Matthias Ohlrogge

Stability Investigation of Large Gate-Width Metamorphic High Electron-Mobility Transistors at Cryogenic Temperature

An investigation of metamorphic high electron mobility transistor stability at cryogenic temperature is presented in this paper. Unlike in the case of two-finger transistors, the measurements of cooled four-finger devices with large gate widths exhibit unstable behavior in the form of steps in the current-voltage characteristics, discontinuities in the transconductance, and reduced gain. This unstable behavior has hampered the reliable realization of low-noise amplifiers for cryogenic applications. We study different gate-width devices with a multiport transistor model, allowing the separation of gate and drain feeder structures from the active part of the transistor. The simulation reveals the presence of resonances in the frequency region of several hundreds of gigahertz. We demonstrate that the resonances disappear when an air bridge is placed across the fingers of the drain feeder structure, and confirm the stabilizing effect of the air bridge both on device and circuit level by cryogenic measurements.

On the Accurate Measurement and Calibration of S-Parameters for Millimeter Wavelengths and Beyond

It is well known that, in the millimeter (mm-wave) and sub-mm-wave range, on-wafer S-parameter measurements are often inaccurate and suffer from serious systematic artifacts. In this paper, we confirm that these artifacts are related to spurious wave modes that are excited and propagate in the substrate. These parasitic wave components may be scattered at neighboring structures on the wafer and cause detrimental crosstalk. While these parasitic components deteriorate the measurement itself, an even more serious complication arises from the fact that these modes are already present in the calibration measurement and are unintentionally imported and superposed to the measurement data. In this paper, we present a new type of RF pad with novel screening features and show that these parasitic modes can be efficiently suppressed by the use of proper on-wafer couple structures. Moreover, a novel calibration substrate and method is presented and demonstrated to be capable to remove spurious artifacts from S-parameter measurements up to 450 GHz.

A scalable compact small-signal mHEMT model accounting for distributed effects in sub-millimeter wave and terahertz applications

In this paper we utilize a new approach for a small signal model which is scalable from very small to rather large transistors in a wide frequency range from 50 MHz up to 500 GHz. We show that with increasing frequency and decreasing transistor size we need to take into account termination effects at the open ends of the transistor electrodes. This new approach is based on a decomposition of the transistor into multiport sections. These sections are simulated individually by an electromagnetic field solver and then parameterized by compact networks. The model is verified by S-parameter measurements up to 450 GHz.

An investigation of millimeter wave switches based on shunt transistors including SPDT SWITCH MMICs up to 300 GHz

This paper reports on a design study for millimeter wave single-pole multiple-throw switches, which are based on shunt transistors. For the investigation of an optimized design flow this study focuses on the three main steps of a millimeter wave switch design: identifying the optimum transistor gate width, large signal modeling of shunt transistors and the MMIC design. Based on the investigations of the optimum transistor gate width, it will be shown that insertion loss and output signal dynamic of a switch are directly correlated to the on-resistance of the utilized semiconductor technology. To prove the feasibility of this study, two RF SPDT switch MMICs, operating in the frequency range from 53 to 150 GHz and from 200 to 330 GHz, respectively, were designed and fabricated. Both switches show low insertion loss, high output signal dynamic, high yield and good agreement to the S-parameter simulations, based on the proposed shunt FET model. The proposed W-band and H-band SPDT switch MMICs achieve an insertion loss of 2 dB and 1.7 dB, respectively, and an output signal dynamic of up to 47 and 20 dB, respectively.

First InGaAs lateral nanowire MOSFET RF noise measurements and model

The first radio frequency (RF) noise measurements on lateral nanowire metal-oxide-semiconductor field-effect transistors (MOSFETs) and a noise model are presented. We have characterized the RF noise and scattering parameters of an indium gallium arsenide (InGaAs) device. A fitted model yields extrapolated ft = 316 GHz current gain cutoff and fmax = 166 GHz maximum oscillation frequency. This device technology is being developed for millimeter wave circuit implementations, targeting a 94 GHz carrier frequency. The modeled intrinsic Fmin < 1dB minimum noise figure obtained promises performance at the target band, given reduction of gate parasitics. In any wireless system, noise and bandwidth limits the performance. Understanding of RF noise in nanowire MOSFET devices is thereby key for realization of future radar and communications systems.

Small signal modelling approach for submillimeter wave III–V HEMTs with analysation and optimization possibilities

In this paper we present a new small signal multiport modelling approach for III-V High Electron Mobility Transistors (HEMT) that is capable for internal transistor analysation and optimization as well as scaleable in gate width and finger-number. The new model decomposes the planar transistor structure into single multiport elements that are separately described by electrical equivalent circuits and connected to each other over discrete ports. With this new modelling topology we only need to extract a couple of multiport elements to predict the correct behavior for a high amount of different planar transistor structures. This point gives the circuit designer a wide range of possibilities to analyze and optimize a given transistor structure according to special needs, like low-noise, input-output matching or cryogenic behavior on a computer based level.