Alberto Santarelli

Empirical modeling of low-frequency dispersive effects due to traps and thermal phenomena in III-V FET's

An empirical approach is proposed which accounts for low-frequency dispersive phenomena due to surface state densities, deep level traps and device heating, in the modeling of the drain current response of III-V FETs. The model, which is based on mild assumptions justified both by theoretical considerations and experimental results, has been applied to GaAs MESFETs of different manufacturers. Experimental and simulation results that confirm the validity of the model are provided in the paper.<<ETX>>

Millimeter-wave FET modeling using on-wafer measurements and EM simulation

Electron device modeling is a challenging task at millimeter-wave frequencies. In particular, conventional approaches based on lumped equivalent circuits become inappropriate to describe complex distributed and coupling effects, which may strongly affect the transistor performance. In this paper, an empirical distributed FET model is adopted that can be identified on the basis of conventional S-parameter measurements and electromagnetic simulations of the device layout. The consistency of the proposed approach is confirmed by robust scaling properties, which enable millimeter-wave small-signal S-parameters to be predicted as a function of the device periphery and number of gate fingers. Moreover, it is shown how the model identified on the basis of standard S-parameter measurements up to 50 GHz can be efficiently exploited in order to obtain reasonably accurate small-signal prediction up to 110 GHz. Extensive experimental validation is presented for 0.2-/spl mu/m pseudomorphic high electron-mobility transistors devices.

Empirical modeling of low-frequency dispersive effects due to traps and thermal phenomena in III-V FETs

An empirical approach is proposed which accounts for low-frequency dispersive phenomena due to surface state densities, deep level traps and device heating, in the modeling of the drain current response of III-V FETs. The model, which is based on mild assumptions justified both by theoretical considerations and experimental results, has been applied to GaAs MESFETs of different manufacturers. Experimental and simulation results that confirm the validity of the model are provided in the paper. >

New pulsed measurement setup for GaN and GaAs FETs characterization

A new setup is proposed for the measurement of current–voltage pulsed characteristics of electron devices. The main advantages of the system consist in: shorter pulse widths through generation in a 50-Ω environment, simple average current monitoring through separation of the direct and alternate current paths, setting of average voltage values independently of pulse amplitudes and duty cycle, and stability of the setup guaranteed by wide-band dissipative terminations. The system is used for the characterization of dispersive effects due to carrier energy traps and thermal phenomena in GaAs and GaN on SiC field effect transistors. The basic differences between the two technologies are highlighted in the paper.

Accurate prediction of PHEMT intermodulation distortion using the nonlinear discrete convolution model

A general-purpose, technology-independent behavioral model is adopted for the intermodulation performance prediction of PHEMT devices. The model can be easily identified since its nonlinear functions are directly related to conventional DC and small-signal differential parameter measurements. Experimental results which confirm the model accuracy at high operating frequencies are provided in the paper.

Small-signal distributed FET modeling through electromagnetic analysis of the extrinsic structure

The paper presents a new approach to the distributed modeling of high frequency transistors suitable for CAD applications. In particular, electromagnetic simulation is adopted to characterize the extrinsic part of the electron device in terms of a multi-port scattering matrix without introducing approximations based on lumped components. Experimental and simulation results for 0.5 /spl mu/m GaAs MESFETs with different gate widths preliminary confirm the consistency of the proposed approach.

Accurate EM-Based Modeling of Cascode FETs

Cascode field-effect transistors (FETs) are widely used in the design of monolithic microwave integrated circuits (MMICs), owing to their almost unilateral and broadband behavior. However, since a dedicated model of the cell is rarely provided by foundries, a suboptimal description built by replicating the standard foundry model for both the common source and common gate device is often adopted. This might limit the success of the MMIC design at the first foundry run. This paper describes an electromagnetic-based empirical model of cascode cells, covering topics from the formulation and identification procedures to the corresponding validation described in an exhaustive experimental section. A MMIC low-noise distributed amplifier case is then presented and the proposed model is used for circuit analysis and instability detection. Clear indication is provided about the improvement in the prediction of critical behaviors with respect to conventional modeling approaches. A cascode cell with a symmetric layout is also successfully modeled.

A C-Band AlGaN-GaN MMIC HPA for SAR

A C-Band MMIC high power amplifier (HPA) has been designed exploiting a 0.25 μm HEMT GaN process on SiC substrate. The HPA is designed for future synthetic aperture radar (SAR) antenna applications. The HPA delivers 16 W output power with PAE over 38% at 6 dB gain compression within a 900 MHz bandwidth around 5.75 GHz. Up to 20 W output power and 40% PAE are obtained at higher gain compression. A comparison with another amplifier, differing only for the layout of the devices in the final stage, points out that the transistor thermal conditions represent the main limitation for this high power density technology.

An Automated Measurement System for the Characterization of Electron Device Degradation Under Nonlinear Dynamic Regime

A fully automated measurement setup is described, which is aimed at investigating the time dispersion (or ldquowalkoutrdquo) of microwave electron-device characteristics. The proposed setup has the original capability of characterizing device degradation under a nonlinear dynamic operation. Such information is invaluable when the success of a project is inherently related to the end-of-life performance of each system component (e.g., military and space applications). Some previous works underline the importance of such information, but they are not oriented to device characterization and, moreover, are focused on a particular application (e.g., power amplifier design). The commonly adopted measurement techniques, which are essentially focused on high-field static operations, are extremely useful to perform accelerated stress, but they cannot be adopted to obtain exhaustive information about the time dispersion of device characteristics under realistic operative conditions. Through the proposed system, the stress procedure can be carried out under static (DC) and nonlinear dynamic (RF) operations to give the maximum flexibility in gathering useful information on the device degradation in different operating regimes. In particular, the device under test (DUT) is fed in rf-stressing conditions by applying a large-amplitude excitation signal at moderately high frequency at either the input (forward mode) or output (reverse mode) port of the device. The system features, in fact, a symmetrical dual-channel architecture. The DUT can be either a bipolar- or a field-effect transistor, and the walkout of its characteristics can be observed both at the end of the stress test and in real-time during the test execution. A special-purpose control software automates the measured data acquisition. Several experiments that were performed using the proposed setup are discussed in the paper.

Equivalent-voltage approach for modeling low-frequency dispersive effects in microwave FETs

In this paper, a simple and efficient approach for the modeling of low-frequency dispersive phenomena in FETs is proposed. The method is based on the definition of a virtual, nondispersive associated device controlled by equivalent port voltages and it is justified on the basis of a physically-consistent, charge-controlled description of the device. Dispersive effects in FETs are accounted for by means of an intuitive circuit solution in the framework of any existing nonlinear dynamic model. The new equivalent-voltage model is identified on the basis of conventional measurements carried out under static and small signal dynamic operating conditions. Nonlinear experimental tests confirm the validity of the proposed approach.