Tunnel Junction-Enabled AlGaN/GaN Heterojunction Bipolar Transistors With All n-Type Contacts

The performance of contemporary GaN heterojunction bipolar transistors (HBTs) is currently limited by challenges associated with the formation of p-type contacts for the base terminal. In this article, we report on the design and demonstration of GaN HBTs with all n-type contacts for the emitter, base, and collector terminals. We show that degenerately doped GaN tunnel junctions (TJs) can be utilized to function as a base contact at reverse bias and as an emitter terminal at forward bias. We demonstrate device operation using a p<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup>/n<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> GaN TJ sandwiched between a graded AlGaN emitter and a p-GaN base layer with an interdigitated emitter/base stripe design. The device displayed a high collector current density (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I} _{C}$ </tex-math></inline-formula>) of 28 kA/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> extracted from the common emitter output (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I} _{\text {C}}$ </tex-math></inline-formula>–<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V} _{\text {CE}}$ </tex-math></inline-formula>) characteristics. This novel approach to HBT design significantly advances the current field of III-nitride HBTs for the next-generation radio frequency (RF) and mm-Wave applications by circumventing the need to fabricate p-type contacts on the base as well as eliminating regrowth of the emitter/base epilayers.