Ali A. Rezazadeh

Empirical low-field mobility model for III-V compounds applicable in device simulation codes

A Caughey–Thomas-like mobility model with temperature and composition dependent coefficients is used in this work to describe the dependence of electron and hole mobilities on temperature, doping concentration, and alloy composition. Appropriate parameter sets are given for a large number of III–V binary and ternary compounds, including: GaAs, InP, InAs, AlAs, GaP, Al0.3Ga0.7As, In0.52Al0.48As, In0.53Ga0.47As, and In0.49Ga0.51P. Additionally, physically justifiable interpolation schemes are suggested to find the mobilities of various ternary and quaternary compounds (such as AlxGa1−xAs, In1−xGaxP, In1−xGaxAs, In1−xAlxAs, and In1−xGaxAsyP1−y) in the entire range of composition. The models are compared with numerous measured Hall data in the literature and very good agreement is observed. The limitations of the present model are also discussed. The results of this work should be extremely useful in device simulation packages, which are currently lacking a reliable mobility model for the above materials.

Changes in the dielectric properties of rat tissue as a function of age at microwave frequencies.

The dielectric properties of ten rat tissues at six different ages were measured at 37 degrees C in the frequency range of 130 MHz to 10 GHz using an open-ended coaxial probe and a computer controlled network analyser. The results show a general decrease of the dielectric properties with age. The trend is more apparent for brain, skull and skin tissues and less noticeable for abdominal tissues. The variation in the dielectric properties with age is due to the changes in the water content and the organic composition of tissues. The percentage decrease in the dielectric properties of certain tissues in the 30 to 70 day old rats at cellular phone frequencies have been tabulated. These data provide an important input in the provision of rigorous dosimetry in lifetime-exposure animal experiments. The results provide some insight into possible differences in the assessment of exposure for children and adults.

Stepping toward standard methods of small-signal parameter extraction for HBTs

An improved HBT small-signal parameter extraction procedure is presented in which all the equivalent circuit elements are extracted analytically without reference to numerical optimization. Approximations required for simplified formulae used in the extraction routine are revised, and it is shown that the present method has a wide range of applicability, which makes it appropriate for GaAs and InP-based single and double HBTs. Additionally, a new method is developed to extract the total delay time of HBTs at low frequencies, without the need to measure h/sub 21/ at very high frequencies and/or extrapolate it with -20 dB/dec roll-off. The existing methods of finding the forward transit time are also modified to improve the accuracy of this parameter and its components. The present technique of parameter extraction and delay time analysis is applied to an InGaP/GaAs DHBT and it is shown that: (1) variations of all the extracted parameters are physically justifiable; (2) the agreement between the measured and simulated S- and Z-parameters in the entire range of frequency is excellent; and (3) an optimization step following the analytical extraction procedure is not necessary. Therefore, we believe that the present technique can be used as a standard extraction routine applicable to various types of HBTs.

3-D low-loss coplanar waveguide transmission lines in multilayer MMICs

Newly developed transmission-line structures using the great flexibility of three-dimensional multilayer technology have been designed and fabricated. In this paper, we demonstrate that monolithic microwave integrated circuit (MMIC) coplanar waveguide transmission lines with a wide range of characteristic impedances can easily be designed using the multilayer technique. Furthermore, this implementation can avoid the well-known current crowding effects on the conductor edges minimizing dissipation loss. The system of three layers of metals and two layers of sandwich polyimide as dielectrics was employed. The fabricated transmission lines have been characterized providing a wide range of impedances from 10 to 70 Omega. In addition, the effects of unintentional horizontal and vertical coupling in multilayer MMICs have been investigated. The results indicated that an optimum separation of 75 mum is necessary for negligible coupling (~-30 dB)

Optically transparent ITO emitter contacts in the fabrication of InP/InGaAs HPT's

An optically transparent emitter InP/InGaAs heterojunction phototransistor (HPT) fabricated using Indium Tin Oxide (ITO) as the ohmic contact is presented for the first time; these devices show similar electrical characteristics to their opaque emitter counterparts and enhanced optical responsivities (5.4 A/W at 780 nm wavelength). Measured spectral response suggests responsivities of up to 30 A/W and 22 A/W at /spl lambda/=1310 nm and 1550 nm respectively. >

Modeling and analysis of the spectral response for AlGaAs/GaAs HPTs for short wavelength optical communication

Detailed spectral response (SR) modeling for heterojunction bipolar phototransistors (HPTs) is presented in this work. All the related physical parameters are taken into account for the resolution of photogenerated excess minority carrier continuity equations in the active layers of the HPT. The layer dependence of the optical flux absorption profile at near-bandgap wavelengths is also investigated and its generalization as a single-exponential has been refuted for HPTs based on GaAs material systems (InGaP-GaAs/AlGaAs-GaAs). The variation in the responsivity of the device with changing base width is analyzed at various wavelengths and a detailed experimental setup for optical characterization of HPTs is also provided. The measured results at 635, 780, 808, and 850 nm show good agreement to the modeled data, validating the newly developed theoretical model.

Direct extraction and numerical simulation of the base and collector delay times in double heterojunction bipolar transistors

A new method is presented to evaluate the base and collector transit times, /spl tau//sub B/ and /spl tau//sub C/ in heterojunction bipolar transistors (BBTs) from the phase and magnitude of the common-base current gain, /spl alpha/(/spl omega/), which itself was directly extracted from measured S-parameter data. The method is applied to InGaP/GaAs single and double HBTs. A smaller cutoff frequency in the latter device is attributed to /spl tau//sub B/ and /spl tau//sub C/ due to two effects: trapping of electrons in the conduction band triangular barrier existing at the base-collector (B-C) heterojunction and smaller saturation velocity of electrons in InGaP as compared to GaAs. Finally, a new B-C design of InGaP/GaAs DNBTs is proposed to partially compensate the transit time effects. Numerical simulation of the cutoff frequency demonstrates the superiority of the proposed structure for high-frequency applications.

Temperature-Dependent DC and Small-Signal Analysis of AlGaAs/InGaAs pHEMT for High-Frequency Applications

Temperature-dependent dc and small-signal analysis have been carried out on 0.5 μm × 200-μm AlGaAs/InGaAs pseudomorphic high-electron mobility transistor over the temperature range from -40 °C to 150 °C by on-wafer S-parameter measurements up to 50 GHz. The thermal behavior of dc and equivalent circuit parameters along with their temperature coefficients was analyzed and reported for the first time using the same device. Most of these parameters show a negative trend with temperature, such as drain-source output current I_ds, extrinsic transconductance g_m, effective electron velocity v_eff, threshold voltage V_T, Schottky barrier height φ_b, gate-drain capacitance C_gd, drain-source capacitance C_ds, intrinsic transconductance g_m0, cutoff frequency f_t, and maximum frequency f_max. On the other hand, Two-dimensional electron gas sheet carrier density n_s, ON-resistance R_ON, series resistance R_series, terminal resistances (R_g, R_s, and R_d), output resistance R_ds, input resistance R_i, gate-source capacitance C_gs, and intrinsic delay time τ show a positive trend with temperature. The results provide some valuable insights for future design optimizations of advanced GaAs-based Monolithic microwave integrated circuits.

Photoresponse Modeling and Analysis of InGaP/GaAs Double-HPTs

Photoresponse of GaAs-based double-heterojunction phototransistors (DHPTs), in surface-illuminated orientation, has been analyzed with a modified small-signal model. The effect of incident optical illumination on various intrinsic parameters has been discussed for In0.49Ga0.51P/GaAs N+p+N- DHPT. Since the primary detecting material is GaAs, the device is optimized to detect short wavelength at 850 nm. A novel formulation for optical flux absorption in DHPTs is also provided along with its comparison with single-heterojunction phototransistors. The analysis of DHPTs presented in this paper can be utilized for performance enhancement through device optimization in sensors, photoreceivers in optical networks, and remote sensing applications employing integrated circuits.

Design and Realization of Spiral Marchand Balun Using CPW Multilayer GaAs Technology

Newly developed spiral Marchand baluns using the great flexibility of three-dimensional multilayer technology have been designed, optimized, fabricated, and measured. This paper demonstrates a monolithic microwave integrated circuits (MMIC) coplanar waveguide (CPW) spiral Marchand balun with good balance performance in a wide frequency range and compact size. Even/odd mode characteristic impedance analysis was applied to optimize the baluns performance. A 2.5D simulator ADS Momentum and a true 3D simulator Ansoft HFSS were used for EM simulations providing useful insight of the components design and optimizations as compared with the measured data. On-wafer RF measurements were carried out from 45MHz to 40GHz. The measured results shows small insertion loss, good amplitude and phase balance in a wide operating frequency band. A very compact size of 0.26 mm2 is achieved