Abstract:
A double heterojunction bipolar transistor on a substrate comprises a collector formed of InGaAsP, a base in contact with the collector, an emitter in contact with the base, and electrodes forming separate electrical contacts with each of the collector, base, and emitter, respectively. A device incorporates this transistor and an opto-electronic device optically coupled with the collector of the transistor to interact with light transmitted therethrough.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 11/007,697 filed on Dec. 7, 2004, the disclosure of which is incorporated herein in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to integrated semiconductor devices, and more particularly to a new bipolar transistor structure such as a double heterojunction bipolar transistor (DHBT) and integrated opto-electronic devices employing such a transistor. 
     BACKGROUND 
     Current InP-based HBTs typically employ InP or InGaAs as the collector. InGaAlAs and InGaAs/graded-InGaAsP/InP composite collectors have also been utilized. No reported literature, however, makes reference to further using the collector material for waveguide applications. InGaAs, which is a commonly used collector material in InP-based HBTs, absorbs light with wavelengths shorter than 1.5 microns, and hence cannot practically be used as a waveguide material. InP is transparent to 1.5 micron light, but cannot be easily clad with a semiconductor to confine light to thereby serve as a waveguide. 
     For a DHBT to function satisfactorily, there should be no spike in the conduction band between the base and the collector. As appreciable by those skilled in the art, the compositions of the alloys forming the base and the collector that enable transistor operation are not obvious and must be experimentally derived for each particular application. Controlling the composition of thick (˜2000 Å thickness used in the collector) mixed group-V alloys in order to maintain good lattice matching with the InP substrate is likewise not a straightforward matter. 
     What is needed is a method and apparatus for providing DHBTs with good waveguide properties to enable the manufacture of highly-integrated opto-electronic devices. The embodiments of the present disclosure answer these and other needs. 
     SUMMARY 
     In a first embodiment disclosed herein, a bipolar transistor on a substrate comprises a collector formed substantially of InGaAsP having a first conductivity type, a base having a second conductivity type in contact with the collector, an emitter of the first conductivity type in contact with the base, and electrodes forming separate electrical contacts with each of the collector, base, and emitter, respectively. 
     In another embodiment disclosed herein, a device comprises at least one bipolar transistor on a substrate, the transistor comprising a collector formed substantially of InGaAsP of a first conductivity type to transmit light therethrough, a base formed substantially of InGaAs or GaAsSb of a second conductivity type in contact with the collector, an emitter of the first conductivity type in contact with the base, and transistor electrodes forming separate electrical contacts with each of the collector, base, and emitter, respectively, and at least one opto-electronic device optically coupled with the collector of the transistor to interact with the light transmitted therethrough. 
     In a further embodiment disclosed herein, the electro-optical device may be butt coupled with the collector of the transistor or evanescently coupled with the electromagnetic field of the collector. The conduction band energy level of the base may be higher than the conduction band energy level of In 0.53 Ga 0.47 As, and the Ga mole fraction in the base may be above 0.47. A graded layer may be formed in the collector in contact with the base and varying in composition to minimize a conduction band discontinuity between the base and the collector. 
     These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a structural diagram of a DHBT that may be formed in accordance with the present disclosure; 
         FIG. 2  is a graph depicting Poisson band bending models for the base collector interface for a baseline DHBT and a DHBT formed in accordance with the present disclosure; 
         FIG. 3  is a graph depicting the room temperature photoluminescence spectrum for an MBE-grown In 0.75 Ga 0.25 As 0.54 P 0.46  film deposited on InGaAs, in accordance with the present disclosure; 
         FIG. 4  is a graph depicting the x-ray rocking curve for a DHBT using a lattice matched In 0.75 Ga 0.25 As 0.54 P 0.46  collector and a strained base layer, in accordance with the present disclosure; and 
         FIG. 5  is a structural block diagram of an integrated opto-electronic device employing a DHBT formed in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a generic DHBT structure  10  is shown for discussion purposes. A transistor such as DHBT  10  is typically formed on a substrate  100 , and includes a collector layer  20  disposed on the substrate, a base layer  30  disposed over the collector layer, and an emitter layer disposed over the base layer  40 . Electrical contacts  21 ,  31 ,  41  provide electrical contact to each of the collector  20 , base  30 , and emitter  40  respectively. As well known to those skilled in the art, the collector and emitter are doped with a dopant of one conductivity type (e.g. n-type) and the base is doped with a dopant of the opposite conductivity type (e.g. p-type). The collector, base and emitter layers may be further formed with various sublayers, as discussed elsewhere herein. 
     A material for forming the base that is currently widely used is an alloy of InGaAs. A particularly favored alloy composition is In 0.53 Ga 0 .47As, because this alloy matches the lattice constant of InP, which is a favored material for the substrate  100 . In accordance with a novel aspect of the present disclosure, a DHBT is described that uses a collector formed of an InGaAsP alloy that exhibits proper semiconductor characteristics as well as good waveguide characteristics. 
     Published data available in the literature were used to perform band bending simulations for the base collector junction. The simulations indicated that the conduction band offset between the In 0.75 Ga 0.25 As 0.54 P 0.46  (collector) and the In 0.44 Ga 0.56 As (strained base) is approximately −0.1 to 0.1 eV, as depicted in the graph of  FIG. 2 . Because a small or negative offset in the base/collector band alignment is highly desirable for the unimpeded electron transport from the base to the collector, DHBTs with an abrupt base collector junction were initially grown and fabricated. However, the DC gain for these abrupt junction transistors was less than 2, indicating the presence of a significant barrier at the base collector interface that was contrary to the theoretical estimates. Subsequent to this finding, DHBT structures were grown with a graded InGaAsP layer  22  as the transition between the base  30  and the collector  20  to eliminate the barrier in the conduction band. 
     Table 1 lists an approximate structure composition of this embodiment. As shown, the collector  20  consists of a In 0.75 Ga 0.25 As 0.54 P 0.46  quaternary layer, including an InGaAs subcollector  26 , an InGaAsP subcollector  24 , an InGaAsP collector layer, and a graded collector layer  22 . In 0.44 Ga 0.56 As, including a spacer layer  32 , coherently strained for a thickness of 350 Å, forms the base  30  of the transistor. The emitter  40  includes chirped super lattices (SLs)  42 . The alloy composition of the collector can be varied while allowing lattice matching to the base and keeping the conduction band offset between the base and the collector to a minimum. For this embodiment, a 450 Å graded collector was employed to achieve a smooth grading in the conduction band energy between the base and the collector. 
     The material structure shown in Table 1 was grown by MBE, and transistors were fabricated. A dry-etch process was developed to etch the quaternary collector  20 , as wet etching was found to produce unacceptable levels of under-cut. The DC gain measured for this embodiment was −25, and could be further increased by optimizing the composition of the quaternary graded layer  22 . The collector breakdown voltage BV, was as high as 6V for the transistor, for a collector thickness of 2000 Å. The base-emitter turn-on voltage (V be ) was measured at 0.6V and the reverse breakdown voltage was measured at 2V. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                   
                 2003-DHBT0.75-I-3463 
               
             
          
           
               
                   
                 DHBT Layer  
                 Thickness 
                 Doping Level 
               
               
                   
                 Description 
                 (Å) 
                 (cm −3 ) 
               
               
                   
                   
               
               
                   
                 InGaAs contact 
                 ~1000 
                 n:2E19 
               
               
                   
                 InAlAs emitter contact 
                 ~400  
                 n:1E19 
               
               
                   
                 InAlAs emitter 
                 ~1000 
                 n:1E18 
               
               
                   
                 Chirped SLs 
                 ~300  
                 n:1E18 
               
               
                   
                 In 0.44 Ga 0.56 As spacer layer 
                 ~50  
                 p:2E18 
               
               
                   
                 In 0.44 Ga 0.56 As base layer 
                 ~300  
                 p:3E18 
               
               
                   
                 In 0.75 Ga 0.25 As 0.54 P 0.46   to 
                 ~450  
                 n:7E16 
               
               
                   
                 In 0.44 Ga 0.56 As graded  
                   
                   
               
               
                   
                 collector layer 
                   
                   
               
               
                   
                 In 0.75 Ga 0.25 As 0.54 P 0.46   
                 ~1500 
                 n:7E16 
               
               
                   
                 collector layer 
                   
                   
               
               
                   
                 In 0.75 Ga 0.25 As 0.54 P 0.46  sub- 
                 ~100  
                 n:2E19 
               
               
                   
                 collector layer 
                   
                   
               
               
                   
                 InGaAs sub-collector 
                 ~3000 
                 n:2E19 
               
               
                   
                 Substrate 
                 6 × 10 7   
                 SI 
               
               
                   
                   
               
             
          
         
       
     
     The DHBT  10  formed as per Table 1 incorporates a coherently strained In 0.44 Ga 0.56 As base layer  30 . Due to the increase in the Ga mole fraction from 47% (lattice matched to InP) to 56%, the energy level of the conduction band is raised by ˜0.1 eV (see  FIG. 2 ), thereby lowering the barrier at the base collector interface. Raising the conduction band energy level proved to be crucial in enabling the transistor to exhibit practical gain. The critical layer thickness for the growth of In 0.44 Ga 0.56 As on InP is ˜500 Å, comparable to the thickness of the base layer in a typical transistor. Hall measurements were performed on 400 Å thick In 0.44 Ga 0.56 As:Be to compare the mobility with the lattice matched alloy that is typically used in transistors. The results for two compositions of strained InGaAs along with the properties of lattice matched reference In 0.53 Ga 0.47 As:Be are listed in Table 2. As expected, the mobility decreased with increasing Ga mole fraction. The sheet resistance of the base layer increased by about 10% as the Ga mole fraction was increased from 47% to 56%. This is a relatively minor effect, since the decrease in mobility can be compensated by doping the alloy with the higher Ga mole fraction more heavily. Thus no sheet resistance penalty is anticipated by using In 0.44 Ga 0.56 As:Be as the base layer. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 Rs  
                   
                   
               
               
                   
                   
                   
                 Rs 
                 (Hall) 
                   
                 mu 
               
               
                   
                   
                   
                 (ohm/ 
                 (ohm/ 
                   
                 (cm 2 / 
               
               
                 Run # 
                 Structure 
                 Boule # 
                 sq) 
                 sq) 
                 n(cm −3 ) 
                 V · s) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1-3245 
                 In .46 Ga .54 As:Be 
                 S615-69 
                 640.5 
                 630.5 
                 3.18E+19 
                 62.2 
               
               
                 1-3247 
                 In .53 Ga .47 As:Be 
                 S615-76 
                 581.2 
                 591.4 
                 3.25E+19 
                 65 
               
               
                 1-3258 
                 In .44 Ga .56 As:Be 
                 S813-13 
                 668.4 
                 653.5 
                 3.30E+19 
                 58.03 
               
               
                   
               
             
          
         
       
     
     The composition of the quaternary layer was determined from x-ray and photoluminescence (PL) measurements. A strong PL signal was measured from InGaAsP layers grown on InGaAs buffer layers indicating the high-quality of the InGaAsP layer.  FIG. 3  shows the measured room temperature PL spectrum for an MBE-grown In 0.75 Ga 0.25 As 0.54 P 0.46  film deposited on InGaAs. The band-gap of this quaternary layer  20  was 0.9 eV. 
     The x-ray rocking curve for a quaternary collector  20  DHBT is shown in  FIG. 4 . As shown in the structure diagram listed in Table 2, this device uses a lattice matched In 0.75 Ga 0.25 As 0.54 P 0.46  collector  20  and a strained base layer  30 .  FIG. 4  shows that the lattice constant of the collector closely matches that of the substrate. Close lattice matching was achieved due to the excellent composition control over the group-V flux achieved by using gas-source MBE. The peak at 32.06 degrees is from the strained In 0.53 Ga 0.47 As base layer. The strong interference oscillations shown confirm abrupt interfaces and high quality of the device structure. 
     The new InP-based DHBT disclosed herein facilitates integration of opto-electronic components, since the quaternary collector can also serve as a waveguide. This enables the monolithic integration of optics and electronics for the fabrication of detectors, modulators and other opto-electronic devices. As shown conceptually in  FIG. 5 , an opto-electronic device  80  such as an amplifier, photodetector, modulator, or light emitter such as a laser or LED, may be formed on the substrate  100  and optically coupled with the collector  20  of the DHBT  10 , and thus be formed as an integral component of the same device as the DHBT. The opto-electronic device  80  could, in one embodiment, be butt coupled with the collector  20  (as depicted in  FIG. 5 ), which offers simple fabrication and low insertion losses. As further shown in  FIG. 5 , the opto-electronic device  80  could be further coupled to the collector  20 ′ of another DHBT, and could thus be adapted to pass light transmitted through the collector  20  on to the collector  20 ′ of another DHBT after detecting, modulating, or amplifying it. As also shown in  FIG. 5 , in another embodiment an opto-electronic device  90  could be evanescently coupled with the electromagnetic field of the collector  20 ′ of a DHBT, and controlled via electrodes  91  and  92 . 
     Thus, the quaternary layer  20  of the present disclosure can serve as a waveguide for light having a wavelength of 1.5 microns, which enables the integration and fabrication of waveguides, detectors, modulators and other opto-electronic components with InP-based transistors. The presence of the quaternary waveguide material all across the substrate wafer  100  facilitates low insertion loss integration of optical components evanescently coupled with electromagnetic fields. 
     Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as disclosed herein.