Low collector contact resistance heterojunction bipolar transistors

Abstract

In certain aspects, a heterojunction bipolar transistor (HBT) comprises a collector mesa, a base mesa on the collector mesa, and an emitter mesa on the base mesa. The base mesa has a tapered sidewall tapering from a wide bottom to a narrow top. The HBT further comprises a collector contact on a portion of the collector mesa and extending to a portion of the tapered sidewall of the base mesa.

What is claimed is: 
     
         1 . A method, comprising:
 providing an HBT comprising a collector mesa, a base mesa on the collector mesa, and an emitter mesa on the base mesa, wherein the base mesa has a tapered sidewall tapering from a wide bottom to a narrow top;   forming a photoresist layer on the HBT;   patterning the photoresist layer with one or more photoresist openings, wherein the one or more photoresist openings expose a first region of the collector mesa and a first region of the tapered sidewall of the base mesa; and   depositing a collector contact metal over the first region of the collector mesa and the first region of the tapered sidewall of the base mesa.   
     
     
         2 . The method of  claim 1 , wherein the collector mesa comprises GaAs. 
     
     
         3 . The method of  claim 1 , wherein the collector mesa comprises a substrate and a first sub-collector on the substrate and wherein the base mesa comprises a second sub-collector on the collector mesa. 
     
     
         4 . The method of  claim 3  further comprising an etch stop layer between the first sub-collector and the second sub-collector. 
     
     
         5 . The method of  claim 3 , wherein the collector contact metal is on a sidewall of the second sub-collector. 
     
     
         6 . The method of  claim 5 , wherein the collector contact metal is on lower portion of the tapered sidewall of the base mesa. 
     
     
         7 . The method of  claim 1  further comprising
 forming a mask layer over the HBT; 
 patterning the mask layer with one or more mask openings exposing a second region of the collector mesa; 
 etching a layer of the collector mesa in the second region of the collector mesa; and 
 growing an metal alloy in the second region of the collector mesa. 
 
     
     
         8 . The method of  claim 7 , wherein the second region of the collector mesa is the same as the first region of the collector mesa. 
     
     
         9 . The method of  claim 7 , wherein the metal alloy has a lower bandgap than that of the collector mesa. 
     
     
         10 . The method of  claim 9 , wherein the metal alloy comprises InAs or InGaAs. 
     
     
         11 . The method of  claim 7 , wherein the one or more mask openings expose a second region of the tapered sidewall of the base mesa. 
     
     
         12 . the method of  claim 11 , wherein the second region of the tapered sidewall is the same as the first region of the tapered sidewall.