Negative differential conductivity of metal-insulator-metal tunneling structures
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Negative Differential Conductivity of Metal–Insulator–Metal Tunneling Structures T. A. Khachaturova and A. I. Khachaturov* Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine, ul. R. Lyuksemburg 72, Donetsk, 83114 Ukraine *e-mail: [email protected] Received May 8, 2008
Abstract—The tunneling characteristics of a metal–insulator–metal junction are calculated in the framework of the two-band model, which takes into account the presence of the valence band for the insulator layer. It is demonstrated that, in the case where the Fermi level EF of the structure under investigation lies below the middle of the band gap of the insulator, the dependence of the tunneling current on the bias voltage across the junction contains portions with a negative differential resistance at V > EF /e. The iron–aluminum oxide–iron magnetoresistive junctions are considered as samples in which the effect under discussion can be observed. It is shown that, in the given case, the appearance of a portion with a negative differential resistance should be expected at voltages exceeding the Fermi energy E F1 for the spin-up electron band. PACS numbers: 73.40.Gk, 73.40.Rw DOI: 10.1134/S1063776108110174
1. INTRODUCTION The negative tunneling resistance or the negative tunneling conductivity, as a rule, is considered to mean the corresponding differential quantity. For example, the differential conductivity σ(V) is the first derivative of the tunneling current I(V) with respect to the bias voltage: σ(V) = dI(V)/dV. The quantity R(V) = (dI(V)/dV)–1, which is the reciprocal of the differential conductivity, is referred to as the differential resistance. Therefore, the presence of a negative portion in the dependence R(V) means that, in this range, the total number of electrons tunneled per unit time through a barrier decreases with an increase in the applied voltage. Esaki [1] emphasized that the negative resistance phenomenon is important not only in solid-state electronics, where it can provide a signal amplification, but also this phenomenon is important by itself because it sheds light on some fundamental features of the tunnel effect. In [1], it was also noted that this effect can be observed only under special conditions and that these conditions can be created in different ways. For example, in the case of semiconductor structures that consist of successive conductor (semiconductor) and insulator layers, the appearance of a portion with a negative resistance in the corresponding dependence is associated with the resonant tunneling when the energy of a tunneling electron coincides with the energy of a discrete electronic quasi-level that arises inside the potential well with the walls formed by the boundaries of neighboring insulator regions [2].
The above conditions can also be created as a result of specific features of the electronic spectrum of electrodes. For example, if the case in point is tunneling from a normal metal to a p-type semiconductor, the Fermi level of the
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