H.R. Chen

Kinetics of compositional disordering of AlGaAs/GaAs quantum wells induced by low‐temperature grown GaAs

Compositional disordering of GaAs/AlGaAs quantum wells due to the presence of low‐temperature grown GaAs (by molecular beam epitaxy) was studied. Ga vacancy enhanced interdiffusion was found to be the mechanism underlying the observed intermixing. Diffusion equations were solved numerically to obtain the band profile after intermixing. The transition energies in the quantum wells under various annealing conditions were solved and agree very well with the observed photoluminescence emission peaks. The diffusivity of Ga vacancies and that of induced Al‐Ga interdiffusion were obtained. The vacancy induced interdiffusion diffusivity was found to have an activation energy of 4.08 eV, which is smaller than the activation energy of interdiffusion diffusivity of normal temperature grown GaAs/AlGaAs heterostructures. This is a clear indication of enhanced interdiffusion due to the presence of low‐temperature grown GaAs.

High current gain, low offset voltage heterostructure emitter bipolar transistors

Heterostructure Emitter Bipolar Transistors (HEBTs) were fabricated with improved characteristics by employing the emitter edge-thinning technique. Optimum design for the n-GaAs emitter and the ledge thickness were used to remove the potential spike and to prevent ledge conduction simultaneously. Current gains as high as 720, which is the highest current gain ever reported for HEBTs, and offset voltages down to 70 mV were obtained. Very large improvement in current gain, especially at low collector current, was obtained by using the emitter edge-thinning technique.<<ETX>>

Correlating drain-current with strain-induced mobility in nanoscale strained CMOSFETs

The correlation between channel mobility gain (Deltamu), linear drain-current gain (DeltaI_dlin), and saturation drain-current gain (DeltaIdsat) of nanoscale strained CMOSFETs are reported. From the plots of DeltaI_dlin versus DeltaI_dsat and ballistic efficiency (Bsat,PSS), the ratio of source/drain parasitic resistance (R_SD,PSS) to channel resistance (R_CH,PSS) of strained CMOSFETs can be extracted. By plotting Deltamu versus DeltaI_dlin, the efficiency of Deltamu translated to DeltaI_dlin is higher for strained pMOSFETs than strained nMOSFETs due to smaller RSD,PSS-to-RCH,PSS ratio of strained pMOSFETs. It suggests that to exploit strain benefits fully, the RSD,PSS reduction for strained nMOSFETs is vital, while for strained pMOSFETs the DeltaI_dlin-to-Deltamu sensitivity is maintained until R_SD,PSS becomes comparable to/or higher than R_CH,PSS

Heterojunction bipolar transistors with emitter barrier lowered by /spl delta/-doping

An npn heterojunction bipolar transistor with a Si-/spl delta/-doped layer at the emitter-base hetero-interface is demonstrated. The Si /spl delta/-doped layer reduces the potential spike at the emitter junction. This design reserves the abruptness of the heterojunction, reduces electron barrier and increases the hole barrier simultaneously. Experimental results show that the HBTs turn-on characteristics are greatly improved while the current gain remains high. The offset voltages as low as 44 mV have been obtained. Very high current gains at very low collector current densities are obtained.<<ETX>>

Channel backscattering characteristics of uniaxially strained nanoscale CMOSFETs

Channel backscattering characteristics of uniaxially strained nanoscale CMOSFETs are reported for the first time. Channel backscattering ratio increases and decreases under uniaxial tensile and compressive strain, respectively. It is found that in sub-100-nm devices, strain-induced modulation of carrier mean-free path for backscattering and reduction in k/sub B/T layer thickness are responsible for the different behaviors of backscattering ratio. Nevertheless, the source-side injection velocity improves irrespective of the strain polarities. The impact of channel backscattering ratio on drive current is also analyzed in terms of ballistic efficiency and injection velocity.

INFLUENCE OF X-VALLEY SUPERLATTICE ON ELECTRON BLOCKING BY MULTIQUANTUM BARRIERS

Four n‐i‐n GaAs/AlGaAs diodes have been used to demonstrate that at room temperature the carrier blocking is influenced by the X‐valley superlattice in a multiquantum barrier. The diode with a Γ‐X crossover multiquantum barrier has a current at least three orders lower than the corresponding diode with only a bulk barrier. However, diodes without Γ‐X crossover barriers have resistance as low as 10 Ω in spite of whether a multiquantum barrier exists or not. This indicates that the X‐valley superlattice plays an important role in blocking the current flowing across/through a multiquantum barrier.

Influence of oxygen on the performance of GaAs/AlGaAs quantum well infrared photodetectors

The influence of oxygen on the performance of GaAs/AlGaAs quantum well infrared photodetectors (QWIP) has been studied. Photoluminescence and secondary ion mass spectroscopy were used to examine the relationship between the quality of AlGaAs and the oxygen content that were then correlated to the performance of QWIPs. It was found that oxygen is the dominant impurity in the GaAs/AlGaAs superlattice. Because oxygen atoms behave likes electron traps, which effectively reduce the carrier concentration in the material, both of the responsivity and the dark current of QWIP are reduced. The detectivities of QWIPs with lower oxygen concentrations are better than those with higher oxygen concentrations.

ASYMMETRIC DARK CURRENT IN QUANTUM WELL INFRARED PHOTODETECTORS

Asymmetric dark current versus voltage characteristics in quantum well infrared photodetectors have been studied. A model based on asymmetrical potential barriers was proposed. The asymmetrical potential barriers, which are most likely due to the accumulation of oxygen impurities at one of the interfaces, cause the asymmetrical I‐V characteristics. The height of the potential spike is found to increase with the Al content in the AlGaAs barriers. Calculations based on our model agree well with experimental results.

The application of delta-doping in heterojunction bipolar transistors

The spl delta/-doping is applied to the EB junction of single heterojunction bipolar transistors(SHBT) and BC junction of double heterojunction bipolar transistors(DHBT). Very good results are obtained. The offset voltage is reduced in SHBT and dramatically current gain increase is obtained in DHBT.

Compositional disordering of AlGaAs/GaAs superlattices by low temperature grown GaAs

The use of low-temperature (/spl tilde/ 200 /spl deg/C) grown GaAs by molecular beam epitaxy to induce compositional disordering of AlGaAs/GaAs superlattice has been studied. After furnace annealing between 700 /spl deg/C and 850 /spl deg/C for 30 minutes, obvious blue shift in the peak wavelength of the superlattice emission is observed by the 77K photoluminescence (PL), indicating that the emission has been changed from that of the GaAs quantum wells to that of the intermixed AlGaAs. The PL shift and the depth profile of Al concentration measured by secondary ion mass spectrometry indicate that the superlattice is nearly totally disordered after 850 /spl deg/C annealing.