Compared to the intense research focus on the optical properties, the transport properties in non-polar and semi-polar III-nitride semiconductors remain relatively unexplored to date. The purpose of this paper is to discuss charge-transport properties in non-polar and semi-polar orientations of GaN in a comparative fashion to what is known for transport in polar orientations. A comprehensive approach is adopted, starting from an investigation of the differences in the electronic bandstructure along different polar orientations of GaN. The polarization fields along various orientations are then discussed, followed by the low-field electron and hole mobilities. A number of scattering mechanisms that are specific to non-polar and semi-polar GaN heterostructures are identified, and their effects are evaluated. Many of these scattering mechanisms originate due to the coupling of polarization with disorder and defects in various incarnations depending on the crystal orientation. The effect of polarization orientation on carrier injection into quantum-well light-emitting diodes is discussed. This paper ends with a discussion of orientation-dependent high-field charge-transport properties including velocity saturation, instabilities and tunneling transport. Possible open problems and opportunities are also discussed.

A novel method for the reduction of subthreshold slope below the room‐temperature Boltzmann limit of 60 mV/dec for a field‐effect transistor based on negative differential capacitance is proposed. This effect uses electric field induced electrostriction of a piezoelectric gate barrier of the transistor. The mechanism amplifies the internal surface potential over the applied gate voltage. This internal voltage gain mechanism provides an opportunity for steep subthreshold slope switching below 60 mV/decade of the transistor current. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A scattering mechanism stemming from the Stark-shift of energy levels by electric fields in semiconductor quantum wells is identified. This scattering mechanism feeds off interface roughness and electric fields, and modifies the well known 'sixth-power' law of electron mobility degradation. This work first treats Stark-effect scattering in rough quantum wells as a perturbation for small electric fields, and then directly absorbs it into the Hamiltonian for large fields. The major result is the existence of a window of quantum well widths for which the combined roughness scattering is minimum. Carrier scattering and mobility degradation in wide quantum wells are thus expected to be equally severe as in narrow wells due to Stark-effect scattering in electric fields. PACS numbers: Valid PACS appear hereHigh-mobility 2DEGs have proven invaluable for fundamental discoveries in condensed matter physics such as the quantum Hall effect, quantized conductance, and ballistic transport among many others [1, 2] and thus continuous improvement in the mobilities and mean free paths of carriers are highly desirable. For high-speed and low-power field-effect transistors (FETs), a high degree of vertical scaling is essential to support lateral (gate length) scaling, requiring one to move towards highly confined 2DEGs in ultrathin quantum wells such as in Silicon-on-Insulator (SOI) and III-V Quantum Well (QW) FETs [3,4]) to avoid short-channel effects. Thus, interface roughness scattering assumes increasing importance in high-performance transistors. In their seminal work in 1987 Sakaki et al. identified the importance of interface roughness scattering on electron transport in 2dimensional electron gases (2DEGs) confined in narrow quantum wells [5]. They showed that in the presence of quantum well width (L w ) variations in the 2D plane, the electron mobility limited by interface roughness (IR) scattering degrades in thin wells as the sixth power of the well-width (µ IR ∼ L 6 w ). Sakaki et al. assumed a QW with no electric field [5]. In typical QW FETs, the electric field indeed goes to zero when the carriers are depleted (when the device is in the 'off' state), and increases to high values in the 'on' state of the device. For high-performance devices, a high 2DEG density is essential for boosting the drive current -which results in high electric fields in the QW. In this work, we show that the electric field in the QW leads to an enhanced quantum-confined 'Stark-effect' scattering that feeds off interface roughness, and degrades electron mobility in rough quantum wells. We first evaluate the effect of Stark-effect scattering in a QW in cases where the potential fluctuation due to the electric field is small enough to be treated as a perturbation. Then, we discuss situations where the field is so large that a perturbative treatment does not do justice, and a modified treatment that treats IR+Stark-effect scattering on equal footing * rjana1@nd.edu captures the role of this mobility degradation mechanism. We note that this form of...

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