Contactless, high resolution characterization of current and voltage waveforms within high power communication amplifiers

Characterisation of high-power communications-based amplifiers (PAs) has generated many thousands of research papers and much of this work assumes the transistors at the heart of these (PAs) to be a ‘large’ holistic entity. Given that high-power communications-based transistors are made up of multiple, parallel transistors on a single substrate, it is this intermediate scale range, within the periphery of the device, but much larger than the geometrical scale of the epitaxy and the lithography, that requires deeper investigation. Raman-based thermography may add a dimension of spatially varying heat dissipation but ‘lifting the bonnet’ of the transistor and making internal contactless measurements of current and voltage is the only way to fully account for the myriads of parasitic effects that have been observed by countless researchers. To date, however, very little research has been conducted on quantifying the individual spatial voltages within the transistor in order to fully characterise it. Miniaturised contactless current and voltage probes are theorised, designed, characterised and optimised in this thesis to deliver a robust and reliable means of transistor characterisation at these internal spatial dimensions. The contactless voltage probe presented in this work has a spatial resolution four times finer than the previously reported voltage probe, with a useful bandwidth up to 7 GHz and a controllable passive gain up to 20 dB at the desired operating frequency. The pinnacle of this thesis delivers a novel shielded contactless current probe, capable of high-resolution scanning, culminating in a ‘quasi-calibrated’ measurement of the distributed currents within a multi-finger LDMOS transistor operating at high power and high frequency. The spatial resolution of this shielded contactless current probe is 62.5 μm with 22.7 dB average rejection ratio to the electric field, and it has a broad bandwidth up to 9 GHz. To date, this type of contactless current measurement has not been reported elsewhere

A novel microwave non contact current probe with high spatial resolution

A high-resolution microwave non-contact current probe is described. Novel techniques are used in order to reduce the intrusive E-field pickup, which has restricted the performance and usefulness of such probes in the past. These include a probe structure that has built-in E-field screening and a high performance broadband differential amplifier. A rigorous testing procedure is described which asserts that the probe is truly responding almost entirely to the H-field. The probe operates up to 9GHz and has been used to profile the current distribution between the multiple cells of a power transistor at 3GHz, the first time such a direct measurement of this kind has been reported

A 25 µm spatial resolution broadband microwave voltage probe

The present generation of voltage probes have demonstrated a spatial resolution of 100 μm at 6 GHz bandwidth. This paper presents results of a contactless voltage probe having 25 μm spatial resolution and instantaneous bandwidth up to 7 GHz. The probe is constructed from a miniaturized open-ended coaxial cable of 20 μm inner conductor diameter and 300 μm outer diameter. Capacitive coupling between the open-end coaxial cable and the device under test (DUT) generates a signal which feeds a miniaturized microwave amplifier. This concept introduces a new dimension into the process of designing microwave circuits which allows high accuracy, spatially resolved, voltage measurements. The improved probe design offers more flexibility in terms of the range of devices that can be measured, extending beyond large power microwave transistors to medium and small scale microwave active devices