Effect of p-i-p+ buffer on characteristics of n-channelheterostructure field-effect transistors

The carrier concentration in heterostructure FETs (HFETs) with a p-i-p⁺ buffer is presented as a function of the gate bias, obtained by self-consistent one-dimensional calculation of 2-D electron density, subband levels, and electrostatic potential. These results quantify certain limitations on the range of geometrical and doping parameters and make it possible to optimize the HFET design and to compare modulation doped FET (MODFET) and doped channel HFET (DCHFET) structures. Monte Carlo simulations demonstrate that this p-i-p⁺ buffer drastically reduces the short-channel effects     

DC and microwave characteristics of sub-0.1-μm gate-lengthplanar-doped pseudomorphic HEMTs

Analytical modeling of these very-short-channel HEMTs (high-electron-mobility transistors) using the charge-control model is given. The calculations performed using this model indicate a very high electron velocity in the device channel (3.2±0.2×10⁷ cm/s) and clearly demonstrate the advantages of the planar-doped devices as compared to the conventional uniformly doped HEMTs. Devices with different air-bridged geometries have been fabricated to study the effect of the gate resistance on the sub-0.1-μm HEMT performance. With reduced gate resistance in the air-bridge-drain device, noise figures as low as 0.7 and 1.9 dB were measured at 18 and 60 GHz, respectively. Maximum available gains as high as 13.0 dB at 60 GHz and 9.2 dB at 92 GHz, corresponding to an f_max of 270 GHz, have also been measured in the device. Using the planar-doped pseudomorphic structure with a high gate aspect-ratio design, a noise figure of less than 2.0 dB at 94 GHz is projected based on expected further reduction in the parasitic gate and source resistances     

Analysis of bias stress on unpassivated hydrogenated amorphoussilicon thin-film transistors

Both the subthreshold slope and the threshold voltage in inverted-staggered amorphous silicon thin-film transistors (a-Si:H TFTs) are vulnerable to metastable changes in the density of states (DOS) due to Fermi level displacement. In previous work, we have used passivated and unpassivated TFTs to distinguish between the effects of bulk states and interface states at the top passivating nitride interface. Here we report the results of experimental measurements and two-dimensional (2-D) simulations on unpassivated TFTs. Since there are no top interface states, all the observed changes are due solely to the bulk DOS. The subthreshold current activation energies in a-Si:H TFTs are compared for n-channel nonpassivated TFTs before and after bias stress. The experimental results agree well with the 2-D simulations, confirming that the dependence of subthreshold current activation energy on gate bias reveals the distribution of the DOS in energy but cannot resolve the magnitude of features in the DOS. This type of analysis is not accurate for TFTs with a top passivating nitride, since the activation energies in such devices are affected by the interfere states     

Modeling frequency dependence of GaAs MESFET characteristics

We present a new method of modeling the output conductance dispersion of GaAs MESFET's. High frequency model parameters are extracted and then used to model high frequency output conductance over a wide range of bias conditions. The model is then used to simulate and analyze the effect of output conductance dispersion on the performance of DCFL and SCFL logic gates. Whereas the DCFL performance is not significantly affected by the high frequency effects, the noise margin of SCFL decreases by almost a factor of 30% above 100 kHz, with an associated decrease in the voltage swing and gate delay     

Near ballistic electron transport in GaAs devices at 77°K

The effect of collisions on near ballistic electron transport in short GaAs terminal devices at 77°K is analyzed in the frame of the model based on the equations of momentum and energy balance where momentum and energy relaxation times are fit to agree with the results of Monte Carlo calculation for static constant electric fields. Solving the equations by the iteration technique yields the criterion of the ballistic transport $\text{(L}{}_{\mathrm{(\mu m}}\text{) ? 0.44 μ (m}{}^2\text{/V.s) V}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(v)}$ for GaAs at low voltages). The computer solution is used to obtain current-voltage characteristics, field, energy, velocity and voltage distributions for GaAs devices at 77°K. The results of the calculation show that the ballistic effects are dominant at 77°K even at relatively high doping levels (such as 10¹⁶ cm^?3) for short devices (～ 0.2 μm long). These effects lead to a higher electron velocity at low voltages and could be utilized in building high speed GaAs integrated circuits.     

Factors affecting defining the quality and functionality of excipients used in the manufacture of dry powder inhaler products

The successful manufacture of a regulatory approved dry powder inhaler (DPI) product is only achievable by applying robust control systems to all aspects of analytical, engineering, and material based processes. Whilst many aspects of DPI drug product manufacturing can be adequately controlled, it is often the control of materials, that is, drug substance and excipients, which can lead to variation in the quality of the final drug product. This article gives an overview of DPI excipients and highlights the challenges of defining and, importantly, understanding the relationships between quality and functionality for excipient components in DPI formulations.