The Dirac surface states of topological insulators offer a unique possibility for creating spin polarized charge currents due to the spin-momentum locking. Here we demonstrate that the control over the bulk and surface contribution is crucial to maximize the charge-to-spin conversion efficiency. We observe an enhancement of the spin signal due to surface-dominated spin polarization while freezing-out the bulk conductivity in semiconducting Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 below 100 K. Detailed measurements up to room temperature exhibit a strong reduction of the magnetoresistance signal between 2 − 100 K, which we attribute to the thermal excitation of bulk carriers and to the electron-phonon coupling in the surface states. The presence and dominance of this effect up to room temperature is promising for spintronic science and technology.

The InGaAs-InAlAs-InP high electron mobility transistor (InP HEMT) is the preferred active device used in a cryogenic low noise amplifier (LNA) for sensitive detection of microwave signals. We observed that an InP HEMT 0.3-14 GHz LNA at 2 K, where the in-going transistors were oriented perpendicular to a magnetic field, heavily degraded in gain and average noise temperature already up to 1.5 T. Dc measurements for InP HEMTs at 2 K revealed a strong reduction in the transistor output current as a function of static magnetic field up to 14 T. In contrast, the current reduction was insignificant when the InP HEMT was oriented parallel to the magnetic field. Given the transistor layout with large gate width/gate length ratio, the results suggest a strong geometrical magnetoresistance effect occurring in the InP HEMT. This was confirmed in the angular dependence of the transistor output current with respect to the magnetic field. Key device parameters such as transconductance and on-resistance were significantly affected at small angles and magnetic fields. The strong angular dependence of the InP HEMT output current in a magnetic field has important implications for the alignment of cryogenic LNAs in microwave detection experiments involving magnetic fields.In many sensitive detection systems, high electron mobility transistor (HEMT) low-noise amplifiers (LNAs) at cryogenic temperatures (1-10 K) are used to read out tiny microwave signals. Some of these systems rely on the presence of a strong magnetic field, e.g. in mass spectrometry 1 or detection of dark matter. 2,3 A potential application for cryogenic LNAs in a magnetic field is magnetic resonance imaging. 4 It has long been known that the sensitivity of the cryogenic LNA is affected by the presence of a magnetic field: when aligned perpendicular to the magnetic field, the noise temperature of a cryogenic AlGaAs-GaAs (GaAs) HEMT LNA was shown to be strongly degraded with increasing magnetic field. 5 However, reports on the electrical behavior in a magnetic field for the cryogenic InGaAs-InAlAs-InP (InP) HEMT LNA -the standard component used in today's most sensitive microwave receivers -have so far been absent. Compared to previous work, 5 we here report that the gain and noise properties for the cryogenic InP HEMT LNA are much more prone for degradation when exposed to a magnetic field. For the first time, we measured the InP HEMT at 2 K as a function of angular orientation with respect to the magnetic field. It is shown that the InP HEMT output current is limited by a strong geometrical magnetoresistance effect (gMR). The results suggest that even small a) Electronic mail: isabelr@chalmers.se b) Present address: Qamcom Research and Technology AB, Falkenbergsgatan 3, SE-412 85 Gothenburg, Sweden

The optimization of graphene field-effect transistors (GFETs) for high-frequency applications requires further understanding of the physical mechanisms concerning charge carrier transport at short channel lengths. Here, we study the charge carrier transport in GFETs with gate lengths ranging from 2 [Formula: see text]m down to 0.2 [Formula: see text]m by applying a quasi-ballistic transport model. It is found that the carrier mobility, evaluated via the drain–source resistance model, including the geometrical magnetoresistance effect, is more than halved with decreasing the gate length in the studied range. This decrease in mobility is explained by the impact of ballistic charge carrier transport. The analysis allows for evaluation of the characteristic length, a parameter of the order of the mean-free path, which is found to be in the range of 359–374 nm. The mobility term associated with scattering mechanisms is found to be up to 4456 cm[Formula: see text]/Vs. Transmission formalism treating the electrons as purely classical particles allows for the estimation of the probability of charge carrier transport without scattering events. It is shown that at the gate length of 2 [Formula: see text]m, approximately 20% of the charge carriers are moving without scattering, while at the gate length of 0.2 [Formula: see text]m, this number increases to above 60%.