β-Ga₂O₃ RF power FETs

A promising candidate for tomorrow’s high-voltage, high-power RF applications, the β-Ga₂O₃-on-SiC RF power FET is already amplifying signals up to 8 GHz with output power density approaching 1 W mm.^-1

BY MIN ZHOU, HONG ZHOU, JINCHENG ZHANG AND YUE HAO FROM XIDIAN UNIVERSITY

There are a number of applications that require amplifiers with a high power and a high efficiency producing mega-watt powers within the UHF to X-band. Those applications where these amplifiers can serve include: airport surveillance; particle accelerators for scientific or industrial systems; and weather, marine and military radar systems.

Since power is the product of current and voltage, the options for realising a high-power, solid-state RF transistor are to use either a very high current or a very high voltage. Of these two, there are many downsides associated with a large current. This approach involves a large gate periphery and hence a substantial chip size – and that brings the downsides of high cost, low load-impedance and increased network loss. Using a high voltage is favoured for several reasons, now discussed.

The benefits of a higher voltage include realising higher powers and minimising network loss. A higher supply voltage leads to a proportionally higher output power, enabling a smaller chip size for the same power output, a major attribute for multiple-input, multiple-output (MIMO) systems.

Figure 1. (a) β-Ga₂O₃ RF power FETs on SiC substrates realise efficient heat extraction, a low on-resistance, a high maximum drain current, a high blocking voltage, and high values for f_T and f_max. (b) Atomic force microscopy image of the piranha-solution-treated β-Ga₂O₃ thin film.

What is more, in accordance with Ohm’s law, operating at a higher voltage results in an equal increase in load-line resistance. To illustrate this benefit, let’s consider a given output power of 1 kW, while, for simplicity, neglecting the knee voltage of the transistor. When the operating voltage increases from 50 V to 200 V, the load impedance increases from 1.25 Ω to 20 Ω. Due to this, the corresponding transformation ratio for matching to 50 Ω is far more favourable, improving from a factor of 40 to just 2.5. In conjunction, there is a decrease in the total gate periphery of the transistor. In turn, this avoids more complicate matching networks for very-high-power devices, and minimises network loss.

The use of high voltages also enables a high efficiency at high frequency. This is most valued, because the delivery of sufficient output power requires a system with thousands of modules, a condition leading to a degradation in efficiency. Another concern is the shunt RC-circuit, created by the combination of load-line resistance and output capacitance – the latter dominates the output impedance at high frequency and results in a quasi-short-circuit.

To design a high-efficiency amplifier, engineers tend to exploit harmonic termination technology. Consider, for example, the class B amplifier, which has a theoretical maximum efficiency of 78.5 percent, realised by shorting all harmonics. When engineers adopt harmonic termination technology they match the network with the second and third harmonic frequency, and possibly higher ones, rather than simply matching the fundamental frequency. Note that it is possible to reach a theoretical maximum efficiency of 100 percent with the likes of a Class E or Class F amplifier.