P-type Base Region Research Articles

Defect Dynamic Model of the Synergistic Effect in Neutron- and γ-Ray-Irradiated Silicon NPN Transistors.

A defect dynamic model is proposed for the positive synergistic effect of neutron- and γ-ray-irradiated silicon NPN transistors. The model considers a γ-ray-induced transformation and annihilation of the neutron-induced divacancy defects in the p-type base region of the transistor. The derived model of the base current predicts a growth function of the γ-ray dose that approaches exponentially an asymptotic value, which depends linearly on the neutron-induced initial displacement damage (DD) and a linear decay function of the dose whose slope depends quadratically on the initial DD. Variable fluence and dose neutron-γ-ray irradiation experiments are carried out, and we find all of the novel dose and fluence dependence predicted by the proposed model are confirmed by the measured data. Our work, hence, identifies that the defect evolution under γ-ray irradiation, rather than the widely believed interface Coulomb interaction, is the dominating mechanism of the synergistic effect. Our work also paves the way for the modification of displacement defects in silicon by γ-ray irradiation.

A DLTS Perspective on Electrically Active Defects in Plated Crystalline Silicon n+p Solar Cells

Laser ablation (LA) has been compared with standard wet etching for contact opening in crystalline silicon n+p solar cells, from a perspective of electrically active defects, assessed by Deep-Level Transient Spectroscopy (DLTS). Copper metallization is employed, including a plated nickel diffusion barrier. It is shown that a hole trap around 0.17 eV above the valence band is systematically present in the depletion region of the junctions, irrespective of the contact opening method. This level could correspond with the substitutional nickel donor level in silicon and indicates that Ni in-diffusion occurs during the contact processing. No clear evidence for the presence of electrically active copper has been found. In addition, two other hole traps H2 and H3, belonging to point defects, have been observed after wet etching and standard LA, while for the highest laser power (hard LA) a broad band develops around 175 K, which is believed to be associated with dislocations, penetrating the p-type base region. Evidence will also be given for the impurity decoration of the dislocations, which enhances their electrical activity.

Electrically Active Defects in Plated Crystalline Silicon n+p Solar Cells: A DLTS Perspective

Laser ablation (LA) has been compared with standard wet etching for contact opening in crystalline silicon n+p solar cells, from a perspective of electrically active defects, assessed by Deep-Level Transient Spectroscopy (DLTS). Copper metallization is employed, including a plated nickel diffusion barrier. It is shown that a hole trap around 0.17 eV above the valence band is systematically present in the depletion region of the junctions, irrespective of the contact opening method. It is believed that this could correspond with the substitutional nickel donor level in silicon and indicates that nickel in-diffusion occurs during the contact processing. No clear evidence for the presence of electrically active copper has been found. In addition, two other hole traps H2 and H3, belonging to point defects, have been observed after wet etching and standard LA, while for the highest laser power (hard LA) a broad band develops around 175 K, which is believed to be associated with laser-ablation-induced dislocations, penetrating the p-type base region. Evidence will also be given for the impurity decoration of the dislocations, which enhances their electrical activity.

Highly efficient epitaxial Ge solar cells grown on GaAs (001) substrates by MOCVD using isobutylgermane

Single-junction Ge solar cells have been epitaxially grown on GaAs (001) substrates by a low pressure metalorganic chemical vapor deposition using isobutylgermane. Various growth conditions have been studied to reduce the high doping level of over 1×1019cm−3 of the p-type Ge epitaxial layer, which is detrimental to the minority carrier collection efficiency. By increasing the growth rate and employing a discontinuous doping technique, the p-type doping level of the epitaxial Ge layer has been lowered to 2×1018cm−3, and the hole mobility increased from 44 to 162cm2/Vs. A high power conversion efficiency of 6.72% can be achieved under AM1.5G illumination by the epitaxial Ge solar cell with the lowered doping level in the p-type Ge base region. The spectral response of the epitaxial Ge solar cell with a 5µm thick base layer is good enough to use for InGaP/GaAs/Ge triple-junction solar cells.

Highly efficient single-junction GaAs thin-film solar cell on flexible substrate.

There has been much interest in developing a thin-film solar cell because it is lightweight and flexible. The GaAs thin-film solar cell is a top contender in the thin-film solar cell market in that it has a high power conversion efficiency (PCE) compared to that of other thin-film solar cells. There are two common structures for the GaAs solar cell: n (emitter)-on-p (base) and p-on-n. The former performs better due to its high collection efficiency because the electron diffusion length of the p-type base region is much longer than the hole diffusion length of the n-type base region. However, it has been limited to fabricate highly efficient n-on-p single-junction GaAs thin film solar cell on a flexible substrate due to technical obstacles. We investigated a simple and fast epitaxial lift-off (ELO) method that uses a stress originating from a Cr/Au bilayer on a 125-μm-thick flexible substrate. A metal combination of AuBe/Pt/Au is employed as a new p-type ohmic contact with which an n-on-p single-junction GaAs thin-film solar cell on flexible substrate was successfully fabricated. The PCE of the fabricated single-junction GaAs thin-film solar cells reached 22.08% under air mass 1.5 global illumination.

Stability of the positively charged manganese centre in GaAs heterostructures examined theoretically by the effective mass approximation calculation near the Γ critical point

This paper describes an n—i—p—i—n model heterostructure with a manganese (Mn)-doped p-type base region to check the stability of a positively charged manganese A+Mn centre with two holes weakly bound by a negatively charged 3d5(Mn) core of a local spin S = 5/2 in the framework of the effective mass approximation near the Γ critical point (k ∼ 0). By including the carrier screening effect, the ground state energy and the binding energy of the second hole in the positively charged centre A+Mn are calculated within a hole concentration range from 1 × 1016 cm−3 to 1 × 1017 cm−3, which is achievable by biasing the structure under photo-excitation. For comparison, the ground-state energy of a single hole in the neutral A0Mn centre is calculated in the same concentration range. It turns out that the binding energy of the second hole in the A+Mn centre varies from 9.27 meV to 4.57 meV. We propose that the presence of the A+Mn centre can be examined by measuring the photoluminescence from recombination of electrons in the conduction band with the bound holes in the A+Mn centre since a high frequency dielectric constant of ∊∞ = 10.66 can be safely adopted in this case. The novel feature of the ability to tune the impurity level of the A+Mn centre makes it attractive for optically and electrically manipulating local magnetic spins in semiconductors.

Transistor laser with emission wavelength at 1544nm

Data are presented demonstrating continuous wave laser operation at −185°C of an InP–InAlGaAs–InAlAs double heterojunction bipolar transistor with strained InGaAs quantum wells incorporated in the p-type base region. The laser exhibits a peak wavelength λ∼1544nm when biased in the forward active mode in the common-emitter configuration. A threshold current IB=10mA is observed.

Room temperature continuous wave operation of a heterojunction bipolar transistor laser

Continuous wave laser operation at 25°C, with simultaneous electrical gain, of an InGaP–GaAs heterojunction bipolar transistor laser, with AlGaAs optical confining layers and an InGaAs recombination quantum well incorporated into the p-type base region, is demonstrated. At laser threshold (IB=40mA, VCB=0, 25°C), the transistor current gain β=ΔIC∕ΔIB in common-emitter operation changes abruptly (2.3→1.2,β>1), with laser modes developing at wavelength λ∼1006nm. Direct three-port modulation of the transistor laser at 3GHz is demonstrated for a device with a 2.2μm emitter width and a 850μm length between cleaved Fabry–Perot facets (which is the performance of an exploratory device and not near the limits).

Microwave operation and modulation of a transistor laser

The microwave operation and modulation (3GHz) of an InGaP∕GaAs heterojunction bipolar transistor (HBT) laser is reported. The HBT layer structure is in the form of an optical waveguide (with cleaved ends) that also includes an InGaAs recombination quantum well in the p-type base region to improve the recombination radiation properties. The shift in HBT laser operation from spontaneous to stimulated emission is manifest as a distinct change in the HBT current–voltage characteristics, specifically a decrease in the common-emitter current gain (βdc=IC∕IB) and the occurrence of even a more striking peak in the small signal (ac) gain βac=ΔIC∕ΔIB.

Laser operation of a heterojunction bipolar light-emitting transistor

Data are presented demonstrating the laser operation (quasicontinuous, ∼200K) of an InGaP–GaAs–InGaAs heterojunction bipolar light-emitting transistor with AlGaAs confining layers and an InGaAs recombination quantum well incorporated in the p-type base region. Besides the usual spectral narrowing and mode development occurring at laser threshold, the transistor current gain β=ΔIc∕ΔIb in common emitter operation decreases sharply at laser threshold (6.5→2.5,β>1).

Hydrogen passivation of n+p and p+n heteroepitaxial InP solar cell structures

High-efficiency, heteroepitaxial (HE) InP solar cells, grown on GaAs, Si or Ge substrates, are desirable for their mechanically strong, light-weight and radiation-hard properties. However, dislocations, caused by lattice mismatch, currently limit the performance of the HE cells. This occurs through shunting paths across the active photovoltaic junction and by the formation of deep levels. In previous work we have demonstrated that plasma hydrogenation is an effective and stable means to passivate the electrical activity of dislocations in specially designed HE InP test structures. In this work, we present the first report of successful hydrogen passivation in actual InP cell structures grown on GaAs substrates by metalorganic chemical vapor deposition (MOCVD). We have found that a 2 hour exposure to a 13.56 MHz hydrogen plasma at 275 C reduces the deep level concentration in HE n+n InP cell structures from as-grown values of approximately 10(exp 15)/cm(exp -3), down to 1-2 x 10(exp 13)/cm(exp -3). The deep levels in the p-type base region of the cell structure match those of our earlier p-type test structures, which were attributed to dislocations or related point defect complexes. All dopants were successfully reactivated by a 400 C, 5 minute anneal with no detectable activation of deep levels. I-V analysis indicated a subsequent approximately 10 fold decrease in reverse leakage current at -1 volt reverse bias, and no change in the forward biased series resistance of the cell structure which indicates complete reactivation of the n+ emitter. Furthermore, electrochemical C-V profiling indicates greatly enhanced passivation depth, and hence hydrogen diffusion, for heteroepitaxial structures when compared with identically processed homoepitaxial n+p InP structures. An analysis of hydrogen diffusion in dislocated InP will be discussed, along with comparisons of passivation effectiveness for n+p versus p+n heteroepitaxial cell configurations. Preliminary hydrogen-passivated HE InP cell results will also be presented.

Deep centers and electroluminescence in 4HSiC diodes with a p-type base region

We have performed electrical and optical measurements to characterize the processes of radiative recombination of carriers for 4H-SiC p-n + junctions. We have detected three levels in the band gap, using capacitance and current deep-level transient spectroscopy, which are responsible for the bands observed in the electroluminescence spectra. We assume that the HK 1 center with an activation energy of E v + 0.229 eV and a hole capture cross-section of 8 × 10 −13 (300/ T) 3 is related to the Al acceptor level.

A comparison of minority electron transport in In0.53Ga0.47As and GaAs

The electron transport properties of heavily doped p-type In0.53Ga0.47As and GaAs have been investigated and the applications in heterojunction bipolar transistors (HBTs) are emphasized. Using the dielectric function formalism, we have characterized minority electron transport in terms of mean-free path and diffusivity. These parameters quantify the nonequilibrium (ballistic) and near-equilibrium (diffusive) transport in the p-type base region of HBTs. Our calculations demonstrate that electron energies above 300 meV provide no benefit for ballistic transport in p-type InGaAs in terms of momentum relaxation mean-free path. Especially for very heavily doped cases (∼1020 cm−3), low-energy electron injection into the p-type InGaAs base is more advantageous for base transport, as well as the succeeding transport in the base-collector depletion region. When diffusive transport is dominant, p-type InGaAs exhibits superior performance over GaAs for a wide range of doping concentrations.

Temperature dependence of the open-circuit voltage of an N +-P-P + silicon solar cell under high illumination levels

The effect of the temperature-dependent solar cell parameters on the open-circuit voltage of n +-p-p + solar cells at medium and high levels of illumination is studied. The effects of temperature variations in the intrinsic carrier concentration, mobility, absorption coefficient, excess carrier lifetime and energy band gap shrinkage are incorporated to analyse the variation in the open-circuit voltage with temperature. To calculate the n +-p and p-p + junction voltages, the condition of equal current densities at the two junctions is numerically solved for the voltages under open-circuit conditions. Using these two voltages and the Dember potential, which appears in the p-type base region of the cell, the total open-circuit voltage is calculated for the temperature range 0 – 100°C. The n +-p junction voltage decreases while the p-p + junction and Dember voltages increase slowly with increasing temperature. The effect of all the voltages is such that the total open-circuit voltage decreases with increasing cell temperature. The decrease in the open-circuit voltage with increasing temperature at low and medium levels of illumination is larger than that at high levels of illumination. This effect is explained in terms of the different behaviours of the open-circuit voltages for n +-p and p-p + junctions.

Measuring Methods of Solar Cell Parameters Using Spectrum Response of Short-Circuit Current

Using the spectrum response in which a photon flux F is required to produce the short-circuit current to be constant, new methods for the determination of the solar cell parameters, i.e. the ratio (Dp/Sp) of the diffusion coefficient Dp of minority carriers in the n-type top region to the surface recombination velocity Sp at the front surface, the junction depth xj, the reflectivity R and the diffusion length Ln of minority carriers in the p-type base region, are proposed. It is obtained that Ln has a tendency to decrease with increasing the doping level and that R of black cells has a tendency to decrease with etching time. It is found experimentally that Dp/Sp of flat-surface cells and of black cells has a tendency to increase with doping level and with etching time, respectively.

A three mask bipolar integrated circuit structure

A new bipolar integrated circuit structure has been fabricated that compares favorably to the MOS structure in terms of fabrication simplicity and performance. The new structure is basically a modified isolated lateral transistor and requires only three photolithographic masking operations up to and including first level of metalization. The fabrication of the structure is as follows: a shallow nonselective p-type base region is diffused into a lightly doped p-type substrate; n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> emitter and collector regions are then simultaneously and selectively diffused into and through the p-type base region thus forming a lateral n-p-n transistor. The second and third masks define the contact holes and the metalization pattern, respectively. Lateral isolation of the structure is obtained by encircling the emitter and base regions with the collector region. Vertical isolation is achieved by the large collector-depletion region that extends beneath the emitter and base regions. Since the substrate is lightly doped a low collector voltage will adequately isolate the emitter and base regions from adjacent devices. The new technology permits the fabrication of transistors, resistors, and crossunders. Transistors with 2-to 3-µm spacings occupy 500 µm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of silicon area and have the following characteristics: <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\beta = 35</tex> , peak <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_{t} = 0.1</tex> GHz at 0.5 mA, BV(SUSTAIN) = 3 to 5 V, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t_{r} = 20</tex> ns, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t_{f} = 120</tex> ns, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t_{s} = 20</tex> ns. Resistors with values as high as 40 kΩ have been fabricated within 600 µm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Active nonlinear loads with effective resistance up to 200 kΩ have been fabricated. TTL gates have been made with power-delay products of 3.6 pJ, and propagation delays of 34 ns.

Defect Clusters in Electron-Irradiated Silicon

Calculations of the formation of disordered regions in silicon due to irradiation by high energy (15-45 MeV) electrons indicate that a sufficient concentration of defect clusters is produced to affect the electrical properties of the material. Isochronal annealing of room temperature radiation-induced degradation in the short-circuit current of silicon solar cells and in the minority carrier lifetime of the p-type base region is studied up to 500°C. The existence of a low temperature (50-200°C) annealing stage is shown to be independent of dopant and oxygen impurity concentration. It is inferred that this stage, which is similar to those observed in fast neutron- and in proton-irradiated silicon, is characteristic of cluster formation.

Silicon n-p-n Grown Junction Transistors

Silicon n-p-n junction transistors have been made from rate-grown single crystals. Using gallium and antimony as doping agents, single crystals of n-type silicon have been grown containing up to five p-regions from 0.0005 to 0.002 inches wide which are suitable for the production of transistors. The ohmic contact to the p-type base region was made by alloying an aluminum wire using techniques similar to those employed in the fabrication of aluminum-silicon diodes. The electrical evaluation of n-p-n transistors produced from these single crystals is described. Alphas in excess of 0.9 at −1 ma emitter current have been obtained, and collector saturation currents of 10−6ampere/cm2 are common. With base-layer widths of about 0.0005 inch, the alpha cutoff occurs at approximately 5 megacycles. The units (approximately 0.04×0.04×0.5 inches in size) have been operated at power levels in excess of 1.5 watts in air with no special provision for heat dissipation.