Robert Neidhard

Very high-power-density CW operation of GaAs/AlGaAs microwave heterojunction bipolar transistors

Thermal instability of multi-emitter high-power microwave heterojunction bipolar transistors (HBTs) was eliminated using a novel heat spreading technique that regulates internal device currents to avoid the formation of hot spots. Devices with 2- and 3- mu m minimum emitter sizes and no intentional ballast resistors showed unconditionally stable CW operation up to the device electronic limitations. A record 10-mW/ mu m/sup 2/ power density was obtained at 10 GHz with 7-dB gain and 60% power-added efficiency. The highest efficiency was 67.2% at 9.3-mW/ mu m/sup 2/ power density. It was shown that stable high-power-density operation can be maintained at multiwatt output power levels.<<ETX>>

Microwave ZnO Thin-Film Transistors

We have developed ZnO thin-film transistor design and fabrication techniques to demonstrate microwave frequency operation with 2-mum gate length devices produced on GaAs substrates. Using SiO2 gate insulator and pulsed laser deposited ZnO active layers, a drain-current ON/OFF ratio of 1012, a drain-current density of 400 mA/mm, a field-effect mobility of 110 cm2/V ldr s, and a subthreshold gate voltage swing of 109 m\/dec were achieved. Devices with Ti-gate metal had current and power gain cutoff frequencies of 500 and 400 MHz, respectively.

High-Frequency ZnO Thin-Film Transistors on Si Substrates

Record microwave frequency performance was achieved with nanocrystalline ZnO thin-film transistors fabricated on Si substrates. Devices with 1.2-mum gate lengths and Au-based gate metals had current and power gain cutoff frequencies of fT = 2.45 GHz and fmax = 7.45 GHz, respectively. Same devices had drain-current on/off ratios of 5 times1010 exhibited no hysteresis effects and could be operated at a current density of 348 mA/mm. The microwave performances of devices with 1.2- and 2.1- mum gate lengths and 50- and 100-mum gate widths were compared.

A Low Voltage Tunable Analog Phase Shifter Utilizing Ferroelectric Varactors

ABSTRACT A distributed transmission line phase shifter was implemented by periodically loading a coplanar transmission line using ferroelectric varactors in a shunt configuration. A single dc bias applied to the phase shifter circuit provides a large relative phase shift. A relative phase shift of ∼ 305 degrees was obtained at 20 GHz for a dc bias of only 5 V.

Nanocrystalline ZnO microwave thin film transistors

Nanocrystalline ZnO films prepared by Pulsed Laser Deposition were used to fabricate the first thin film transistors that operate at microwave frequencies. Unlike more conventional amorphous Si and organic thin film transistors, which are only suitable for low speed applications, ZnO-based thin film transistors exhibit figure-of-merit device numbers that are comparable to single crystal transistors. These include on/off ratio of 1012, current density of >400mA/mm and field effect mobility of 110 cm2/V.s. Parameters, including film growth temperature, gate insulators, and device layout designs were examined in detail to maximize performance. We have achieved current gain cut-off frequency, fT, and power gain cut-off frequency, fmax, values of 2.9GHz and 10GHz, respectively with 1.2μm gate length devices demonstrating that ZnO-based TFTs are suitable for microwave applications.

Bias, frequency, and area dependencies of high frequency noise in AlGaAs/GaAs HBT's

The noise figure of the heterojunction bipolar transistor (HBT) in the microwave frequency range is studied, and an improved physical noise model is developed. Unlike the conventional high frequency noise model, which considers only the bias current dependence, the present model includes both the effects of voltage and current on the noise behavior. In addition, the frequency- and area-dependent natures of the HBT noise at very high frequencies are incorporated in the model. It is found that the voltage dependence of the high frequency noise in the HBT results from the self-heating effect, which gives rise to a higher HBT lattice temperature than the ambient temperature. Also, the free-carrier transport delay time must be considered to properly model the frequency dependence of noise since the inverse of this time is comparable with the frequency. Furthermore, the area dependence of noise is dominated by changes in the base resistance and emitter-base junction capacitance. Results for the minimum noise factor calculated from the model compare favorably with those obtained from measurements.

Design of a frequency agile X-band LNA using BST varactor based voltage tunable impedance matching networks

Current advancements in military and wireless applications create the need for increased functionality with reduced cost and size. In this paper a highly integrated tunable electronics are necessary to meet these new requirements. Barium strontium titanium oxide (BST) is a viable technology for these applications. BST technology offers significant benefits with its high tuning range, high power capabilities and low control voltages. While there is great interest in the development of BST technology, little research has been published regarding integrated matching networks using BST thin film parallel plate capacitors for an X-band low noise amplifier (LNA).

Thermally stable AlGaAs/GaAs microwave power HBT's

Summary form only given. Novel thermal shunt and thermal lens techniques have been used in the design of multiple-emitter AlGaAs/GaAs power HBTs (heterojunction bipolar transistors). Thermal stabilization was achieved by the use of a thick metal bridge connecting all emitters. The thickness of the bridge was designed to assure a virtual thermal short between emitters. The use of such a thermal shunt element redistributes temperature-dependent internal device currents to force an equalization of device junction temperature. The bridge further serves to spread heat from each emitter over a larger GaAs surface (thermal lens), thereby reducing overall device thermal resistance. Multiemitter devices with 2 mu m and 3 mu m minimum emitter feature sizes were fabricated and tested at microwave frequencies to demonstrate the thermally stable operation. >

RF filters in SiGe BiCMOS technology and fully depleted silicon-on-insulator CMOS technology

This paper presents two integrated non-reflective bandpass filters. The filters are implemented in a silicon germanium (SiGe) BiCMOS technology and fully depleted silicon on insulator (FDSOI) CMOS technology. The purpose of these circuits is to explore the feasibility of passive filter applications on silicon substrates while maintaining low insertion loss and 50 Ohm impedance matching. The SiGe-based filter achieved 3.3-4.2 dB insertion loss across 3.5-4.5 GHz with input return loss better than -10 dB from 1-10 GHz. The FDSOI filter simulation yielded an insertion loss of 4.5 dB across the design frequency of 3.7-4.3 GHz

A New Ferroelectric Varactor Shunt Switch for Microwave and Millimeterwave Reconfigurable Circuits

Abstract : This paper presents a ferroelectric varactor shunt switch which can be useful for microwave/millimeter wave switching as well as for the design of reconfigurable circuits. The device operation is based on nonlinear dielectric tunability of a ferroelectric thin-film sandwiched between two metal layers in the parallel plate configuration. A CPW based design allows for MMIC compatible shunt switches with low insertion loss and high isolation. Experimental performance of the varactor shunt switch indicates good switching performance with ^24 dB isolation 41 GHz, and insertion loss below 7 dB up to 45 GHz.