Riichi Katoh

Self-consistent particle simulation for (AlGa)As/GaAs HBTs with improved base-collector structures

A one-dimensional self-consistent particle simulator was developed for (AlGa)As/GaAs heterojunction bipolar transistors (HBTs) to investigate how far the device performance can be improved by positively utilizing non-equilibrium electron transport phenomena under a heavily doped base condition. Computation was thus carried out for HBTs with various categories of base and collector structures. The electron transport mechanisms are discussed in detail according to the results in conjunction with the reduction in base-to-collector transit time.

Self-consistent particle simulation for (AlGa)As/GaAs HBTs under high bias conditions

Nonequilibrium electron transport phenomena in the emitter and collector regions under high bias conditions were investigated for standard N-p-n (AlGa)As/GaAs heterojunction bipolar transistors (HBTs) by utilizing a previously developed one-dimensional self-consistent particle simulator. A dramatic increase in the cutoff frequency was observed for a lightly doped collector HBT as the current density increased over 10/sup 5/ A/cm/sup 2/, where the collector transit time was reduced due to the extension of the velocity overshoot region in the collector corresponding to the decrease in electric field near onset of the Kirk effect. A saturation tendency was seen in the collector current versus base-to-emitter bias voltage (V/sub BE/) characteristic for high V/sub BE/, where V/sub BE/ exceeded the base-to-emitter built-in voltage of the conduction band. Simulations indicate that this feature is caused by electron velocity saturation in the neutral n-type (AlGa)As emitter region. >

A NOVEL QUANTUM CELLULAR AUTOMATA LOGIC WITH LOOP STRUCTURES

The effects of additional cells on the circuit operation of quantum cellular automata (QCA) logic, proposed by Lent et al., are numerically studied using the extended Hubbard model self-consistently. We investigate the differences in logic circuit operation between Lents inverter and our original 4-cell one characterized by a loop structure in the cases of 4 and 5 quantum dot cells (QDC). We find that the 4-cell inverter with loop structure composed of 4-QDC exhibits favorable characteristics in QCA logic operation.

A model-based comparison of AlInAs/GaInAs and InP/GaInAs HBT's: a Monte Carlo study

The high-speed performances of AlInAs/GaInAs and InP/GaInAs heterojunction bipolar transistors (HBTs) are investigated using a one-dimensional self-consistent particle simulator. Optimum alloy compositions for a graded-gap base structure are obtained for both transistors through the tradeoff between the emitter-charging time and base transit time. The saturation velocity in the GaInAs n-type collector is found to be smaller than that in InP, which has been attributed to the diffusion of a large number of hot back-scattered Gamma -valley electrons in the GaInAs collector. The difference in the collector transit time in p-type collectors is trivial, since the maximum electron velocity was restricted to below 1.2*10/sup 8/ cm/s due to a strong nonparabolicity effect. The cutoff frequency for the former and the latter are estimated to be 2 and 1.5 times higher, respectively, than for AlGaAs/GaAs HBTs. These results are attributed to a larger bandgap difference between the emitter and base, to yield a high base built-in field, rather than a larger Gamma -L band separation energy in the collector to enhance the velocity overshoot effect. >

Charge-control analysis of collector transit time for (AlGa)As/GaAs HBTs under a high injection condition

Charge-control analysis of collector transit time ( tau /sub C/) for (AlGa)As/GaAs n-p/sup +/-n heterojunction bipolar transistors has been carried out under the restriction of low-frequency approximation to explain the remarkable tau /sub C/ reduction at the onset of the Kirk effect. The fundamental idea of the model is that the increment of the current density must depend not only on the increment of the carrier density but also on the increment of the carrier drift velocity in the collector space-charge region. The obtained model well explains the previous Monte Carlo results, thus indicating the validity of the above idea as well as the possibility that the actual tau /sub C/ is by far smaller than that estimated from the conventional model. It also leads to a concept for the reduction of transit times, i.e. that an increasing rate of the electron overshoot velocity is as essential for the reduction of tau /sub C/ as the magnitude of the overshoot velocity itself. >

A self-consistent particle simulation for (AlGa) As/GaAs HBTs with improved base-collector structures

A one-dimensional self-consistent particle simulator was developed for (AlGa)As/GaAs heterojunction bipolar transistors (HBTs) to investigate how far the device performance can be improved by positively utilizing non-equilibrium electron transport phenomena under a heavily doped base condition. Computation was thus carried out for HBTs with various categories of base and collector structures. The electron transport mechanisms are discussed in detail according to the results in conjunction with the reduction in base-to-collector transit time.

Ring oscillator circuit simulation with physical model for GaAs/GaAlAs heterojunction bipolar transistors

Switching performance is simulated for GaAs/GaAlAs heterojunction bipolar transistors (HBTs) by combining a realistic physical device model that involves numerical solutions for carrier transport equations and Poissons equation with our own circuit simulator that enables direct access to the device model embedded in arbitrary circuits. Based on simulated results for five-stage ring oscillators, discussion is given as to how the switching performance depends on the circuit configuration such as current mode logic (CML) without and with emitter follower, and direct-coupled transistor logic (DCTL), inclusion or exclusion of external base areas, and choice for single-or double-heterojunction transistors.

Numerical CML switching analyses for heterojunction GaAs/(GaAl)As bipolar transistors

Abstract The switching performance of GaAs/Ga 0.7 Al 0.3 As n - p - n heterojunction bipolar transistors (HBTs) has been investigated for current-mode-logic circuit operation using a hybrid device model composed of numerical one-dimensional transistor and diode models interconnected through resistances corresponding to the real device structure. Switching time is discussed in conjunction with parasitic effects, external circuit conditions and doping profiles. Ultra-high-speed switching of less than 10 ps has been shown to be attainable by scaling the device pattern dimensions down to 1 μm order of magnitude.

Self-consistent particle simulation of heterojunction bipolar transistors under high temperature operating conditions

The high-temperature operation of an AlGaAs-GaAs heterojunction bipolar transistor (HBT) has been investigated quantitatively using a one-dimensional self-consistent particle simulator. Emitter charging time is found to have little temperature dependence below 10/sup 5/ A/cm/sup 2/ current density, while it decreases with an increase in temperature above 10/sup 5/ A/cm/sup 2/ because of the decrease in transconductance. The base transit time is found to decrease slightly as temperature rises, because of the reduction in ionized impurity scattering for thermally excited high-energy electrons in the base. A drastic increase in collector transit time is observed with an increase in temperature, which is attributed not to LO phonon scattering but to intervalley scattering. Nonequilibrium electron transport effects, such as velocity overshoot, become less effective as temperature rises, especially in the p-collector, thus deteriorating the high-speed performance. Therefore, the choice of materials with larger Delta E/sub Gamma -L/, such as InP, is recommended to take advantage of the velocity overshoot in the collector even under high-temperature operation.<<ETX>>