We have observed that the shot noise of tunnel current, I, in GaSb-AlSb-InAs-AlSb-GaSb double-barrier structure under a magnetic field can exceed 2qI. The measurements were done at T=4K in fields up to 5T parallel to the current. The noise enhancement occurred at each of the several negativedifferential conductance regions induced by the tunneling of holes through Landau levels in the InAs quantum well. The amount of the enhancement increased with the strength of the negative conductance and reached values up to 8qI. These results are explained qualitatively by fluctuations of the density of states in the well, but point out the need for a detailed theory of shot noise enhancement in resonant tunneling devices.PACS Numbers: 73.40.Gk, 73.50.Td, 73.50.Jt, 73.20.Dx Since shot noise is sensitive to correlation effects that result from Pauli exclusion principle and the Coulomb interaction between charged particles, noise measurements can give information about the kinetic of electrons in a conductor. The realization of this fact has spurred recent interest in shot noise in a variety of systems 1 , including the double-barrier resonant-tunneling diode (RTD).The current-voltage characteristic (I-V) of an RTD usually has a quasi-triangular shape, with an initial region of positive differential conductance (PDC) before the current peak, followed by a sharp region of negative differential conductance (NDC) just after the peak. So far, most studies of shot-noise have focused on the PDC region, probably because experimentally the NDC region is often masked by an external instability that depends on the circuit to which the diode is connected. Measurements in GaAlAs-GaAs-GaAlAs RTDs have shown a significant noise suppression relative to the "full" shot noise 2qI (q is the electron charge and I the tunneling current), which has been explained by correlation between tunneling electrons 2 . The deviation of the actual shot noise from 2qI is quantified by a Fano factor, defined as the ratio of the noise spectral density, S I (ω), to the full shot noise. Using either a quantum-transport 3 or a semiclassical theory 4 it has been shown that the Fano factor can be expressed in terms of the tunneling probabilities T 1 and T 2 , through the two potential barriers:The value for γ ranges from γ = 1, for very asymmetric barriers, to γ = 1/2, when T 1 = T 2 . For completely incoherent transport we can view the two barriers as two resistances in series and if each resistance generates full shot noise then tunneling probabilities in (1) are replaced by differential resistances of the barriers 5 . As Landauer 6 has pointed out, the maximum noise suppression is readily obtained then for two identical diodes in series.The few results available in the NDC region exhibit a behavior very different from the suppression found in the PDC region. Li et al.2 showed enhancement of noise in the NDC region, although, their focus being on noise suppression in the PDC region, they did not discuss that result in detail. Brown 7 has reported an enhancemen...

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The lifetime of the lowest quasibound state localized between the barriers of a GaAs/AlGaAs double-barrier structure is calculated as a function of barrier and well dimensions. The results are consistent with high-frequency experiments.

A mid-IR ͑3.3-3.5 m͒ type-II interband cascade laser has been demonstrated at temperatures up to 300 K in pulsed mode and 150 K in cw mode. Threshold current densities as low as 13.2 A/cm 2 and power efficiencies as large as 17% have been achieved under cw conditions at 80 K.

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Midinfrared (3.6–3.8 μm) interband cascade lasers based on InAs/GaInSb type-II quantum wells have been demonstrated in the continuous-wave (cw) mode with low-threshold current densities (e.g., ∼56 A/cm2 at 80 K) and power efficiencies exceeding 9%. At a relatively low current of 0.4 A, we observed ∼100 mW/facet of optical power out at 80 K (124 mW at 60 K) from lasers mounted epilayer-side up with uncoated facets. These lasers were able to operate in the cw mode at temperatures up to 127 K. Also, in the pulsed mode, devices lased at temperatures up to 250 K and displayed, at 80 K, a peak power efficiency exceeding 11%.