R W Kelsall

An investigation of graded and uniform base Ge/sub x/Si/sub 1-x/ HBT's using a Monte Carlo simulation

A fully self-consistent Monte Carlo simulation has been used to investigate electron transport through the base-collector region of various Ge/sub x/ Si/sub 1-x/ based heterojunction bipolar transistors. By considering a range of Ge content in the base of such devices we have shown that the base transit time decreases significantly as the Ge content of the base is increased from 0% to 15% but remains essentially unchanged by a further increase to 30%. Furthermore, it is shown that high current densities can beneficially affect the field distribution in the collector, substantially reducing the collector transit time. A modified form of the simulation has been used to investigate a graded base heterojunction bipolar transistor, with a maximum Ge content of 30%. By including the variation of effective masses across the base we have been able to show how deviations from linear grading can be modelled and to prove that such configurations can produce improvements for base transit times. >

Monte Carlo simulation of hole mobilities in an InGaAs/GaAs strained layer quantum well

The mobility and velocity-field characteristic of holes in an In0.18Ga0.82As/GaAs strained quantum well have been obtained using a Monte Carlo simulation, for lattice temperatures of 77 K and 4.2 K. The simulation incorporates a four-band Luttinger-Kohn bandstructure calculation to account for the effects of heavy-light mixing on the subband energy dispersions and wavefunctions, intra- and inter-subband phonon scattering and intrasubband alloy scattering processes are considered. The simulated 77 K phonon limited hole mobility shows a 600% enhancement over the measured and simulated values in GaAs heterostructures, but most of this enhancement is removed by alloy scattering. At 4.2 K alloy scattering is again primarily responsible for the order of magnitude difference observed between the hole mobilities in GaAs and InGaAs quantum wells. The effect of other scattering processes-impurity, plasmon-phonon and interface roughness scattering-on the hole mobility in the InGaAs/GaAs system is also discussed.

Theory of Auger recombination in a quantum well wire

Abstract Calculations are presented for the rate of Auger recombination in a semiconductor quantum well wire. An expression is given for the rate of the CHSH Auger process in which the carriers remain in their ground subbands, and the rates of other processes such as CHCC may be obtained by simple algebraic substitution. Numerical calculations that consider all possible bound-to-bound intra- and inter-subband transitions are reported. Results are given for the rate of the CHCC process as a function of wire width and wire axis direction for an InGaAsP/InP quantum well wire.

Matrix elements for hole-phonon scattering in a semiconductor quantum well

Matrix elements for hole-phonon scattering in a quantum well are calculated using a four-band k.p method. The method provides a realistic description of the quantum confined valence states, including the effects of heavy-light hole mixing. This mixing precludes the operation of any symmetry rules for phonon scattering, and the matrix elements are dependent on the specific character of the scattering states involved. For optical phonons, the matrix elements exhibit a strong dependence on the in-plane wavevectors of the scattering states, especially for states lying near the so-called anticrossing regions of the valence subbands. For acoustic (deformation potential) phonons, the matrix elements for intrasubband processes are relatively independent of the in-plane wavevector. In both cases, the larger matrix elements are generally those for intrasubband scattering, with the dominant intersubband processes being those involving adjacent and anticrossing bands.

A comparison of transient velocity overshoot in Si and GaAs structures

The authors have used a self-consistent Monte Carlo simulation to investigate the effect of high currents on the base-collector region of a heterostructure bipolar transistor. By considering identical device structures in Si and GaAs they are able to show that an accurate description of carrier response to field is necessary in order to obtain the correct interdependence of velocity overshoot, field configuration and self-consistent effects.

Monte Carlo simulations of field and carrier density dependent hole transport in an InGaAs/GaAs strained layer quantum well

The authors have carried out Monte Carlo simulations of hole transport in a range of InGaAs/GaAs strained quantum well structures. The simulations include phonon, impurity and alloy scattering, with rates obtained using k.p band structure. The simulations give good agreement with the experimentally determined magnitude and carrier density dependence of the low field hole mobility, for samples with carrier densities ps<or=4.0*1011 cm-2. The results suggest that alloy scattering is primarily responsible for the low mobilities observed in these structures. At ps=4.0*1011 cm-2 strong coupling of optical phonon and plasmon modes leads to enhanced scattering and consequent reduction of the hole drift velocity, but in the current model this effect still does not fully account for the strong saturation of the drift velocity observed experimentally.

Monte Carlo simulations of low-field hole transport in strained InGaAs quantum wells

The authors report on Monte Carlo simulations of low-field hole transport at 77 K in InGaAs-AlGaAs quantum wells of different widths and alloy compositions. The valence subband structure is obtained using a k.p method within the infinite well approximation, which accounts for mixing between heavy and light hole states. The effect of alloy, impurity and phonon scattering are included in the transport simulations. Although the infinite well approximation is only expected to be reliable for barriers with an aluminium fraction greater than about 0.4, for which the heavy hole well is sufficiently deep, the results show good agreement with experimental measurements for a finite 90 AA In0.18Ga0.82As-GaAs quantum well. A study of hole transport in 90 AA InxGa1-xAs wells (0.10<or=x<or=0.25) predicts a mobility which increases with indium concentration since the reduction in the effective mass of the highest HH1 subband due to strain more than compensates for the greater alloy scattering rate. An analysis of wells with 18% indium content and widths in the range 50-150 AA indicates a general increase in hole mobility with well width but with a local minimum around 90 AA due to intersubband scattering from the HH1 subband to the heavier HH2 subband.

Phonon scattering and mobility of holes in a GaAs/AlAs quantum well

Rates for hole-phonon scattering in a GaAs/AlAs quantum well are calculated using an eight-band k.p method. The method includes the effects of heavy hole-light hole mixing on the scattering metrix elements, subband energy dispersions and densities of states. The scattering rates exhibit distinct structure; arising on the one hand, from the strong k/sub /// dependence of the matrix elements (due to band mixing), and on the other, from large peaks in the densities of states at subband energy minima which are displaced from the zone centre. The rates for intrasubband scattering by both optical and acoustic phonons are larger than the principal intraband rates for holes in bulk GaAs. However the rates for intersubband scattering are reduced by band mixing effects, and are all considerably smaller than the principal bulk interband rate. The scattering rates are used as a database for Monte Carlo simulations of steady state hole transport in the GaAs/AlAs quantum well. The low field 2D hole mobility at 77 K is estimated to be some 30% lower than the phonon-limited bulk mobility, and this is attributed to the stronger acoustic phonon scattering in the quantum well. At higher fields, strong intrasubband polar optical scattering is evident, giving rise to an anomalous repopulation of the highest valence subband.

Monte Carlo simulation of electron transport in GaAs/Ga1-xAlxAs quantum wells using different phonon models

The authors calculate electron-LO phonon intersubband scattering rates and intrasubband scattering rates in a 70 AA GaAs/Ga0.7Al0.3As quantum well considering the phonons to be described by the hydrodynamic model, the slab mode model and the bulk phonon approximation. These rates for the different phonon models are used in a Monte Carlo simulation of the electron drift velocity in the quantum well under the influence of an electric field.

Investigation of transit times in SiGe structures: a Monte-Carlo approach

Self-consistent Monte Carlo simulations of the base-collector regions of Ge/sub x/Si/sub 1-x//Si heterostructure bipolar transistors are discussed. High-current effects, which cause inversion of the collector field, are shown to decrease the base-collector transit time from 9.3 ps to 5.7 ps. Base and base-collector transit times are investigated for a range of Ge content in the base. The base transit time is found to decrease as the Ge content of the base is increased from 0% to 30%. The effects of the electron injection boundary conditions on transit times are studied, and it is shown that a complete knowledge of the valley occupancy on injection is unimportant for many aspects of the simulation of carriers passing through the device but that it is necessary to have a good knowledge of the momentum distribution within individual valleys.<<ETX>>