Edward T. Croke

X- and Ku-band amplifiers based on Si/SiGe HBT's and micromachined lumped components

A double mesa-structure Si/SiGe heterojunction bipolar transistor (HBT) and novel micromachined lumped passive components have been developed and successfully applied to the fabrication of X- and Ku-band monolithic amplifiers. The fabricated 5/spl times/5 /spl mu/m/sup 2/ emitter-size Si/SiGe HBT exhibited a DC-current gain /spl beta/ of 109, and f/sub T/ and f/sub max/ of 28 and 52 GHz, respectively. Micromachined spiral inductors demonstrated resonance frequency of 20 GHz up to 4 nH, which is higher than that of conventional spiral inductors by a factor of two. Single-, dual-, and three-stage X-band amplifiers have been designed, based on the extracted active- and passive-device model parameters. A single-stage amplifier exhibited a peak gain of 4.0 dB at 10.0 GHz, while dual- and three-stage versions showed peak gains of 5.7 dB at 10.0 GHz and 12.6 dB at 11.1 GHz, respectively. A Ku-band single-stage amplifier has also been designed and fabricated, showing a peak gain of 1.4 dB at 16.6 GHz. Matching circuits for all these amplifiers were implemented by lumped components, leading to a much smaller chip size compared to those employing distributed components as matching elements.

Monolithically integrated multichannel SiGe/Si p-i-n-HBT photoreceiver arrays

A low-power, short-wavelength eight-channel monolithically integrated photoreceiver array, based on SiGe/Si heterojunction bipolar transistors, is demonstrated. The photoreceiver consists of a photodiode, three-stage transimpedance amplifier, and passive elements for feedback, biasing and impedance matching. The photodiode and transistors are grown by molecular beam epitaxy in a single step. The p-i-n photodiode exhibits a responsivity of 0.3A/W and a bandwidth of 0.8 GHz at /spl lambda/=0.88 /spl mu/m. The three-stage transimpedance amplifier demonstrates a transimpedance gain of 43 dB/spl Omega/ and a -3 dB bandwidth of 5.5 GHz. A single channel monolithically integrated photoreceiver consumes a power of 6 mW and demonstrates an optical bandwidth of 0.8 GHz. Eight-channel photoreceiver arrays are designed for massively parallel applications where low power dissipation and low crosstalk are required. The array is on a 250-/spl mu/m pitch and can be easily scaled to much higher density. Large signal operation up to 1 Gb/s is achieved with crosstalk less than -26 dB. A scheme for time-to-space division multiplexing is proposed and demonstrated with the photoreceiver array.

Si-based multi-finger n-MODFET RF power devices

We report, for the first time, on the fabrication and characterizations of multi-finger n-type Si/SiGe modulation doped FETs (MODFET) for RF power amplifications. Under bias of VDS=5V, V GS=0V and continuous wave operation, load-pull measurements at 2 GHz from a 10-finger n-MODFET with a gate width of 500 (750) mum show that a maximum output power of 12 (14) dBm, power gain at 1 dB compression of 15 (16) dB, and maximum power added efficiency of 12 (15) % of MODFET device have been achieved

High Mobility SiGe/Si n-MODFET Structures and Devices on Sapphire Substrates

Si/Ge/Si n-type modulation doped field effect structures and transistors (n-MODFETs) have been fabricated on r-plane sapphire substrates. Mobilities as high as 1380 cm(exp 2)/Vs were measured at room temperature. Excellent carrier confinement was shown by Shubnikov-de Haas measurements. Atomic force microscopy indicated smooth surfaces, with rms roughness less than 4 nm, similar to the quality of SiGe/Si n-MODFET structures made on Si substrates. Transistors with 2 micron gate lengths and 200 micron gate widths were fabricated and tested.

Materials optimization for high power silicon germanium heterostructure bipolar transistors at X-band frequencies

SiGe base layers with high (>0.15 Ge) can potentially extend the range of high power HBTs to X-band and higher frequencies, but in practice the actual performance of these devices is usually poorer than the anticipated results. Optimization and reproducibility of materials properties is essential for the successful development of these devices, and this paper reports microstructural characterization of high-Ge content films, simulates the performance of an HBT structure, and provides insights into how material defects impact device performance. High resolution X-ray diffraction indicates that the crystalline quality of SiGe layers is highly degraded with increasing Ge content, and interfacial relaxation becomes apparent for Ge>0.15. The SiGe layer thicknesses and Ge contents were measured using both optical ellipsometry and SIMS, and the results were comparable. Simulated data of an HBT transistor shows that minor variations in Ge content alter the cutoff frequencies and gain of the transistors, and base recombination times less that /spl ap/5/spl times/10/sup -7/ seconds severely impair device performance.

High mobility SiGe/Si n-type structures and field effect transistors on sapphire substrates

SiGe/Si n-type modulation doped field effect transistors (MODFETs) fabricated on sapphire substrates are characterized at microwave frequencies for the first time. The highest measured room temperature electron mobility is 1380 cm/sup 2//V-sec at a carrier density of 1.8/spl times/10/sup 12/ cm/sup -2/ for a MODFET structure. At room temperature, a two finger, 2/spl times/200 micron gate n-MODFET has a peak transconductance of 37 mS/mm at a drain-to-source voltage of 2.5 V and an f/sub max/ of 2.45 GHz. Microwave performance of the transistor improved with decreasing temperatures, with an f/sub max/= 5.25 GHz at 100 K.