M. H. Lee

Room-temperature electroluminescence from electron-hole plasmas in the metal–oxide–silicon tunneling diodes

An electron-hole plasma recombination model is used to fit the room-temperature electroluminescence from metal–oxide–silicon tunneling diodes. The relatively narrow line shape in the emission spectra can be understood by the quasi-Fermi level positions of electrons and holes, which both lie in the band gap. This model also gives a narrower band gap than that of bulk silicon. The surface band bending in the Si/oxide interface is responsible for this energy gap reduction.

Ge outdiffusion effect on flicker noise in strained-Si nMOSFETs

The flicker noise characteristics of strained-Si nMOSFETs are significantly dependent on the gate oxide formation. At high temperature (900/spl deg/C) thermal oxidation, the Si interstitials at the Si/oxide interface were injected into the underneath Si-SiGe heterojunction, and enhanced the Ge outdiffusion into the Si/oxide interface. The Ge atoms at Si/oxide interface act as trap centers, and the strained-Si nMOSFET with thermal gate oxide yields a much larger flicker noise than the control Si device. The Ge outdiffusion is suppressed for the device with the low temperature (700/spl deg/C) tetraethylorthosilicate gate oxide. The capacitance-voltage measurements of the strained-Si devices with thermal oxide also show that the Si/oxide interface trap density increases and the Si-SiGe heterojunction is smeared out due to the Ge outdiffusion.

Mechanically strained strained-Si NMOSFETs

The drain-current enhancement of the mechanically strained strained-Si NMOSFET device is investigated for the first time. The improvements of the drain current are found to be /spl sim/3.4% and /spl sim/6.5% for the strained-Si and control Si devices, respectively, with the channel length of 25 /spl mu/m at the external biaxial tensile strain of 0.037%, while the drain-current enhancements are /spl sim/2.0% and /spl sim/4.5% for strained-Si and control Si devices, respectively, with the channel length of 0.6 /spl mu/m. Beside the strain caused by lattice mismatch, the mechanical strain can further enhance the current drive of the strained-Si NMOSFET. The strain distribution due to the mechanical stress has different effect on the current enhancement depending on the strain magnitude and channel direction. The smaller current enhancement for strained-Si device as compared to the control device can be explained by the saturation of mobility enhancement at large strain.

A novel photodetector using MOS tunneling structures

A metal/oxide/p-Si structure with ultrathin oxide is utilized as a photodetector. At positive gate bias, the dark current of the photodetector is limited by the thermal generation of minority carriers in the inversion layer. The high growth temperature (1000/spl deg/C) of the gate oxide can reduce the dark current to a level as low as 3 nA/cm/sup 2/. As biased in the inversion layer, the tunneling diode works in the deep depletion region with soft pinning of oxide voltage, instead of the pinning of surface potential, very different from the conventional MOS diode with thick oxide.

Package-strain-enhanced device and circuit performance

The hole mobility enhancement can be as high as /spl sim/18% for SiO/sub 2/ and /spl sim/20% for high-k HfO/sub 2/ gate stack dielectrics with the uniaxial compressive strain (0.2%) parallel to the channel. The highest drain current of /spl sim/22% at saturation and /spl sim/30% at linear region is observed for the bulk Si PMOS with high-k gate stacks. The drain current and hole mobility of bulk Si PMOS are degraded under the small biaxial tensile strain, while substrate-strained Si device displays the opposite. The nonoptimized ring oscillator has the speed enhancement of /spl sim/7% under the uniaxial tensile strain parallel to NMOS channel. Proper package strain also gives the drive-current as well as mobility enhancement at 100/spl deg/C.

A comprehensive study of inversion current in MOS tunneling diodes

The gate current of MOS tunneling diodes biased at inversion region with different substrate doping is investigated. For p-type substrate (1-5 /spl Omega/-cm) devices, the tunneling diode works in the deep depletion region and the inversion current is dominated by the thermal generation rate of minority electrons via traps at Si/SiO/sub 2/ interface and in the deep depletion region. The activation energy is approximately equal to half of the silicon bandgap independent of gate voltage. For devices on p/sup +/ substrate (0.01-0.05 /spl Omega/-cm), the band-to-traps tunneling and band-to-band tunneling are the dominating current components at inversion bias, and reveal a strong field dependence and a weak temperature dependence. The band-to-traps and band-to-band current components are even more significant in the devices on the p/sup ++/ substrate (0.001-0.0025 /spl Omega/-cm). Finally, the effects of temperature and light illumination on inversion current of MOS tunneling diodes will be also discussed.

Roughness-enhanced electroluminescence from metal oxide silicon tunneling diodes

An approximate two-order increase in magnitude in electroluminescence was observed for the metal-oxide-silicon tunneling diodes with oxide grown at 900/spl deg/C, as compared to 1000/spl deg/C. The X-ray reflectivity revealed that the oxide grown at 900/spl deg/C has rougher interface than that grown at 1000/spl deg/C. The role of interface roughness can be understood in a model composed of phonons and interface roughness. An external quantum efficiency of /spl sim/10/sup -6/ was obtained using Al electrodes.

Temperature dependence of the electron-hole-plasma electroluminescence from metal-oxide-silicon tunneling diodes

The temperature performance of metal–oxide–silicon tunneling light-emitting diodes was studied. An electron–hole-plasma model can be used to fit all the emission spectra from room temperature to 98 K. At constant voltage bias in the accumulation region, the normalized integral emission intensity slightly increases at low temperature with activation energy as low as 12 meV. From room temperature down to 98 K, the extracted band gaps are ∼80 meV lower than the value of Varshni equation, and the linewidth drops from 65 to 30 meV. The transverse optical and longitudinal optical phonons are involved in the light-emission process due to the reduction of extracted band gaps and the resemblance between electroluminescence and photoluminescence spectra at similar temperature.

Comprehensive low-frequency and RF noise characteristics in strained-Si NMOSFETs

Due to the mobility enhancement of strained-Si channels, the strained-Si MOSFET has reportedly a great improvement in DC characteristics. The improvement in the cut-off frequency ( f/sub T/) of strained-Si devices is demonstrated in this work. The strained-Si device has the same flicker noise (1/f) as the control Si device as long as no threading dislocation exists in the channel and the thermal budget is properly controlled. The large defect density in the relaxed SiGe buffer layers shows no effect on the flicker noise. The threading dislocation penetrating into the strained-Si channel and Ge outdiffusion can degrade the flicker noise of strained-Si NMOSFETs. A thicker strained-Si channel layer can reduce the roughness scattering from the underlying strained Si/relaxed SiGe heterojunction, and yields a higher mobility, higher f/sub T/, and lower noise figures, as compared to the thin strained-Si channel.

Wireless charging system of mobile handset using metamaterial-based cavity resonator

A metamterial-based cavity resonator using artificial magnetic conductor surfaces is presented and applied to realize a wireless charging system of mobile handset. This wireless charging system demonstrates the advantages of broad bandwidth, small size, long transmitting range, high transferring efficiency and EMI-free feature. The holistic architecture of wireless charging system for a smartphone is established to experimentally validate the usefulness of the metamaterial cavity resonator.