Karen J. Nordheden

HEMT degradation in hydrogen gas

The effect of hydrogen treatment on both GaAs pseudomorphic HEMTs and InP-based HEMTs, in order to simulate the hermetic seal environment in a Kovar package, is reported for the first time. Under the 270/spl deg/C, 4% H/sub 2/ in Ar atmosphere, significant changes in both types of HEMTs were observed within several minutes. While the drain current at a fixed gate bias and the pinchoff voltage of the GaAs PHEMT consistently decreased under the influence of the hydrogen gas, they were found to either increase or decrease with the InP HEMT. The change of device characteristics resulting from exposure to the hydrogen environment is not permanent; partial recovery of device characteristics was observed under either nitrogen or hydrogen at both elevated and room temperatures. The change in HEMT DC characteristics seems to be primarily resulted from the change in the gate built-in potential. Any device changes due to the Si-donor neutralization by atomic hydrogen, and therefore a reduction in channel carrier concentration, were found to be insignificant.<<ETX>>

Mixed-conducting oxygen permeable ceramic membranes for the carbon dioxide reforming of methane

Due to the high economic, environmental, and safety costs associated with pure oxygen, mixed-conducting oxygen-permeable ceramic membranes are being explored as an alternative oxygen source for hydrocarbon conversion reactors. This work reports a dramatic improvement in catalyst performance when an oxygen-permeable SrFeCo0.5Ox ceramic membrane is used in conjunction with a conventional powder Pt/ZrO2 catalyst for the CO2 reforming of CH4. Experiments comparing catalyst performance with up to 2% co-fed oxygen to catalyst performance with oxygen from the ceramic membrane demonstrated a conversion three times higher with the membrane than with any amount of co-fed oxygen. The results suggest that membrane oxygen is more beneficial for catalyst activity and stability than molecular gas-phase oxygen.

Reactive ion etching of via holes for GaAs high electron mobility transistors and monolithic microwave integrated circuits using Cl2/BCl3/Ar gas mixtures

A process for through‐the‐wafer via hole connections to each individual source contact for monolithic microwave and millimeter wave integrated circuits and power high electron mobility transistors has been developed using reactive ion etching in Cl2/BCl3/Ar gas mixtures. Placing vias in this manner eliminates the need for source airbridges, which minimizes source inductance and results in increased gain and efficiency. The GaAs etch rate and resultant etch profiles have been studied as functions of bias voltage, gas mixture, flow rate, via mask dimension, and etch time.

Characterization of BCl3/N2 plasmas

Optical emission spectroscopy, quadrupole mass spectrometry, and electron density measurements were used to study the effect of the percentage of N2 on the characteristics of BCl3/N2 plasmas and their resulting etch processes. The etch rate of GaAs increased from 80 A/min in pure BCl3 to over 1000 A/min in a 40:60 BCl3:N2 mixture (15 mTorr, 50 W, 20 sccm). The optical emission intensities of both molecular and atomic chlorine exhibited maxima near 30% N2, and an argon actinometer indicated a large increase in argon emission as a function of the increase in N2 percentage. Microwave measurements indicated that the average electron density increased only slightly with an increase in nitrogen percentage up to 60% N2. Mass spectrometric analysis of the plasmas showed that both the dissociation of BCl3 and the production of molecular chlorine were significantly enhanced by the addition of N2. These results suggest that an increase in the electron temperature as a result of electron attachment heating (and possi...

GaAs Etch Rate Enhancement with SF 6 Addition to BCl3 Plasmas

A dramatic increase in the GaAs etch rate has been observed with the addition of SF 6 to BCl 3 plasmas. The etch rate increases from 70 A/min in pure BCl 3 to 4000 A/min with 70% SF 6 in the total flow. Optical emission intensities of both molecular and atomic chlorine were observed to increase with SF 6 addition, and the peak intensity of the atomic chlorine emission coincided with the peak in the etch rate. Argon was added to the mixture as an actinometer, and the argon emission intensity at 750 nm increased significantly with the addition of SF 6 However, microwave measurements indicated that the average electron density decreases with increasing SF 6 addition. It is believed that the increased production of etch species is due to an increase in the average electron temperature as a result of electron attachment heating.

Triple‐crystal x‐ray diffraction analysis of reactive ion etched gallium arsenide

The effect of BCl3 reactive ion etching on the structural perfection of GaAs has been studied with diffuse x‐ray scattering measurements conducted by high‐resolution triple‐crystal x‐ray diffraction. While using a symmetric 004 diffraction geometry revealed no discernible differences between etched and unetched samples, using the more surface‐sensitive and highly asymmetric 113 reflection revealed that the reactive ion etched samples etched displayed less diffusely scattered intensity than unetched samples, indicating a higher level of structural perfection. Increasing the reaction ion etch bias voltage was found to result in decreased diffuse scattering initially, until an apparent threshold voltage was reached, after which no further structural improvement was observed. Furthermore, we have shown that this reduction in process‐induced surface structural damage is not due merely to the removal of residual chemical‐mechanical polishing damage.

Undergraduate research and intellectual property rights

Most universities today have developed intellectual property policies which clearly establish ownership rights in any commercially valuable process developed by professors and graduate students in a university laboratory. Increasingly, undergraduates are also being encouraged to take an active part in research since this often encourages them to go on to graduate school. It is also, in itself, a valuable learning experience. However, little thought has been given to the ownership rights of these undergraduate researchers. Universities must recognize that undergraduates may have ownership rights in research and must deal with this fact. A number of strategies for doing so are presented.

Manufacturing technology for high performance HBT linear power amplifiers

A straightforward HBT MMIC process technology based on an emitter-to-base realigned approach and single-step thick emitter metallization technique has produced highly linear efficient HBTs and power MMICs with high yields. The power transistor cell achieved 72% PAE with 0.17 W at 6 GHz. The two tone saturated power of the MMIC achieved 36.4% PAE at 10 GHz. The IMD is 30 dBc at 5 dB back-off from 2 dB gain compression.

Inductively coupled plasma etching of ZnO

The etching characteristics of ZnO epitaxial layers in Oxford Plasmalab 100 ICP 180 and 380 systems are investigated. Etch rates are studied as a function of gas composition, ICP power and RF bias power. Surface profilometry and scanning electron microscopy are used to characterize etch rates and surface morphologies. Highlights from other recently published results are also discussed.

High performance heterojunction bipolar transistors grown by molecular-beam epitaxy using novel growth method

A new molecular‐beam epitaxial (MBE) growth method is proposed to study the Be diffusion problem in the base of Npn AlGaAs/GaAs heterojunction bipolar transistors (HBTs). The base layer was grown at As‐stabilized conditions under minimum V/III beam equivalent pressure of 8 to enhance Be migration and reduce the concentration of nonradiative recombination centers. For a base doping of 4×1019 cm−3 (Be) and emitter dimensions of 2.0 μm×10 μm, the device demonstrates excellent direct‐current as well as microwave performance as a power transistor. The unit current gain cut‐off frequency fT is 54 GHz and the maximum frequency of oscillation fmax is above 85 GHz. For power transistors with large emitter periphery (120 μm2), 66% power‐added‐efficiency and 11 dB power gain have been achieved at 4.5 GHz. After the device was continuously stressed at current densities of 2, 4, and 8×104 A/cm2, its current gain, hfe (measured at stressed current density), turn‐on voltage Vbe and junction ideality factor n remain almo...