N-Polar Deep Recess MISHEMTs With Record 2.9 W/mm at 94 GHz

Abstract

W-band power performance is reported on an N-polar GaN HEMT for the first time, resulting in a record output power density for any GaN device on a sapphire substrate. This result is achieved using an N-polar GaN deep recess MISHEMT structure grown by metal-organic chemical vapor deposition on the sapphire substrates. The key component in this device design is the addition of an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> unintentionally doped GaN epitaxial passivation layer in the access regions of the transistor. This GaN layer functions both to control DC-to-RF dispersion as well as to increase the conductivity in the access regions of the HEMT. Devices with very low dispersion and a simultaneous <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{\text {max}}/f_{t}$ </tex-math></inline-formula> combination of 276/149 GHz are demonstrated. Load pull measurements at 94 GHz give a peak power added efficiency (PAE) of 20% with an associated output power density of 1.73 W/mm at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\text {DS}} = 8$ </tex-math></inline-formula> V. A record 2.9-W/mm maximum output power density with an associated 15.5% PAE at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\text {DS}} = 10$ </tex-math></inline-formula> V is achieved despite the low thermal conductivity of the sample’s sapphire substrate.