A. F. Shulekin

Soft breakdown of MOS tunnel diodes with a spatially non-uniform oxide thickness

Abstract The effect of electric stress on the characteristics of Al/SiO 2 /p + -Si MOS tunnel diodes ( d ox =2.5–3 nm) is studied. Along with the gradual current changes, superimposed by the soft-breakdown-related steps, a non-trivial abrupt decrease in current is also revealed during the constant voltage stress. The latter occurred predominately under high bias and may be considered as an unusual appearance of the same soft breakdown events. In case of substantial spatial oxide thickness deviation, this effect is important even if it occurs within a small area.

Threshold energies in the light emission characteristics of silicon MOS tunnel diodes

Abstract Hot electron luminescence of MOS tunnel structures with sub-3 nm oxide layer on p-Si is experimentally studied. Radiation spectra are shown to exhibit thresholds whose positions depend on the initial energy E of injected electrons. Simultaneously, a threshold-like increase of the light intensity at a selected wavelength as a function of E is revealed, and attributed to the onset of different luminescence mechanisms.

Electron tunneling through a double barrier in a reverse-biased metal-oxide-silicon structure

A number of effects in metal/(tunnel-thin SiO2)/p+-Si structures associated with electron tunneling from the valence band of bulk Si into a metal have been studied. The tunneling occurs through two successively arranged tunnel-transparent barriers: that of the depleted space charge region in Si and the SiO2 barrier, with the possible intermediate involvement of a quantum well formed by the Si conduction band. The current-voltage characteristics of the structures are calculated in terms of a simple model that considers these mechanisms for the purely depletion mode, i.e., with the inversion layer charge neglected. The relationship between the structure parameters (p-Si doping level and oxide thickness) and the relative contributions of nonresonant and resonant (via quantum-well levels in the Si conduction band) tunneling to the overall current through an MOS structure is discussed. The conditions most favorable for the observation of resonance effects are formulated.

On the effect of transverse quantum confinement on the electrical characteristics of a submicrometer-sized tunnel MOS structure

The changes in the characteristics of the Al/SiO2/n-Si MOS structure as a result of a decrease in its transverse dimensions in the range from micrometers to several nanometers are studied. The characteristics are simulated with regard to quantum confinement of the motion of carriers in the plane orthogonal to the direction of tunneling. For making the model more illustrative, the MOS structure is considered as an injector (tunnel MOS emitter) of a bipolar transistor. It is shown that the reduction in size does not produce radical changes in the electrical characteristics of the device, but results in some decrease in the collector current and, most notably, in the gain. The results are important as such and, at the same time, in the context of scaling of field-effect transistors, since the transistor involving a tunnel MOS emitter provides one of the practically interesting operation modes of the conventional field-effect transistor.

Impact of the band–band tunneling in silicon on electrical characteristics of Al/SiO2/p+-Si structures with the sub-3 nm oxide under positive bias

Abstract Measurements of current–voltage and capacitance–voltage characteristics were performed for metal––tunnel oxide––silicon structures on p+-silicon wafers doped to 2×1018–2×1019 cm−3. Obtained results show at least two striking points. First, the forward (metal “+”) current is amazingly high and comparable in value with the reverse-bias current (metal “−”). Second, capacitance–voltage curves exhibit a distortion in the depletion regime and a hump indicating the onset of inversion. The hump is accentuated with decreasing frequency. These features are suggested to appear due to a tunneling of the valence band electrons within the forbidden gap of silicon and further into metal. Eventually, such electrons may enter the conduction band of silicon where a fraction of them can be temporarily captured at the interface and impurity defects forming thereby an inversion layer.

Photocurrent amplification in Au/SiO2/n-6H-SiC MOS structures with a tunnel-thin insulator

The first observation of amplification of the photogeneration current in Au/SiO2/n-6H-SiC structures with a tunnel-thin insulator is reported. This effect can be used to increase the efficiency of existing UV-range 6H-SiC-based photodiodes. It also shows that bipolar SiC transistors with a MOS tunnel emitter can be produced.

A MOS tunnel emitter transistor as a tool for determining the effective hole mass in a thin film of SiO2

The effective mass of holes in a tunnel-thin (2–3 nm) SiO2 layer was experimentally determined: mh = (0.32–0.33)m0. The use of this value enables an adequate simulation of a direct-tunneling hole current in MOS devices. The mathematical processing of characteristics of a MOS tunnel emitter transistor has been applied for the first time in the effective mass determination; this method makes it possible to obtain the precise value of the effective thickness of the oxide, because the electron effective mass for SiO2 is well known from literature. The calculations were performed under the assumption that the probability of tunneling across the barrier depends only on the Ez component of the particle energy, related to the motion in the direction of tunneling.

Current-voltage characteristics of Al/SiO2/p-Si MOS tunnel diodes with a spatially nonuniform oxide thickness

The effect of nonuniform distribution of the insulator thickness on the behavior of Al/SiO2/p-Si MOS tunnel structures with a (1–4)-nm-thick insulator is studied. The character and magnitude of the effect depend on the applied bias. In any conditions, the nonuniformity of the SiO2 thickness enhances the total through currents as compared to those flowing across a uniform oxide layer of the same nominal thickness. Further, the potential of the inversion layer changes in the inversion mode. The calculations performed take into account the tunnel transport between the Si conduction band and the metal, that between the Si valence band and the metal (including in the inversion mode, the resonant transport, which is less clearly pronounced because of the thickness nonuniformity), and the band-to-band tunneling in the semiconductor.

Influence of insulator thickness nonuniformity on the switching of the Al/SiO2/n-Si tunnel MOS structure at reverse bias

Current-voltage characteristics of the reversely biased Al/SiO2/n-Si MOS structure are calculated taking into account the nonuniformity of oxide thickness distribution over an area at a nominal thickness of 1–3 nm. It is known that the characteristics are S-shaped in a certain range of average SiO2 thickness, which suggests that a device is bistable. Holding and threshold voltage shifts, caused by statistical thickness variations, were predicted. In response to electrical stress, the root-mean-square deviation of the SiO2 thickness increases, which results in a shift of the threshold voltage to higher values. The calculations are complemented by experimental data.

Degradation of MOS tunnel structures at high current density

The stability of tunneling-thin (2–3 nm) SiO2 films during prolonged flow of high-density currents (102–103 A/cm2) was investigated. A sharp increase in the charge which a tunneling MOS structure is capable of transmitting without degradation on switching from Fowler-Nordheim injection to direct tunneling (103 C/cm2 and 107 C/cm2, respectively) was observed. The degradation of SiO2 films was investigated using Al/SiO2/n-Si/p+-Si thyristor structures with a positive bias on the semiconductor, i.e., with reverse bias of the MOS structure. The use of these devices accounted for the uniformity of the current distribution over the area and made it possible to monitor the state of the insulator layer by measuring the device gain in the phototransistor mode.