Breakdown Mechanism of AlGaN/GaN HEMT on 200-mm Silicon Substrate With Silicon Implant-Assisted Contacts

We present an access technology suitable for scaled gallium nitride (GaN) high electron mobility transistor (HEMT) in Ka-band. The comparison between OFF-state characteristics of a silicon implant-assisted contact and a conventional recessed Ti/Al-based Ohmic contact is presented. The transistor with source/drain extension by Si implantation has a low contact resistance with ${R}_{C}$ down to $0.4 ~\Omega \cdot {\mathrm {mm}}$ and a sheet resistance of the implanted layer of $67~ \Omega $ /sq. In addition to promising contact performance, transistors with source and drain extension sustain high breakdown voltage (BV) with short dimensions for high-frequency applications. The systematic study of gate–source, gate–drain, and gate length variations shows a new breakdown mechanism for implanted access technology with current flowing beneath the channel leading to an unusual correlation between source–drain spacing and BV. With a conventional titanium-alloyed contact, a punchthrough effect is responsible for the BV. Cross-sectional transmission electron microscopy and secondary ion mass spectroscopy (SIMS) characterizations on both wafers highlight a degradation of the AlGaN-based back-barrier and a high silicon concentration deep into the epitaxial stack on the implanted wafers indicating a way to improve BV with an adapted process flow.