Masaaki Tomizawa

0.05-μm-Gate InAlAs/InGaAs High Electron Mobility Transistor and Reduction of Its Short-Channel Effects

In this paper, we discusse the advantages of thinning the channel on short-channel effects for lattice-matched InAlAs/InGaAs high electron mobility transistors (HEMTs) with sub-0.1-µm-long gates with regard to the performance of a 0.05-µm-gate device. To fabricate a sub-0.1-µm gate, the opening shape of the gate-footprint is controlled by using a bilayer dielectric film system and RIE side etching. The device shows a current gain cutoff frequency of 300 GHz and g m/g d ratio of 15. Thinning the channel and the barrier down to 100 A improves carrier confinement and subthreshold characteristics and is indispensable for reducing the short-channel effects in the sub-0.1-µm-gate-length region.

An analysis of the kink phenomena in InAlAs/InGaAs HEMT's using two-dimensional device simulation

Kink phenomena in InAlAs/InGaAs HEMTs are investigated using a two-dimensional (2-D) device simulation that takes into account impact ionization, including nonlocal field effects, and the surface states in a side-etched region at the gate periphery. The simulation model enables us to represent the kink, and it is found that the accumulation of holes generated by the impact ionization has the channel electron density in the side-etched region increase at the bias point where kink appears. When the electron density in the side-etched region is small, the hole accumulation causes a significant increase in that electron density, resulting in a large kink. The simulation results suggest a model in which the kink is described in terms of the modification of the parasitic source resistance induced by the hole accumulation. This model implies a way to eliminate the kink, that is, keeping the electron density in the side-etched region high.

Two-dimensional electron-gas density in AlXGa1−XN/GaN heterostructure field-effect transistors

We have calculated maximum two-dimensional electron-gas densities in AlXGa1−XN/GaN heterostructure field-effect transistors with wurtzite crystal structures in (0001) orientation, by self-consistently solving Schrodinger’s and Poisson’s equations, taking the piezoelectric effect into account. In order to obtain a guideline for increasing electron densities in the devices, we have examined dependences of the maximum electron densities on both Al compositions of AlXGa1−XN layers and lattice relaxations at the heterointerfaces. The maximum electron density was found to depend more strongly on the lattice relaxation than on the Al composition, which determines the conduction-band discontinuity. Controlling the lattice relaxation is shown to be crucial for obtaining high electron densities in the devices.

A comparison of numerical solutions of the Boltzmann transport equation for high-energy electron transport silicon

In this work we have undertaken a comparison of several previously reported computer codes which solve the semiclassical Boltzmann equation for electron transport in silicon. Most of the codes are based on the Monte Carlo particle technique, and have been used here to calculate a relatively simple set of transport characteristics, such as the average electron energy. The results have been contributed by researchers from Japan, Europe, and the United States, and the results were subsequently collected by an independent observer. Although the computed data vary widely, depending on the models and input parameters which are used, they provide for the first time a quantitative (though not comprehensive) comparison of Boltzmann Equation solutions. >

Accurate modeling for submicrometer-gate Si and GaAs MESFET's using two-dimensional particle simulation

Device characteristics, including nonstationary carrier-transport effects such as velocity overshoot phenomena in submicrometergate Si and GaAs MESFETs, are presented in detail by two-dimensional full Monte Carlo particle simulation. Accurate current-voltage characteristics and transient current response are successfully obtained without relaxation time approximation. Moreover, the carrier dynamics influence on device operation is clarified in a realistic device model, compared with the conventional simulation. It can be pointed out that such nonstationary carrier transport is acutely important for accurate modeling of submicrometer-gate GaAs MESFETs, but is not as important for that of Si MESFETs.

A three-dimensional analysis of semiconductor devices

An accurate three-dimensional analysis of semiconductor devices based on the general transport equations is carried out. In this analysis, the finite difference formulation and ICCG (Incomplete Choleski and Conjugate Gradient) methods are utilized to reduce computational time and memory requirements. The algorithms are applied to a wide variety of devices, including a bipolar n-p-n transistor, an Integrated Injection Logic (ILL), and a Static Induction Transistor (SIT). Calculated results are compared to those obtained using a conventional two-dimensional simulator. Several three-dimensional effects are modeled successfully. These analyses make it clear that three-dimensional calculation is indispensable for accurate device modeling.

Three mechanisms determining short-channel effects in fully-depleted SOI MOSFETs

Mechanisms determining short-channel effects (SCE) in fully-depleted (FD) SOI MOSFETs are clarified based on experimental results of threshold voltage (V/sub T/) dependence upon gate length, and analysis using a two-dimensional (2-D) device simulator. Drain-induced barrier lowering (DIBL) effect is a well known mechanism which determines the SCE in conventional bulk MOSFETs. In FDMOSFETs, two more peculiar and important mechanisms are found out, i.e., the accumulation of majority carriers in the body region generated by impact ionization, and the DIBL effect on the barrier height for majority carriers at the edge of the source near the bottom of the body. Due to these peculiar mechanisms, V/sub T/ dependence upon gate length in the short-channel region is weakened. It is also shown that floating body effects, the scatter of V/sub T/, and transient phenomena are suppressed due to the SCE peculiar to FD MOSFETs.

Temperature dependence of hot carrier effects in short-channel Si-MOSFETs

Full-band Monte Carlo simulations were carried out to investigate hot carrier effects associated with impact ionization under the lateral electric field profiles typical of submicrometer Si-MOSFETs. It is shown that the temperature dependence of the band-gap energy of Si plays an important role for hot carrier suppression at low temperature in submicrometer devices. On the other hand, as the device size shrinks into the sub-0.1 regime, in which the high-field region is comparable in size to or smaller than the energy relaxation length, the number of electrons with energy below the supply drain voltage becomes less sensitive to temperature. As a result, the suppression of impact ionization at low temperature in sub-0.1 /spl mu/m devices could be ascribed to both quasi-ballistic transport characteristics and temperature-dependent band-gap energy.

Accurate modeling of AlGaAs/GaAs heterostructure bipolar transistors by two-dimensional computer simulation

An accurate modeling of Al0.3Ga0.7As/GaAs heterostructure bipolar transistors (HBTs) has been carried out using a two-dimensional numerical simulator. By comparing an HBT with a GaAs homotransistor, the potential advantages of the HBT, such as a high injection efficiency, a low voltage drop in the base region, and an improvement in high-frequency operation, were well confirmed, not only by the calculated transistor performance but also by the carrier and the potential distributions. It was also demonstrated that the injected electrons spread more and more conspicuously into the extrinsic base region, even for the HBTs with the increase in the collector current. Taking into account the near ballistic transport in the base region, it is possible to realize an abrupt HBT with a maximum cutoff frequency of above 130 GHz and current gains of up to 3200. The HBT was confirmed to be a promising device for the realization of ultrahigh-frequency integrated circuits.

A numerical analysis of a heterostructure InP/InGaAs photodiode

Electrical properties of a heterostructure InP/In0.53Ga0.47- As photodiode have been analyzed numerically. A device simulator, which can handle a heterostructure up to a high voltage operation, was developed for this work. The numerical simulator explains the experimental results obtained regarding photocurrent. The band-gap discontinuities, i.e., 0.22 eV for the conduction band and 0.37 eV for the valence band, were confirmed to be plausible. The photocurrent switching mechanism in the photodiode by bias voltage was clarified through carrier and potential distributions. It was revealed that the photo-excited holes accumulate at the heterointerface, and the switching voltage for photocurrent varies according to the incident optical power level. It is expected that this simulator will be a powerful tool for optimum heterostructure photodiode design.