Interplay of Various Charge Sources in AlGaN/GaN Epi-Stack Governing HEMT Breakdown

In this work, we have revealed the interplay of various charge sources (surface, polarization, and buffer) and their relative concentrations across the AlGaN/GaN epi-stack governing the electric field distribution and the breakdown mechanism in high electron mobility transistors (HEMTs). The investigations are carried out for Schottky, metal-insulator-semiconductor (MIS), and p-GaN gate stacks while accounting for possible GaN buffer types (Fe-doped and C-doped). A strong correlation between 3-terminal (3T) breakdown voltage, gate design, and relative concentration of various charges was found. % increase in 3T breakdown voltage when physical dimensions were doubled was also found to be strongly correlated with the relative concentration of various charges for both Fe- and C-doped buffer HEMTs. On the other hand, the 2T (source-drain, without gate, and GaN channel below gate) breakdown voltage was independent of all other parameters except buffer properties and physical dimensions. Physical insights are developed to explain the dependence of electric field distribution, carrier injection, and HEMT breakdown on the surface states, polarization charge, and buffer traps, as well as their relative concentrations for both the buffer types and all three gate types. These insights will help to design efficient surface passivation schemes and resolve ambiguities, often observed in experiments, in terms of location of peak electric field (drain side, or gate side or both) as well as OFF-state conduction and breakdown mechanism (gate injection, or punchthrough, or parasitic conduction through buffer or avalanche generation).