowire, and the type of ohmic selectivity employed at each contact. The analysis showed that there were ranges of values for each parameter that supported good to excellent photoresponses, with certain combinations of experimentally attainable material properties yielding internal energy conversion efficiencies at the thermodynamic limit for a single junction cell. The merits of the discrete-contact nanowire cell were contrasted with “conventional” nanowire photovoltaic cells featuring a uniform conformal contact and also with planar point-contact solar cells. The unique capacity of the discrete-contact nanowire solar cell design to operate at useful energy conversion efficiencies with low quality semiconductor nanowires (i.e., possessing short charge-carrier lifetimes) with only light doping is discussed. This work thus defines the impetus for future experimental work aimed at developing this photovoltaic architecture.

Theoretical simulation of photovoltaic response of graphene-on-semiconductors

Using the fundamental models for voltage and current, we report on the photovoltaic behavior of graphene-on-semiconductor-based devices. The graphene-n-Si and graphene-n-GaAs systems are studied for open-circuit voltage (VOC) and short-circuit current density (JSC) under low- and high-level injection conditions. The effects of semiconductor doping density and surface recombination velocity on the VOC of both systems are investigated. The VOC for graphene-n-Si under low- and high-level injection conditions are found to be 0.353 V and 0.451 V, respectively, whereas the VOC for graphene-n-GaAs under low- and high-level injection conditions are 0.441 V and 0.471 V, respectively. The JSC for graphene-n-Si under low- and high-level injection conditions are calculated as 3 mAcm−2 and 4.78 mAcm−2, respectively, whereas the JSC for graphene-n-GaAs under low- and high-level injection conditions are 5.2 mAcm−2 and 6.68 mAcm−2, respectively. These results are in good agreement with the reported experimental work.

Analysis of the operation of thin nanowire photoelectrodes for solar energy conversion

The solar energy conversion properties of thin Si and GaP nanowire photoelectrodes in photoelectrochemical cells have been examined through sets of finite-element simulations. A discussion describing the motivation behind nanostructured, high aspect ratio semiconductor photoelectrode designs and a brief survey of current experimental results reported for nanostructured semiconductor photoelectrodes in photoelectrochemical cells are presented first. An analysis is then shown that outlines the primary recombination pathways governing the steady-state current-potential behaviors of thin, cylindrical nanowire photoelectrodes, with explicit expressions detailing the differences between planar and cylindrical photoelectrodes arising from the solution of carrier fluxes in planar and cylindrical geometries. Results from finite-element simulations used to model the key features of thin nanowire photoelectrodes under low-level injection conditions are shown that illustrate which recombination pathway(s) is operative under various experimental conditions. Specifically, the respective effects of non-uniform doping, tapering along the length, variation in charge carrier mobilities and lifetimes, changes in nanowire radius, and changes in the density of surface defects on the observable photocurrent-potential responses are reported. These cumulative results serve as guides for future experimental work aimed at improving the attainable solar energy conversion efficiencies of doped semiconductor nanowire photoelectrodes. Lastly, separate simulations that model lightly doped nanowire photoelectrodes under high-level injection conditions are discussed. These results suggest discrete, ohmic-selective contacts may afford a way to circumvent the stringent doping requirements discussed herein for thin nanowire photoelectrodes.

On an AlGaInP Light-Emitting Diode With a Modulation-Doped Multiquantum-Well (MD-MQW) Structure

An interesting AlGalnP multiquantum-well (MQW) light-emitting diode (LED) with an n-type modulation-doped (MD) structure, grown by low-pressure metal-organic vapor-phase epitaxy (LP-MOVPE), is fabricated and studied. This n-type MD-MQW LED exhibits lower dynamic resistance, higher luminescence, and higher luminous efficiency than those of a conventional undoped-MQW LED. Experimental results show a higher luminous efficiency of 16.05 lm/W and higher luminescence of 2.53 lm are obtained for the MD-MQW LED which are superior to the corresponding values of 14.49 lm/W and 2.04 lm for the undoped-MQW LED under dc operation. In addition, the n-type MD-MQW LED exhibits a higher quantum efficiency of 7.2% under dc operation as compared with the 6.7% of the undoped-MQW LED. The reduced junction temperature of 12degC at 200 mA is also acquired for the MD-MQW LED. Moreover, the brightness reliability of this new device is found to be comparable to the undoped-MQW LED. These positive results could be attributed to the presence of a higher electron concentration in the active region of the MD-MQW structure which causes the suppression of electron thermal velocity especially at a high level injection condition, and a reduced junction heating effect.

Effects of Interfacial Energetics on the Effective Surface Recombination Velocity of Si/Liquid Contacts

Photoconductivity decay data have been obtained for NH4F(aq)-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of methanol, tetrahydrofuran (THF), or acetonitrile containing either ferrocene+/0 (Fc+/0), [bis(pentamethylcyclopentadienyl)iron]+/0 (Me10Fc+/0), iodine (I2), or cobaltocene+/0 (CoCp2+/0). Carrier decay measurements were made under both low-level and high-level injection conditions using a contactless rf photoconductivity decay apparatus. When in contact with electrolyte solutions having either very positive (Fc+/0, I2/I-) or relatively negative (CoCp2+/0) Nernstian redox potentials with respect to the conduction-band edge of Si, Si surfaces exhibited low effective surface recombination velocities. In contrast, surfaces that were exposed only to N2(g) ambients or to electrolyte solutions that contained a mild oxidant (such as Me10Fc+/0) showed differing rf photoconductivity decay behavior depending on their different surface chemistry. Specifically, surfaces that pos...

Extraction of eleven model parameters for consistent reproduction of lateral bipolar snapback high-current I-V characteristics in NMOS devices

A series of literature models originally devoted to the second breakdown trigger current I/sub t2/ in a grounded-gate NMOS transistor can further find promising potential in handling high-current I-V due to lateral bipolar snapback. This is achieved primarily by building significant linkage between bipolar current-gain /spl beta/-related parameters: 1) the collector-to-base junction voltage dependencies A/sub 1/ and A/sub 2/ of the medium-level injection /spl beta/ roll-off factor; 2) the high-level /spl beta/ roll-off factor A/sub 3/; and 3) the collector-to-base junction voltage dependencies A/sub 4/ and A/sub 5/ of the collector corner current at the onset of high-level /spl beta/ roll-off. The new parameters A/sub 1/ to A/sub 5/ enable a consistent I-V solution along with other existing six model parameters such as the substrate resistance R/sub sub/ and its conductivity modulation factor A/sub /spl tau//, the impact ionization coefficients K/sub 1/ and K/sub 2/, and the emitter series resistance R/sub e/ and collector series resistance R/sub c/. Parameter extraction except R/sub c/ is thoroughly performed using only the parametric analyzer, and opposed to the traditional procedure, impact ionization coefficients and current gains are all assessed without entering the snapback regime. Remarkably, not only excellent agreements are obtained, but also bipolar snapback I-V measured under the current pulsing condition can be separated into two distinct parts: medium- and high-level injection region. This is quite effective under R/sub e/=R/sub c/. Series resistance, although having very low value, is not to be absent under the high-level injection conditions.

Preparation of air-stable, low recombination velocity Si(111) surfaces through alkyl termination

A two-step, chlorination/alkylation procedure has been used to convert the surface Si–H bonds on NH4F(aq)-etched (111)-oriented Si wafers into Si–alkyl bonds of the form Si–CnH2n+1 (n⩾1). The electrical properties of such functionalized surfaces were investigated under high-level and low-level injection conditions using a contactless rf apparatus. The charge carrier recombination velocities of the alkylated surfaces were <25 cm s−1 under high-level and low-level injection conditions, implying residual surface trap densities of <3×109 cm−2. Although the carrier recombination velocity of hydrogen-terminated Si(111) surfaces in contact with aqueous acids is <20 cm s−1, this surface deteriorates within 30 min in an air ambient, yielding a high surface recombination velocity. In contrast, methylated Si(111) surfaces exhibit low surface recombination velocities in air for more than 4 weeks. Low surface recombination velocities were also observed for Si surfaces that had been modified with longer alkyl chains.

Recombination processes in doubly capped antimonide-based quaternary thin films

Recombination processes in antimonide-based materials for thermophotovoltaic (TPV) devices have been investigated using a radio-frequency (rf) photoreflectance technique, in which a Nd–YAG pulsed laser is used to excite excess carriers, and the short-pulse response and photoconductivity decay are monitored with an inductively coupled noncontacting rf probe. Both lattice-matched AlGaAsSb and GaSb have been used to double cap InGaAsSb active layers to evaluate bulk lifetime and surface recombination velocity with different active layer thicknesses. With an active layer doping of 2×1017 cm−3, effective bulk lifetimes of 95 ns and surface recombination velocities of 1900 cm/s have been obtained. As the laser intensity is increased the lifetime decreases, which is attributed to radiative recombination under these high-level injection conditions. Similar measurements have been taken on both TPV device structures and starting substrate materials for comparison purposes.

Behavior of Si Photoelectrodes under High Level Injection Conditions. 3. Transient and Steady-State Measurements of the Quasi-Fermi Levels at Si/CH3OH Contacts

Real-time measurements of the photovoltage rise and decay at the back of lightly doped, thin, long lifetime Si photoelectrodes were recorded subsequent to a variety of spatial and temporal carrier generation impulses. The functional form of the rising portion of the photovoltage signal is sensitive to charge transport processes, and this signal was used to validate experimentally the hypothesis that charge transport in these samples under high level injection is primarily driven by diffusion, as opposed to drift. The decay of the photovoltage signal back to its equilibrium value yielded information concerning the surface recombination velocity, Sf, of the various Si/CH3OH redox couple contacts. These data validated the relatively high surface quality of the Si/liquid interface in contact with a variety of redox species. Furthermore, the low surface recombination velocities are in agreement with prior theoretical and experimental estimates of interfacial charge-transfer rate constants for semiconductors in contact with nonadsorbing, outer-sphere, redox species. The front surface recombination velocity data also provided a needed boundary condition for modeling the carrier concentration dynamics and allowed quantification of the difference between the quasi-Fermi levels at the back and front surfaces of the samples at all times of experimental interest. Digital simulation and analytical modeling were performed to compute the gradients in the quasi-Fermi levels for samples operated under steady-state, open-circuit, high level injection conditions. In no case was the difference between the quasi-Fermi level value at the back of the sample and its value at the solid/liquid contact greater than 10 meV. These data, combined with those described in parts 1 and 2, comprise a relatively complete picture of the transport and recombination processes that occur at these types of semiconductor/liquid contacts.

Behavior of Si Photoelectrodes under High Level Injection Conditions. 2. Experimental Measurements and Digital Simulations of the Behavior of Quasi-Fermi Levels under Illumination and Applied Bias

Use of thin, nearly intrinsically doped Si electrodes having implanted, interdigitated n+ and p+ back contact points has allowed electrical control over the potential of either electrons or holes in the solid. During potential control at the n+ point contacts, the open-circuit potential of holes could be monitored, while during potential control of the p+ point contacts, the open-circuit potential of electrons was measured. In combination with current density−voltage measurements of either electrons or holes passing through the back contact points, these data allowed a comparison of the behavior of a given carrier type when generated by an applied bias (i.e., as majority carriers) relative to their behavior when generated with band gap illumination of the solid (as minority carriers). Data have been collected for Si/CH3OH junctions having 1,1‘-dimethylferrocene+/0, decamethylferrocene+/0, methyl viologen2+/+, and cobaltocene+/0 as redox couples. These data have been used to validate certain key prediction...

Behavior of Si Photoelectrodes under High Level Injection Conditions. 1. Steady-State Current−Voltage Properties and Quasi-Fermi Level Positions under Illumination

The behavior of the quasi-Fermi levels of electrons and holes at various semiconductor/liquid interfaces has been probed through the use of thin, high purity, low dopant density single crystal Si photoelectrodes. Since standard Air Mass 1.5 illumination is sufficient to produce high level injection conditions in such samples, minimal electric fields can be present near the solid/liquid interface. Under these conditions, efficient charge separation relies on establishment of kinetic asymmetries at the back contacts while effectively sustaining photogenerated carrier concentration gradients in the photoelectrode. These conditions were achieved for Si/CH3OH interfaces in contact with the 1,1‘-dimethylferrocene+/0, cobaltocene+/0, methyl viologen2+/+, and decamethylferrocene+/0 redox couples. For redox couples having energies near the top of the Si valence band, such as 1,1‘-dimethylferrocene+/0, the sample acted like an n-type photoelectrode, yielding large photovoltages for collection of electrons at the ba...