Method of making high mobility multilayered heterojunction device employing modulated doping

Abstract

The mobility of a relatively narrow bandgap semiconductor material can be significantly enhanced by incorporating it into a multilayered structure (10) comprising a first plurality of relatively narrow bandgap layers (12) of the material and a second plurality of wider bandgap semiconductor layers (14) interleaved with and contiguous with the first plurality. The wide bandgap and narrow bandgap layers are substantially lattice-matched to one another, and the wide bandgap layers are doped such that the impurity concentration-thickness product therein is greater than the same product in the narrow bandgap layers. The fabrication of the structure by MBE to enhance the mobility of GaAs is specifically described. In this case, the narrow bandgap layers (12) comprise GaAs and are unintentionally doped to about 10 14 /cm 3 , whereas the wide bandgap layers (14) comprise AlGaAs doped n-type to about 10 16 to 10 18 /cm 3 . The incorporation of this structure in an FET is also described.

We claim: 
     
       1. A method of enhancing the mobility of a narrow bandgap semiconductor material comprising the steps of: (a) forming said narrow bandgap material is a first plurality of spaced apart, narrow bandgap semiconductor layers,   (b) forming a second plurality of wide bandgap semiconductor layers interleaved with and contiguous with said first plurality, and   (c) forming said wide bandgap layers from a material which (i) is substantially lattice-matched to that of said narrow bandgap layers (ii) forms a conductor or valence band step at the interfaces with said narrow bandgap layers of sufficient magnitude to confine carriers, and (iii) is doped such that the impurity-concentration-thickness product thereof exceeds that of said narrow bandgap layers.   
     
     
       2. The method of claim 1 wherein said forming steps include growing said layers by molecular beam epitaxy in an ultra high vacuum chamber wherein said first and second pluralities of layers are grown alternately on a semiconductor substrate. 
     
     
       3. The method of claim 2 wherein said chamber includes an oven carrying a dopant source which is used to generate a donor beam for doping said wide bandgap layers n-type and which is shuttered closed during the growth of said narrow bandgap layers so that impurities are incorporated in said narrow bandgap layers primarily from background contamination in said chamber. 
     
     
       4. The method of claim 3 wherein said forming step (a) is effective to grow said first plurality of .[.GaSa.]. .Iadd.GaAs .Iaddend.layers having an impurity concentration of about 10 14  or less, and said forming steps (b) and (c) are effective to grow said second plurality of n-type Al x  Ga 1-x  As layers 0.02≅x having a donor concentration of at least 10 16  /cm 3 . 
     
     
       5. The method of claim 1 wherein said forming steps (b) and (c) are effective to dope only a central portion of each of said wide bandgap layers with donors, thereby leaving a thin, undoped buffer zone in said wide bandgap layers adjacent said narrow bandgap layers. .Iadd. 
     
     
       6.  A method of enhancing the mobility of a narrow bandgap semiconductor material comprising the steps of: (a) forming said narrow bandgap material as a narrow bandgap first layer,   (b) forming an essentially undoped wide bandgap semiconductor second layer contiguous with one side of said first layer,   (c) forming a wide bandgap semiconductor third layer contiguous with said second layer,   (d) forming an essentially undoped wide bandgap semiconductor fourth layer contiguous with the other side of said first layer,   (e) forming a wide bandgap semiconductor fifth layer contiguous with said fourth layer,   (f) forming said layers from materials which (i) form a conduction or valence band step at the interfaces with said first layer of sufficient magnitude to confine carriers, and (ii) are doped such that the impurity-concentration thickness product at least one of of said third layer and said fifth layer exceeds that of said first layer, and   (g) forming electrode means electrically coupled to said layers and capable of causing said carriers to flow in said first layer in a direction essentially parallel to said interfaces. .Iaddend. .Iadd.   
     
     
       7.  A method of fabricating a field effect transistor comprising the steps of: (a) forming a narrow bandgap semiconductor first layer which includes the channel of said transistor,   (b) forming a wide bandgap semiconductor second layer on one side of said first layer,   (c) forming a wide bandgap semiconductor third layer on the other side of said first layer,   (d) forming said layers from materials which (i) form a conduction or valence band step at the interfaces with said first layer of sufficient magnitude to confine carriers, and (ii) are doped such that the impurity-concentration thickness product of at least one of said second and third layers exceeds that of said first layer, and   (e) forming electrode means on said transistor including source and drain electrode means which are electrically coupled to said channel and gate electrode means for controlling the flow of carriers in said channel. .Iaddend. .Iadd.   
     
     
       8.  The method of claim 7 wherein said forming steps (a), (b), (c) and (d) are effective to dope only a portion of the doped ones of said second and third layers, thereby leaving thin undoped buffer zones in said second and third layers adjacent to said first layer. .Iaddend.