Vertical GaN Power Electronics - Opportunities and Challenges

Tremendous progress has been made in wide-bandgap (WBG) power electronic devices. Most WBG devices today are based on silicon carbide (SiC) or gallium nitride (GaN). However, the most mature GaN-based power device, the high electron mobility transistor (HEMT), falls short of the full potential of GaN for several reasons: (1) internal stress due to the growth of thick buffer layers on non-native substrates; (2) the lateral architecture of the HEMT produces a non-ideal internal electric field distribution; and (3) avalanche ruggedness is lacking. GaN-based HEMTs are thus unacceptable for higher-voltage applications such as the electric grid and vehicle drivetrain electrification. In contrast to GaN HEMTs, vertical GaN power devices grown on native GaN substrates do not suffer from these shortcomings. However, materials challenges exist for vertical GaN, such as the quality of native substrates and the epitaxial growth of thick (tens of microns or more), low-doped (< 10¹⁶ cm^-3 n-type) drift layers required for high breakdown voltage. Processing challenges also exist, such as the ability to selectively dope GaN and to activate buried p-layers. This talk will present progress in the design, fabrication, and evaluation of vertical GaN power devices. The fundamental material properties relevant for such devices will be discussed, including how they enable devices superior to those fabricated from other semiconductors. Challenges and progress associated with substrates and epitaxial growth will be covered, as will design trade-offs and processing challenges (including yield and reliability) for various types of power devices.