Photodiodes made from III-V group semiconductor alloys have been found to exhibit anomalously high dark currents. We present evidence that tunneling is the dominant source of dark current in many cases. The tunneling current becomes substantial at peak junction electric fields as low as 105 V/cm due to the small direct energy gaps and small effective masses of the materials tested. Tunneling sets limits on the magnitude of the electric field attainable in these materials, and therefore has serious implications on photodetector design and performance.
Several mechanisms producing negative differential conductance (NC) in semiconductors and insulators have been investigated recently, in particular (a) field-dependent excitation to bands with lower effective masses (Gunn effect)/ (b) field-dependent capture cross section of recombination centers/ (c) emission of acoustical phonons if the electron drift velocity exceeds the velocity of sound/ and (d) field quenching.^ In this paper we examine a new mechanism arising from interaction of photoexcited carriers with LO phonons which is capable not only of producing NC, but also a "total nega-E,-tive conductance" (TNC)/>^ For TNC the average drift velocity v{S) is, by definition, in a direction opposite to the electrical force lv(S) <0], while for NC, we have BviS)/BS<0.Briefly, the effect requires the following: (1) injection of electrons (or holes) by a monochromatic light source in the conduction band (valence band) at an energy E^ just below the threshold energy for LO phonon emission,"^ where Ei^Eg-¥zfiu)Q^ 0.92<>2 <1.00; (2) energy losses by acoustic phonon interaction during the lifetime of an electron sufficiently small so that the electron distribution remains nearly monoenergetic; (3) electron density sufficiently low so that e^e interaction is small compared with other scattering mechanisms. Upon the application of an electric field 5, the momentum losses, because of the threshold character of the LO phonon emission, are preferentially in the direction opposite to the drift velocity, as illustrated in Fig. 1. A detailed calculation, as summarized below, shows that the distribution can become such that the average velocity is negative.The Boltzmann equation for the distribution function/(^)^/o(fe)+/i(fe)cos0 (after expansion in spherical harmonics) leads to the following system of differentio-functional equations* (for a parabolic energy band):^eS 1 8 3n k FIG. 1. Illustration of basic mechanism producing total negative conductance. Top: Conventional picture of the conduction band. Middle: Plane section in momentum space in a direction parallel to the electric force F, Bottom: Distribution function along the direction (k^,0^ 0). Electrons are transferred from a discrete impurity level EQ to the conduction band by photon absorbtion (1) to an energy ^Bj just below one LO phonon energy. The electrons gain energy from the electric field on the right-hand side (2), but lose energy on the left-hand side, since their initial velocity is in the opposite direction. Electrons with E >^WQ on the righthand side quickly make a transition to the bottom of the band by emission of one LO phonon (4), The electrons are finally removed (5) from the conduction band by, e.g., a transition to excited impurity states. Elastic scattering due to impurities (3) is the dominant scattering mechanism for E
Switching phenomena take place in thick bulk samples of semiconducting glass, once a path of devitrified material is established. Potential probe and infrared microradiometer measurements reveal that the switching action takes place in a small region somewhere along this path. Application of voltage pulses can move this region to a different position. Evidence of partial devitrification and melting is also found in thin film switches made from many different glass compositions. Memory switching has also been observed in all bulk and thin film experiments to date. Since the characteristics of bulk and thin film switching are remarkably similar, doubt is cast upon the interpretation of switching phenomena as due to electronic properties of amorphous semiconductors.
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