Raf Vandersmissen

Improvement of AlGaN∕GaN high electron mobility transistor structures by in situ deposition of a Si3N4 surface layer

We have made AlGaN∕GaN high electron mobility transistors with a Si3N4 passivation layer that was deposited in situ in our metal-organic chemical-vapor deposition reactor in the same growth sequence as the rest of the layer stack. The Si3N4 is shown to be of high quality and stoichiometric in composition. It reduces the relaxation, cracking, and surface roughness of the AlGaN layer. It also neutralizes the charges at the top AlGaN interface, which leads to a higher two-dimensional electron-gas density. Moreover, it protects the surface during processing and improves the Ohmic source and drain contacts. This leads to devices with greatly improved characteristics.

State-space modelling of slow-memory effects based on multisine vector measurements

Non-linear microwave devices and circuits often exhibit slow-memory effects. When subjected to two-tone, or more general multisine excitations, the characteristics of these devices and circuits depend on the offset frequency between the tones. Since modulated excitations are an integral part of telecommunication systems, models aimed for circuit and system design should be able to accurately represent slow-memory behaviour. In this work, we develop a modelling procedure based on the state-space modelling approach to accurately incorporate these slow-memory effects. The technique is experimentally demonstrated on a high electron mobility transistor (HEMT).

Efficient construction of a large-signal behavioural HEMT model from automated vectorial large-signal measurements

Large-signal models for microwave devices are classically derived via a small-signal detour using S-parameter measurements. Due to the recent advances in the metrology area of vectorial large-signal measurements, several novel modelling methodologies circumventing this small-signal detour are being developed. An important aspect that these methods have in common is the minimisation of the number of required measurements. As in the case of S-parameter measurements, the goal of the large-signal measurements is to adequately cover the predefined operating region of the device-under-test. In this work, we present a method that enables us to automatically calculate the excitation signals to be applied in order to efficiently cover the (V/sub 1/, V/sub 2/) voltage plane of two-port microwave devices. We illustrate this method on a HEMT, where we obtained a significant (>90%) reduction in measurements. Finally, we show that this limited set of vectorial large-signal measurements is sufficient to construct an accurate time-domain behavioural model.

Feedback amplifier based on an embedded HEMT in thin-film multilayer MCM-D technology

In this paper a feedback amplifier circuit integrated in MCM-D (MultiChip Module with Deposited thin films) on glass technology is presented. The active device of the amplifier is a thin-film Ge (germanium) -based HEMT. The HEMT is embedded in the bottom dielectric layer of the MCM-D. The combination of passive MCM-D technology and HEMTs on Ge allows for efficient semimonolithic integration of active devices and realization of MCMs with embedded passive and active components for amplifier circuits.

Large-signal state-space model for Ge-based MHEMTs: construction and validation by an amplifier in multilayer thin-film technology

We construct a large-signal state-space model for thin-film metamorphic HEMTs based on germanium, directly from time-domain large-signal measurements. These thin-film HEMTs are used in a feedback amplifier circuit, designed as a multichip module with deposited thin layers (MCM-D) on glass. For the first time, we show that this type of state-space model can accurately predict the large-signal behaviour of a feedback amplifier.