Monolithic Integration of GaN Transistors for Higher Efficiency and Power Density in DC-DC Converters

Power converters are constantly trending towards higher output power, higher efficiency, and higher power density. To provide improved performance better power devices are required. For silicon (Si) power devices, the gains in performance have slowed as the technology has matured and approaches its theoretical limits [1]. Gallium nitride (GaN) devices have emerged as a possible replacement for silicon devices in various power conversion applications and as an enabler of new applications not previously possible [1]-[4]. In this paper we will discuss the latest eGaN FETs developments, including a major improvement with the latest generation of discrete devices and introduce a new family of monolithic half bridge ICs offering unmatched high frequency performance. These new families of eGaN FETs are widening the performance gap with the aging power MOSFET in high frequency power conversion by providing significant gains in key switching figures of merit, continued reductions of performance limiting in-circuit parasitics, and improved thermal performance. 1. Rapid Improvements in GaN Performance The GaN technology journey is just beginning, and we are still far from theoretical performance limits [1]. It is quite reasonable to expect a rapid rate of improvement reminiscent of Moore’s Law, which predicted the growth of microprocessor technology – doubling of product performance every two to four years for at least the next decade. The latest generation (circa 2014) of discrete high current eGaN FET power transistors demonstrates over a two-fold reduction in hard switching figure of merit (FOMHS) [5] over the previous generation (circa 2011), as shown on the left in figure 1. The latest generation of eGaN FETs also have over a two-fold reduction in on-resistance compared to their predecessor, increasing the current capability of GaN transistors [5]. The latest generation family of eGaN FETs, when compared to the state-of-art Si power MOSFETs, reduces FOMHS by 4.8 times, 8 times, and 5 times respectively for 40 V, 100V, and 200 V devices as plotted on a log scale shown on the right in figure 1. Fig.1. Hard switching FOMHS comparison of generation 2 (circa 2011) and generation 4 (circa 2014) high current eGaN FETs (left) and eGaN FETs and state-of-the-art Si MOSFETs (right) for drain-tosource voltages at half of the rated voltage, and a drain-to-source current of 20 A 0 5 10 15 20 25 30 35 40 45 50 0 50 100 150 200 250 F O M H S = (Q G D + Q G S 2 )· R D S (o n ) (p C ·Ω ) Drain-to-Source Voltage (V) 1.4x 2.4x 2.4x eGaN FETs circa 2011 eGaN FETs circa 2014 1 10 100 0 50 100 150 200 250 F O M H S = (Q G D + Q G S 2 )· R D S (o n ) (p C ·Ω ) Drain-to-Source Voltage (V) EPC Gen 4