2017.05.25 Si Power Device
From this point, we discuss super-junction MOSFETs, which in recent years have become representative of MOSFETs with high voltages.
Features and Positioning of Power Transistors
Let us begin by reviewing the power and frequency ranges covered by the principal power transistors of recent years, which are Si-MOSFETs, IGBTs, and SiC-MOSFETs. Hereafter we will be focusing on super-junction MOSFETs, but it will be useful to understand the position of Si-MOSFETs in the market in order to be able to understand how they are used selectively according to their features and characteristics.
The following graphic illustrates the power and frequency regions that can be handled by the different types of power transistors. We see that Si-MOSFETs lag behind IGBTs and SiC-MOSFETs somewhat with respect to ON-resistance and rated voltage, but are well-suited to high-speed operation at lower to intermediate power levels.
Planar MOSFET and Super-junction MOSFET
Si-MOSFETs can be classified as planar MOSFETs and super-junction MOSFETs according to the manufacturing processes used. Put simply, in the field of power transistors, the super-junction structure was developed in order to transcend the limits of planar structures.
As indicated in the graphic below, a planar structure constitutes a flat or planar transistor. This structure has had the drawback that if the rated voltage is raised, the drift layer becomes thicker, and so the ON-resistance is increased. In contrast, a super-junction structure is a structure in which multiple vertical pn junctions are arranged, as a result of which a low ON-resistance RDS(ON) and reduced gate charge Qg are realized while maintaining a high voltage.
In addition, the reverse current irr and the reverse recovery time trr of the internal diode are parameters that need to be studied for the turn-off switching characteristics of a transistor. As indicated in the waveform diagram below, in essence a super-junction MOSFET has a larger pn junction area than a planar MOSFET, and so trr is faster than for a planar MOSFET, but a larger irr flows.
This characteristic, one issue with super-junction MOSFETs, is steadily undergoing improvement, and such features as fast operation and low noise endow super-junction MOSFETs with considerable variety. In the following sections, we will review features for a variety of MOSFETs.
・Si-MOSFETs are positioned as elements capable of fast operation at low to intermediate power levels.
・The super-junction structure achieves a lower ON-resistance RDS(ON) and reduced gate charge Qg, while maintaining a high voltage.
・A super-junction MOSFET has a characteristic that irr is larger and trr is faster than for a planar MOSFET.