On the reduction of base resistance in GaN-based heterojunction bipolar transistors

The high resistivity of p-type GaN presents a significant barrier to the development of bipolar electronic and optoelectronic devices. Specifically with regard to N-p-n heterojunction bipolar transistors (HBTs), the resistance of the p-type base manifests itself in three distinct ways: the intrinsic base resistance under the active region of the device, as well as the base access resistance, consisting of contact resistance and extrinsic base resistance. Methods of combating each of these parasitics are presented. A new type of low resistance metal semiconductor contact is proposed and demonstrated, based on strong internal electric fields induced by polarization effects. Results on photoelectrochemical etching as a low damage, dopant selective technique of etching GaN-based HBT structures are presented. Very smooth interfaces and low contact resistances are obtained in GaN/SiC HBT structures. Another etch technique involves crystallographic etching of GaN, producing undercut cross-sections. This enables the use of self-aligned base contacts, further reducing base access resistance. Finally, increasing the conductivity of the p-type material itself is realized by the use of AlGaN/GaN superlattices. Lateral hole conductivity is improved in these structures, but effective electron transport perpendicular to the layers remains to be proven.