GaN and related materials (especially AlGaN) have recently attracted a lot of interest for applications in high power electronics capable of operation at elevated temperatures. Although the growth and processing technology for SiC, the other viable wide bandgap semiconductor material, is more mature, the AlGaInN system offers numerous advantages. These include wider bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy) for producing very thick layers or even quasi-substrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices, including heterostructure field effect transistors (HFETs), Schottky and p–i–n rectifiers, heterojunction bipolar transistors (HBTs), bipolar junction transistors (BJTs) and metal-oxide semiconductor field effect transistors (MOSFETs). This review focuses on the development of fabrication processes for these devices and the current state-of-the-art in device performance, for all of these structures. We also detail areas where more work is needed, such as reducing defect densities and purity of epitaxial layers, the need for substrates and improved oxides and insulators, improved p-type doping and contacts and an understanding of the basic growth mechanisms.

Fabrication and performance of GaN electronic devices

It is controversial whether specific tension (the ratio between muscle strength and size) declines with aging. Therefore, contractile muscle volume was estimated separately from the intramuscular noncontractile tissue by magnetic resonance imaging, and maximum isometric torque was measured in the knee extensors and flexors of 10 young (22.8 +/- 5.7 years) and 10 older (69.5 +/- 2.4 years) healthy active women. Specific tension was lower in the older women both in the extensors (93.1 +/- 20.1 kN x m(-2) vs. 112.1 +/- 12.3 kN x m(-2); P < 0.05) and in the flexors (100 +/- 31 kN x m(-2) vs. 142.7 +/- 23.9 kN x m(-2); P < 0.01). This was accompanied by an increase in the percentage coactivation of the knee flexors during knee extension. These data suggest that the lower level of muscle torque in the older women can be explained not only by smaller contractile muscle mass but also by increased coactivation of the antagonist muscles during knee extension.

Contractile muscle volume and agonist-antagonist coactivation account for differences in torque between young and older women

Basic scientific interest in using a semiconducting electrode in molecule-based electronics arises from the rich electrostatic landscape presented by semiconductor interfaces. Technological interest rests on the promise that combining existing semiconductor (primarily Si) electronics with (mostly organic) molecules will result in a whole that is larger than the sum of its parts. Such a hybrid approach appears presently particularly relevant for sensors and photovoltaics. Semiconductors, especially Si, present an important experimental test-bed for assessing electronic transport behavior of molecules, because they allow varying the critical interface energetics without, to a first approximation, altering the interfacial chemistry. To investigate semiconductor-molecule electronics we need reproducible, high-yield preparations of samples that allow reliable and reproducible data collection. Only in that way can we explore how the molecule/electrode interfaces affect or even dictate charge transport, which may then provide a basis for models with predictive power.

To consider these issues and questions we will, in this Progress Report, 


review junctions based on direct bonding of molecules to oxide-free Si.


describe the possible charge transport mechanisms across such interfaces and evaluate in how far they can be quantified.


investigate to what extent imperfections in the monolayer are important for transport across the monolayer.


revisit the concept of energy levels in such hybrid systems.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.200901834

Molecules on si: electronics with chemistry.

OBJECTIVE —Diabetic neuropathy is a length-dependent process that leads to reduced muscle strength and atrophy of leg muscles in some patients. We hypothesized that intrinsic foot muscles are atrophied in diabetic neuropathy and that the degree of atrophy is a measure of motor dysfunction closely related to the neuropathic process. RESEARCH DESIGN AND METHODS —Consecutive cross-sectional magnetic resonance images of the nondominant foot were obtained for stereological determination of the total volume of the intrinsic foot muscles (VFM) in 23 long-term diabetic patients with ( n = 15) and without ( n = 8) chronic neuropathy and in 23 matched healthy nondiabetic control subjects. Based on clinical examination, nerve conduction studies, and quantitative sensory examination, a neuropathy rank-sum score was calculated for each patient. RESULTS —Total VFM was 86 ± 52, 165 ± 34, and 168 ± 42 cm 3 in neuropathic patients, nonneuropathic patients, and healthy control subjects, respectively ( P r = −0.75, P −5 ). CONCLUSIONS —Total volume of the foot muscles is halved in patients with diabetic neuropathy. Atrophy of foot muscles is closely related to the severity of neuropathy and reflects motor dysfunction.

https://care.diabetesjournals.org/content/diacare/27/10/2382.full.pdf

Atrophy of Foot Muscles: A measure of diabetic neuropathy

Objective: the present study examined which of muscle volume (MV) and cross-sectional area (CSA) is appropriate for evaluating the relation with elbow flexor muscle strength in young and elderly individuals. Methods: the subjects were 52 young (20‐34 year; 30 men and 22 women) and 51 elderly individuals (60‐77 year, 19 men and 32 women). The MV and maximal anatomical CSA (ACSA) of elbow flexors were determined by magnetic resonance imaging. The torque developed during maximal voluntary contraction of isometric elbow joint flexion was converted to force by dividing it by the forearm length of each subject. Results: torque was significantly correlated with MV in young and elderly individuals (r = 0.564‐0.926). Similarly, force was also significantly correlated with ACSA in each of them (r = 0.637‐0.906). However, the y-intercepts of the regression lines for the ACSA-force relationship in young men and women were significantly higher than zero. There was no age effect on torque per MV, whereas force per ACSA was significantly higher in young adults than in elderly individuals. Conclusion: for elbow flexors, MV compared to ACSA is appropriate for evaluating the size‐strength relationship and the existence of age-related difference in muscle strength per size.

Muscle volume compared to cross-sectional area is more appropriate for evaluating muscle strength in young and elderly individuals

The performance capabilities of InP-based pnp heterojunction bipolar transistors (HBT's) have been investigated using a drift-diffusion transport model based on a commercial numerical simulator. The low hole mobility in the base is found to limit the current gain and the base transit time, which limits the device's cutoff frequency. The high electron majority carrier mobility in the n/sup +/ InGaAs base allows a reduction in the base doping and width while maintaining an adequately low base resistance. As a result, high current gain (>300) and power gain (>40 dB) are found to be possible at microwave frequencies. A cutoff frequency as high as 23 GHz and a maximum frequency of oscillation as high as 34 GHz are found to be possible without base grading. Comparison is made with the available, reported experimental results and good agreement is found. The analysis indicates that high-performance pnp InP-based HBT's are feasible, but that optimization of the transistor's multilayer structure is different than for the npn device.

Simulation and design of InAlAs/InGaAs pnp heterojunction bipolar transistors

Inclusion of tunneling and ballistic transport effects in an analytical approach to modeling of NPN InP-based heterojunction bipolar transistors

Hot-carrier-induced degradation in commercially prepared silicon-gate MOSFETs incorporating ammonia annealed, nitrided oxides as the gate dielectric is examined and compared with the degradation observed in similar devices incorporating conventional oxides. Nitridation at 1100 degrees C for 2 h is observed to reduce the rate of transconductance degradation and threshold voltage increase by nearly half, compared to the oxide for stressing at both low and high gate bias, and to modify the effects of stressing on the substrate current characteristics. In contrast, nitridation at 1150 degrees C produces both improvements and degradations in device stability depending on the parameter examined and the stress conditions. While ammonia annealing introduces nitrogen, it also appears to incorporate excess hydrogen in the dielectrics that alters charge trapping and interface-state generation so that the performance of the dielectric under electrical stress depends on the concentrations of both species. >

Hot-carrier-induced degradation in nitrided oxide MOSFETs

Starting with the $4\ifmmode\times\else\texttimes\fi{}4$ Luttinger-Kohn Hamiltonian, we develop a scattering-matrix approach to study coherent hole transport through the valence-band energy profile across the emitter-base junction of typical abrupt and graded $\mathrm{Pnp}$ heterojunction bipolar transistors. The tunneling and reflection coefficients of heavy and light holes are calculated for the upper and lower $2\ifmmode\times\else\texttimes\fi{}2$ Hamiltonians obtained through a unitary transform of the $4\ifmmode\times\else\texttimes\fi{}4$ Luttinger-Kohn Hamiltonian. The probability for a transition from heavy (light) to light (heavy) hole while tunneling across the emitter-base junction is calculated as a function of the value of the wave vector parallel to the emitter-base heterointerface for both abrupt and graded heterojunctions. For holes injected from the emitter into the base, the probability of heavy- to light-hole conversion is shown to be quite different when calculated with the upper and lower Hamiltonians. On the other hand, the probability of light- to heavy-hole conversion is nearly the same for the upper and lower Hamiltonians. The energy dependence of the heavy- and light-hole tunneling coefficients is shown to be quite different from those calculated using a parabolic-band model, in which the effects of mixing and anisotropy in the valence band are neglected.

Hole tunneling through the emitter-base junction of a heterojunction bipolar transistor

Starting with Burt’s envelope function theory, we calculate the transmission coefficients of holes across an InP/In1−xGaxAsyP1−y/In0.53Ga0.47As heterointerface while varying the width and gallium and arsenic fractions of the InP lattice-matched quaternary compound (x=0.47y). While comparing our results to the case of an abrupt InP/In0.53Ga0.47As interface, we find that the transmission coefficients of both heavy- and light-holes can be enhanced significantly for a 60-A-wide quaternary layer with an arsenic fraction y=0.4 (x=0.188). This should lead to an enhanced hole injection efficiency of Pnp heterojunction bipolar transistors using the heterointerface analyzed here as an improved design of the emitter-base junction.

Design of a InP/In1−xGaxAsyP1−y/In0.53Ga0.47As emitter-base junction in a Pnp heterojunction bipolar transistor for increased hole injection efficiency

K.P. Roenker

Papers

Simulation and design of InAlAs/InGaAs pnp heterojunction bipolar transistors

Inclusion of tunneling and ballistic transport effects in an analytical approach to modeling of NPN InP-based heterojunction bipolar transistors

Hot-carrier-induced degradation in nitrided oxide MOSFETs

Hole tunneling through the emitter-base junction of a heterojunction bipolar transistor

Design of a InP/In1−xGaxAsyP1−y/In0.53Ga0.47As emitter-base junction in a Pnp heterojunction bipolar transistor for increased hole injection efficiency