Three terminal edge illuminated epilayer waveguide phototransistor

Abstract

A three terminal edge illuminated epilayer waveguide phototransistor including a subcollector layer formed of an epitaxially grown quaternary semiconductor material, such as heavily doped InGaAsP. A collector region of undoped InGaAs is epitaxially grown on the subcollector layer. A base region, including a heavily doped InGaAs base layer and a very thin undoped InGaAs spacer layer, is epitaxially grown on the collector layer. An emitter region, including a doped InGaAsP layer, a doped InP layer, and a heavily doped InGaAs emitter contact layer, is epitaxially grown on the base layer. The various layers and regions are formed so as to define an edge-illuminated facet for receiving incident light.

1 . Edge illuminated epilayer waveguide phototransistor comprising: 
 a subcollector layer formed of an epitaxially grown quaternary semiconductor;    a collector region epitaxially grown on the subcollector layer;    a base region epitaxially grown on the collector layer;    an emitter region epitaxially grown on the base layer; and    the subcollector layer, the collector region, the base region, and the emitter region being formed so as to define an edge illuminated facet for receiving incident light.    
     
     
         2 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 1  wherein the subcollector layer is epitaxially grown on an InP substrate.  
     
     
         3 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 2  wherein the subcollector layer is composed of InGaAsP.  
     
     
         4 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 3  wherein the InGaAsP subcollector layer includes a composition that is transparent at the optical wavelengths of interest.  
     
     
         5 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 4  wherein the InGaAsP subcollector layer includes a InGaAsP composition that corresponds to a band gap wavelength of 1.15 μm.  
     
     
         6 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 1  wherein the subcollector layer has a thickness in a range of approximately 0.80 μm to 0.90 μm.  
     
     
         7 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 6  wherein the subcollector layer is doped to provide a sheet resistance value in a range of 20 Ω/square to 30 Ω/square.  
     
     
         8 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 1  wherein the collector region includes an undoped InGaAs layer with a thickness chosen to optimize the transit frequency, breakdown voltage, base-collector capacitance, and rate of optical absorption.  
     
     
         9 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 8  wherein the collector region thickness is in a range of 0.3 μm to 0.5 μm.  
     
     
         10 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 8  wherein the collector region thickness is approximately 0.4 μm.  
     
     
         11 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 8  wherein the collector region thickness is chosen to provide a transit frequency of approximately 130 GHz.  
     
     
         12 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 8  wherein the collector region has a length selected to provide an internal quantum efficiency greater than 90%.  
     
     
         13 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 1  wherein the base region includes a doped base layer and an undoped spacer layer.  
     
     
         14 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 13  wherein the collector region is undercut below the base region, reducing the width of the collector region to minimize base-collector capacitance.  
     
     
         15 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 3  wherein the emitter region includes a layer of InGaAsP and a layer of InP.  
     
     
         16 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 15  wherein the layer of InGaAsP has a thickness in a range of approximately 0.05 μm to 0.15 μm.  
     
     
         17 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 16  wherein the InP emitter layer has a thickness large enough to prevent optical absorption loss in the top InGaAs emitter contact layer.  
     
     
         18 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 17  wherein the InP emitter layer has a thickness of approximately 0.5 μm.  
     
     
         19 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 15  wherein the emitter region further includes a contact layer of InGaAs.  
     
     
         20 . Edge illuminated epilayer waveguide phototransistor comprising: 
 a subcollector layer formed of doped InGaAsP with a thickness in a range of 0.80 μm to 0.90 μm;    a collector layer of undoped InGaAs with a thickness in a range of 0.3 μm to 0.5 μm epitaxially grown on the subcollector layer;    a base region including a doped InGaAs layer epitaxially grown on the collector layer having a thickness of approximately 0.05 μm and an undoped InGaAs layer having a thickness of approximately 50 Å, epitaxially grown on the doped InGaAs layer;    an emitter region including a doped InGaAsP layer having a thickness in a range of 0.05 μm to 0.15 μm and epitaxially grown on the undoped InGaAs layer of the base region, a doped InP layer having a thickness in a range of 0.3 μm to 0.7 μm epitaxially grown on the doped InGaAsP layer, and a doped InGaAs emitter contact layer epitaxially grown on the doped InP layer; and    the subcollector layer, the collector layer, the base region, and the emitter region being formed so as to define an edge illuminated facet for receiving incident light.    
     
     
         21 . Edge illuminated epilayer waveguide phototransistor as claimed in  claim 20  wherein the subcollector layer, the collector layer, the base region, and the emitter region define a mesa having a width in a range of 1.0 μm to 5.0 μm and a length long enough to achieve a greater than 90% internal optical absorption efficiency.  
     
     
         22 . A method of fabricating an edge illuminated epilayer waveguide phototransistor comprising the steps of: 
 providing a semiconductor substrate defining a surface;    epitaxially growing a subcollector layer formed of a quaternary semiconductor material on the semiconductor substrate;    epitaxially growing a collector region on the subcollector layer;    epitaxially growing a base region on the collector layer;    epitaxially growing an emitter region on the base layer; and    forming the subcollector layer, the collector region, the base region, and the emitter region to define an edge illuminated facet for receiving incident light.    
     
     
         23 . A method as claimed in  claim 22  wherein the step of epitaxially growing the subcollector layer includes growing the subcollector layer with a quaternary composition that corresponds to a band gap wavelength that is transparent to the optical wavelengths of interest.  
     
     
         24 . A method as claimed in  claim 22  wherein the step of epitaxially growing the base region includes the step of growing a doped base layer on the collector layer and an undoped spacer layer on the base layer.  
     
     
         25 . A method as claimed in  claim 24  including the steps of etching the collector region to expose a surface portion of the undoped spacer layer, depositing a base metal electrode on the exposed surface portion, and using the base metal electrode as a mask, undercutting the collector layer to reduce base-collector capacitance.