A. G. Chynoweth

Bio: A. G. Chynoweth is an academic researcher from Bell Labs. The author has contributed to research in topics: Space charge & Field electron emission. The author has an hindex of 12, co-authored 12 publications receiving 2119 citations.

Ionization Rates for Electrons and Holes in Silicon

Abstract: The ionization rates for holes and electrons in silicon have been determined over the following ranges of field: for holes, (2.5-6.0)\ifmmode\times\else\texttimes\fi{}${10}^{5}$ volts ${\mathrm{cm}}^{\ensuremath{-}1}$; for electrons, (2.0-5.0)\ifmmode\times\else\texttimes\fi{}${10}^{5}$ volts ${\mathrm{cm}}^{\ensuremath{-}1}$. The ionization rate for electrons is higher than that for holes. The results suggest that the field dependence of the ionization rate for holes and, probably, for electrons also, can be expressed by $a\mathrm{exp}(\ensuremath{-}\frac{b}{E})$, where $E$ is the field. The constants $a$ and $b$ are different for electrons and holes.

Photon Emission from Avalanche Breakdown in Silicon

Abstract: Visible light is emitted from reverse-biased silicon $p\ensuremath{-}n$ junctions at highly localized regions where avalanche breakdown is taking place. The emission occurs in both grown and diffused junctions. By using junctions diffused to a depth of only 2 microns below the crystal surface, it was established that the light sources are randomly spaced over the whole area of the junction as well as around the periphery where the junction intercepts the surface. The light sources are too small to be resolved under a high-power microscope. Their sites are reproducible with current cycling and their intensity and color are relatively insensitive to the field distribution, to the junction width, and to temperature. The number of light spots increases with the current rather than individual spots growing brighter. It is concluded that all the breakdown current is carried through the junction by these localized light-emitting spots.The spectral distribution of the light is continuous with a long tail extending to photon energies greater than 3.3 ev. It is concluded that recombination between free electrons and free holes within the junction region is responsible for the light at the shorter wavelengths, the carrier energies in excess of the energy gap being supplied by the field. At longer wavelengths there appears to be a considerable contribution to the emission from intraband transitions.A tentative figure for the emission efficiency over the visible spectrum is one photon for every ${10}^{8}$ electrons crossing the junction. The recombination cross section required is reasonable, being about ${10}^{\ensuremath{-}22}$ ${\mathrm{cm}}^{2}$.

Surface Space-Charge Layers in Barium Titanate

Abstract: Above the Curie temperature, pyroelectric currents can be produced in single crystals of barium titanate even though there is no electric field applied. The polarization that remains at these temperatures is ascribed to space-charge fields in the crystal. From studies of the wave forms of the pyroelectric current signals it is concluded, tentatively, that space-charge layers of up to ${10}^{\ensuremath{-}5}$ cm in thickness reside at the crystal surface and that these space charges also produce a field through the interior of the crystal. Further evidence for space-charge fields is provided by the occurrence of an associated photovoltaic effect and by asymmetrical hysteresis loops. The space-charge fields vary considerably in magnitude from crystal to crystal. In general, they can be modified by suitable heat treatment but return to their original condition when fields are applied above the Curie point.The space charge fields apparently influence the direction in which the domains polarize when the crystal is cooled through the Curie point. They will also affect capacity measurements above the transition and will influence the actual temperature of the transition. It is quite possible that the fields play an important role in the process of domain nucleation.

Excess Tunnel Current in Silicon Esaki Junctions

Abstract: At low forward biases, a high current flows in Esaki junctions due to band-to-band tunnelling. At sufficiently high biases the current flows by normal forward injection. Between these two bias ranges, the current is unexpectedly high and has been called the excess current. A comprehensive experimental study has been made of this excess current in silicon junctions. It is shown that the properties of the excess current observed so far can be accounted for by a mechanism originally suggested by Yajima and Esaki, in which carriers tunnel by way of energy states within the forbidden gap. Based on this model, the following expression for the excess current, ${I}_{x}$, is proposed: ${I}_{x}\ensuremath{\sim}{D}_{x}\mathrm{exp}{\ensuremath{-}(\frac{{\ensuremath{\alpha}}_{x}{W}_{1}{e}^{\frac{1}{2}}}{2})[\ensuremath{\epsilon}\ensuremath{-}e{V}_{x}+0.6e({V}_{n}+{V}_{p})]},$ where ${D}_{x}$ is the density of states in the forbidden gap at an energy related to the forward bias, ${V}_{x}$, and the Fermi energies on the $n$ and $p$ sides are ${V}_{n}$ and ${V}_{p}$, respectively, $e$ is the electron charge, $\ensuremath{\epsilon}$ is the energy gap, ${W}_{1}$ is the junction width constant, and ${\ensuremath{\alpha}}_{x}$ is a constant containing a reduced effective mass, ${m}_{x}$. This formula describes the observed dependence of ${I}_{x}$ (i) on ${D}_{x}$, observed by introducing states associated with electron bombardment, (ii) on $\ensuremath{\epsilon}$, studied by the temperature variation of the diode characteristics, (iii) on ${V}_{x}$, verified from semilogarithmic plots of the forward characteristics, and (iv) on ${W}_{1}$, tested by using junctions of different widths. From these experiments, ${m}_{x}=0.3{m}_{0}$ to within a factor of 2.The origins of the states in the band gap are not known for certain though they are most likely the band edge tails inherent to heavily doped semiconductors. It is probable that the tunnelling-via-local-states model for the excess current in silicon is applicable to excess currents in other materials.

Internal Field Emission in Silicon p-n Junctions

Abstract: Internal field emission is believed to occur in narrow $p\ensuremath{-}n$ junctions in silicon. This conclusion is based on the following observations: (i) as in wide junctions, the multiplication characteristic has a positive temperature coefficient whereas the reverse characteristic, unlike wide junctions, possesses a negative temperature coefficient; (ii) the forward and reverse currents are relatively insensitive to temperature; (iii) radiative transitions of energetic carriers in the high-field region result in the emission of visible light, the pattern of which shows that the current flows more or less uniformly through the whole extent of the junction but that the array of fewer and more intense light spots characteristic of avalanche breakdown is absent; (iv) the noise associated with the onset of avalanche breakdown is absent. It is concluded that in these narrow junctions the junction space charge field due to the built-in potential alone is sufficient to result in a rate of carrier generation much greater than that of the normal thermal processes. By assuming a reasonable form for the voltage dependence of the field-generated current, it proves possible to account qualitatively for the complex forward-bias characteristics of these junctions. The reverse characteristics show the onset of multiplication of field-emitted carriers at a reasonable threshold. Possible causes of the softness of the reverse characteristics are discussed.

Detailed Balance Limit of Efficiency of p‐n Junction Solar Cells

Abstract: In order to find an upper theoretical limit for the efficiency of p‐n junction solar energy converters, a limiting efficiency, called the detailed balance limit of efficiency, has been calculated for an ideal case in which the only recombination mechanism of hole‐electron pairs is radiative as required by the principle of detailed balance. The efficiency is also calculated for the case in which radiative recombination is only a fixed fraction fc of the total recombination, the rest being nonradiative. Efficiencies at the matched loads have been calculated with band gap and fc as parameters, the sun and cell being assumed to be blackbodies with temperatures of 6000°K and 300°K, respectively. The maximum efficiency is found to be 30% for an energy gap of 1.1 ev and fc = 1. Actual junctions do not obey the predicted current‐voltage relationship, and reasons for the difference and its relevance to efficiency are discussed.

Fundamentals of Modern VLSI Devices

Abstract: Learn the basic properties and designs of modern VLSI devices, as well as the factors affecting performance, with this thoroughly updated second edition. The first edition has been widely adopted as a standard textbook in microelectronics in many major US universities and worldwide. The internationally-renowned authors highlight the intricate interdependencies and subtle tradeoffs between various practically important device parameters, and also provide an in-depth discussion of device scaling and scaling limits of CMOS and bipolar devices. Equations and parameters provided are checked continuously against the reality of silicon data, making the book equally useful in practical transistor design and in the classroom. Every chapter has been updated to include the latest developments, such as MOSFET scale length theory, high-field transport model, and SiGe-base bipolar devices.

Low-Voltage Tunnel Transistors for Beyond CMOS Logic

Abstract: Steep subthreshold swing transistors based on interband tunneling are examined toward extending the performance of electronics systems. In particular, this review introduces and summarizes progress in the development of the tunnel field-effect transistors (TFETs) including its origin, current experimental and theoretical performance relative to the metal-oxide-semiconductor field-effect transistor (MOSFET), basic current-transport theory, design tradeoffs, and fundamental challenges. The promise of the TFET is in its ability to provide higher drive current than the MOSFET as supply voltages approach 0.1 V.

High‐voltage bulk photovoltaic effect and the photorefractive process in LiNbO3

Abstract: Photocurrents in doped LiNbO3 crystals are shown to be due to a bulk photovoltaic effect with saturation voltages in excess of 1000 V (∼105 V/cm). This effect accounts for the light‐induced index changes in LiNbO3. An explanation of the photovoltaic effect, based on the asymmetry of the lattice, is proposed.

Organohalide lead perovskites for photovoltaic applications

Abstract: There are only few semiconducting materials that have been shaping the progress of third generation photovoltaic cells as much as perovskites. Although they are deceivingly simple in structure, the archetypal AMX3-type perovskites have built-in potential for complex and surprising discoveries. Since 2009, a small and somewhat exotic class of perovskites, which are quite different from the common rock-solid oxide perovskite, have turned over a new leaf in solar cell research. Highlighted as one of the major scientific breakthroughs of the year 2013, the power conversion efficiency of the title compound hybrid organic–inorganic perovskite has now exceeded 18%, making it competitive with thin-film PV technology. In this minireview, a brief history of perovskite materials for photovoltaic applications is reported, the current state-of-the-art is distilled and the basic working mechanisms have been discussed. By analyzing the attainable photocurrent and photovoltage, realizing perovskite solar cells with 20% efficiency for a single junction, and 30% for a tandem configuration on a c-Si solar cell would be realistic.