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High field current fluctuations in n-type germanium Hart, Laurence Gilbert


The work reported here is an experimental and theoretical investigation of high-frequency electrical noise generated in extrinsic single-crystal n-type germanium at high electric fields. The electric field was pulsed so that the lattice temperature remained near 77°K. During the pulse, the electrons quickly reach a non-equilibrium steady-state due to their gaining energy from the electric field and brought to a steady-state by means of collisions with the lattice vibrations. Previous work has been concerned with noise measurements made at right angles to the electric field direction, where anisotropic behaviour was observed. The present measurements, made in the direction of the electric field, also show a high degree of anisotropy. The electrical noise generated is described by the noise temperature, Tn, obtained by adapting the Nyquist formula to the non-equilibrium case. Measurements of Tn, performed at frequencies of 70Mc/s and 30Mc/s, indicated a uniform noise spectrum in this frequency range for all the samples used. The anisotropy of Tn suggested that Tn was explainable on the basis of the many-valley model of the conduction band of germanium, established by previous experimental investigations of the high-field mobility anisotropy. A feature of the many-valley model is that electrons in different valleys of the conduction band, will in general, exhibit different transport behaviour and as a result, transitions between these valleys will result in a noise phenomenon described as "intervalley noise". However, for measurements of Tn in the <100> direction, the "intervalley noise" will vanish, allowing a direct measure of the electron "heating" due to the electric field, the “hot electron noise”. In the <111> an <110> directions, both intervalley and hot electron noise are expected. Both contributions to Tn are evaluated by means of Barrie's extension to the case of many-valley germanium of Stratton's high-field transport theory.

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