Heterojunction bipolar transistors (HBTs) are widely used in high speed and high frequency applications. The heterojunction bipolar transistor (HBT) offers much higher speeds of operation than the more prevalent metal-oxide-semiconductor field-effect transistors (MOSFETS) or even conventional homojunction bipolar transistors, such as npn or pnp silicon transistors. The HBT offers an alternative technology to metal semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs) when a high degree of linearity is desirable. The use of different materials of differing bandgaps for the collector, base and emitter provides for * additional design flexibility. The HBT is a layered structure that includes a semiconductor substrate, a subcollector, a collector, a base and an emitter stacked one on top the other in an integral assembly. Metal contacts are formed to connect power and other circuitry to the emitter, the base and the subcollector. The largest limitation to the operational frequency and speed of the HBT is the base-collector capacitance. The base-collector capacitance is largely due to the collector-base interface area. Reduction of the base area can introduce higher base resistance due to the reduction of the area of the base contact.
A common HBT technology is based on Gallium Arsenide (GaAs)/Gallium Aluminum Arsenide (GaAlAs) based family. The GaAs based HBT suffers from base-collector capacitance due to the area of the interface between the base and collector layers. One method of reducing the base collector capacitance of a GaAs based HBT is to implant protons into an area of the collector surrounding the emitter so as to electrically insulate the implanted area. Another HBT technology is based on the Indium Phosphide (InP)/Indium Gallium Arsenide (InGaAs) material family. The InP/InGaAs HBT also suffers from base-collector capacitance. However, proton implantation has proved ineffective in rendering InP to be sufficiently insulating or semi-insulating.
Miyamoto et al. addresses this problem by performing a selective etchant to substantially etch the collector layer under the base layer so as to undercut the edges of the base layer of InP-based HBTs in “Reduction of Base-Collector Capacitance by Undercutting the Collector and Subcollector in GaInAsInP DHBT's,” IEEE Electron Device Letters, vol. 17, March 1996, pp. 97-99. The undercuts are then backfilled with polyimide to provide mechanical integrity. The reduced size of the collector, together with the lower dielectric constant of the polyimide, reduces the base-collector capacitance. However, this approach does not reduce the base-collector capacitance component from under the base contact. Other methods to reduce base-collector capacitance include using an electrically insulating region of iron (Fe) doped InP replacing part of the collector to reduce the base-collector area.
Another method of reducing the base-collector capacitance has been to reduce the base area and form a base metal micro-bridge to a base contact disposed away from the active portion of the HBT. The base metal micro-bridge is typically formed by depositing the base metal and then etching away the semiconductor from underneath a portion of the base metal. Song et al. demonstrate this technique for an HBT with an InGaAs collector layer in “Reduction of Extrinsic Base-Collector Capacitance in InP/InGaAs SHBTs Using a New Base Pad Design, INP and Related Materials Conference, 2002, pp. 165-168.” However, it is difficult to fabricate the base metal micro-bridge for HBTs that have phosphorus based collector layers and arsenic based base layers due to the unusual etching characteristics of phosphorous based materials. A similar technique is demonstrated for removing InP subcollector layers (but not the collector layer) using a cumbersome double base contact pad by Ida et al. in “InP/InGaAs DHBTs with 341-GHz for a big current density over 800 kA/cm2, IEEE Electron Device Meeting, 2001, pp. 35.4.1-35.4.4.”