Microwave performance improvement of AlGaAs/GaAs heterojunction bipolar transistor using the selective lateral etching technique

Abstract

The new method for improving microwave performance of $Al_{0.3}Ga_{0.7}As/GaAs$ heterojunction bipolar transistor (HBT) was investigated. In this method, extrinsic base-collector capacitance was decreased using selectively lateral etching technique with redox solution ($I_2$/KI). The redox solution used to etch selectively the $Al_{0.3}Ga_{0.7}As$ relative to GaAs has highly preferential etching property and a high etching rate. With optimum etching condition, the undercut width of $Al_{0.3}Ga_{0.7}As$ relative to GaAs was approximately measured up to 3㎛ at the condition of [$I_2$]/[KI]=0.67 and pH=3.0 for [100] directional stripe. The undercut width was found to be remarkably increased with increasing the stirring speed near the chemically neutral etching solution. To verify selectively lateral etching effect, the conventional and lateral-etched HBTs were fabricated upon the HBT epitaxial layer structure with a composite subcollector layer, composing of $n^+-GaAs$ and $N^+-Al_{0.3}Ga_{0.7}As$. As the result, dc characteristics of the lateral etched HBT were very similar to those of the conventional HBT except the collector current density for maximum dc current gain. For 3㎛×20㎛ emitter size, although the maximum current gain is nearly the same for both devices, the dc current gain of lateral etched HBT was reduced a little than that of conventional HBT which has the gain of 25 at the collector current density (Jc) of 9kA/㎠, resulting in 21 at the same collector current density. From Gummel plots, ideality factors of base and collector current were estimated as 1.5 and 1.01 for both samples, respectively. Although both samples showed the similar dc characteristics, microwave characteristics such as maximum oscillation frequency($f_max$) and unity current gain cutoff frequency($f_T$) revealed the large difference. The microwave properties of HBTs were examined by cutoff mode S-parameters measurement. From the simulation for both device structures, about 60% red...