Won-Seong Lee

Effect of emitter-base spacing on the current gain of AlGaAs/GaAs heterojunction bipolar transistors

An investigation of the effect of surface recombination and emitter-base-contact spacing on the DC current-gain of AlGaAs/GaAs heterojunction bipolar transistor (HBT) using thin AlGaAs emitter structures is discussed. The selectively-etched, thin-AlGaAs-emitter layer has been used to prevent an exposed extrinsic base region, which has previously limited current gain because of high surface recombination. It is found that a factor of approximately 50 improvement in the current gain can be achieved by proper surface passivation and emitter-base-contact spacing.<<ETX>>

The Effect of Trapped Charge Distributions on Data Retention Characteristics of nand Flash Memory Cells

We present this letter on the combining effect of tunnel-oxide degradation and narrow width effect on the data retention characteristics of NAND flash memory cells. Due to severe boron segregation in shallow-trench isolation (STI) corner, the cell transistor suffers from intense VTH shift on STI corner in data retention mode. Independent of enhancing the tunnel-oxide quality, the data retention characteristics are improved by designing a cell transistor that isolates the region where Fowler-Nordheim stress mainly occurs in tunnel oxide away from STI corner. Experimental results show that VTH shift is reduced by 0.3 V or more in retention mode as the tunneling is separated from the isolation edge.

Temperature-Dependent Characteristics of Cylindrical Gate-All-Around Twin Silicon Nanowire MOSFETs (TSNWFETs)

The characteristics of cylindrical gate-all-around twin silicon nanowire field-effect transistors with a radius of 5 nm have been measured in temperatures T ranging from 4 to 300 K. The dependence of the off-current suggests that thermal generation in the channel is the main leakage mechanism. The dependence of the subthreshold swing exhibits no body effects but shows degradations due to slight differences in the threshold voltages and in the body effect constants of the twin nanowires. The T dependence of the peak normalized transconductance gm /VDS gives a clue of 1-D phonon scattering and suggests that surface roughness scattering at the nanowire wall is dominant at low values.

Effect of hydrogen on undoped and lightly Si‐doped molecular beam epitaxial GaAs layers

This letter describes a series of experiments in which pure hydrogen gas (at up to 1.2×10−6 Torr partial pressure) was introduced during molecular beam epitaxial (MBE) growth of low‐doped and undoped GaAs at a substrate temperature of 580 °C. A major improvement in the electrical properties of the epitaxial layer has been observed. Electron mobilities at 77 K and deep level transient spectroscopy (DLTS) spectra are presented. A sharp increase in electron mobility and a dramatic reduction of M1 and M4 deep electron traps as shown by DLTS have been achieved with small amounts of H2 gas at 1.2×10−6 Torr partial pressure.

Accurate measurement of MBE substrate temperature

The behavior of real substrate temperature for radiatively heated MBE holders is measured using changes in the bandgap and refractive index with temperature. The infra-red spectroscopy techniques utilized are not sensitive to window coating and have no adjustable parameters. Using bandgap changes with temperature, we observed that the change in real substrate temperature is approximately 60% that of the indicated thermocouple change and that variations as large as 15°C occurred between “nominally identical” substrate holders. The real substrate temperature also changes by up to 10°C during growth and with opening and closing of high temperature doping furnaces.

Reduction of gallium‐related oval defects

A novel gallium effusion cell consisting of a sapphire crucible in a variable temperature profile furnace was used to grow GaAs layers. The effect of the gallium source temperature profile on gallium source‐related oval defects was investigated. By minimizing gallium droplets in the orifice, the gallium‐related oval defect density was reduced to <50 cm−2 for a 1‐μm‐thick layer. These results indicate that oval defect densities can be further reduced by eliminating gallium droplets in the crucible orifice.

Influence of buffer layer thickness on DC performance of GaAs/AlGaAs heterojunction bipolar transistors grown on silicon substrates

The DC performance of GaAs/AlAs heterojunction bipolar transistors (HBTs) grown on silicon substrates with buffer layers ranging from 0 to 5 mu m was investigated. Current gain, collector-emitter breakdown voltage, emitter-base and collector-base diode ideality factors, and breakdown voltages were measured as the buffer layer thickness was varied between 0 and 5 mu m. The current gain steadily increases with increasing buffer layer thickness until the layer reaches 3 mu m. However, the other DC parameters are relatively insensitive to the buffer layer thickness. A small-signal current gain of 60 is typically achieved for devices with 6*6- mu m/sup 2/ emitters at a density of 6*10/sup 4/ A/cm/sup 2/ when the buffer layer is >or=3 mu m.<<ETX>>

Effect of bulk recombination current on the current gain of GaAs/AlGaAs heterojunction bipolar transistors in GaAs-on-Si

The effect of bulk recombination current on the current gain of heterojunction bipolar transistors (HBTs) in GaAs-on-Si has been studied. A thin AlGaAs emitter layer is used to prevent an exposed extrinsic base region, which has previously masked the effect of the bulk recombination current on the current gain in HBTs. It is found that once the surface recombination current is removed, the current grain due to bulk recombination alone is approximately ten times lower for the HBTs in GaAs-on-Si than for HBTs in GaAs. The difference in the current gain is probably due to electrically active dislocations in the base in the HBTs in GaAs-on-Si, where the density is about 10/sup 8/ cm/sup -2/ as measured by transmission electron microscopy.<<ETX>>

Microwave characteristics of MBE grown resonant tunneling devices

Resonant tunnel devices grown by molecular-beam epitaxy (MBE) have been measured experimentally using network analysis techniques from 130 MHz to 20 GHz. A circuit model for the devices has been extracted for two different InGaAs well structures at a fixed bias point, which fits the measured data well and is useful for circuit design. Additionally, the device impedance has been measured as a function of bias at a fixed frequency point. Complicated capacitance characteristics were observed for the devices with large-indium-content wells.<<ETX>>

Uniform, high-gain AlGaAs/In/sub 0.05/Ga/sub 0.95/As/GaAs P-n-p heterojunction bipolar transistors by dual selective etch process

AlGaAs/InGaAs/GaAs P-n-p heterojunction bipolar transistors (HBTs) have been fabricated using a dual selective etch process. In this process, a thin AlGaAs surface passivation layer surrounding the emitter is defined by selective etching of the GaAs cap layer. The InGaAs base is then exposed by selective etching of the AlGaAs emitter. The resulting devices were very uniform, with current gain varying by less than +or-10% for a given device size. Current gain at a given emitter current density was independent of device size, with gains of over 200 obtained at current densities above 5*10/sup 4/ A/cm/sup 2/.<<ETX>>