Abstract:
Disclosed are a heterojunction bipolar transistor and a method of fabricating the same. A first dielectric layer easily etched is deposited on the overall surface of a substrate before an isolation region is defined. The first dielectric layer and a sub-collector layer are selectively etched, and then a second dielectric layer etched at a low etch rate is deposited on the overall surface of the substrate. Via holes are formed in the first and second dielectric layers, and then the first dielectric layer is removed using a difference between etch characteristics of the first and second dielectric layers. Accordingly, a reduction in power gain, generated at the interface of a compound semiconductor and a dielectric insulating layer (the second dielectric layer), can be eliminated.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korea Patent Application No. 2003-94071 filed on Dec. 19, 2003 in the Korean Intellectual Property Office, the content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     (a) Field of the Invention  
         [0003]     The present invention relates to a heterojunction bipolar transistor and a method of fabricating the same. More specifically, the present invention relates to a heterojunction bipolar transistor using a compound semiconductor and a method of fabricating the same.  
         [0004]     (b) Description of the Related Art  
         [0005]     A heterojunction bipolar transistor using a compound semiconductor such as GaAs and InP is actively used as an essential element having various functions for communications because the heterojunction bipolar transistor has very high speed, very high frequency, large current driving capability, signal linearity, and uniform operation voltage. For instance, the heterojunction bipolar transistor is used as a high-efficiency large-power amplifier in an output amplifier of a mobile terminal. Furthermore, a mixed signal heterojunction bipolar transistor technique greatly affects the construction of an optical communication system. The heterojunction bipolar transistor is disclosed in Korean Patent No. 347520.  FIG. 1  is a cross-sectional view of the heterojunction bipolar transistor.  
         [0006]     To fabricate the heterojunction bipolar transistor shown in  FIG. 1 , a sub-collector layer  102 , a collector layer  103 , a base layer  104 , an emitter layer  105 , and an emitter cap layer  106  are sequentially epitaxial-grown on a compound semiconductor substrate  101 . An emitter electrode  111  is formed on the emitter cap layer  106 . Then, the emitter cap layer  106  and the emitter layer  105  are etched into a mesa, and base electrodes  112  are formed on the base layer  104 . Subsequently, the base layer  104  and the collector layer  103  are etched into a mesa, and collector electrodes  113  are formed on the sub-collector layer  102 . Then, a predetermined isolation region is defined, and a dielectric insulating layer  121  is formed on the overall surface of the substrate. Finally, via holes are formed in the insulating layer  121  to expose the emitter, base, and collector electrodes, and a metal  122  is deposited or plated on the exposed emitter, base, and collector electrodes to form transmission lines. The transmission lines  122  are connected to other active or passive elements of the transistor.  
         [0007]     In the above-described conventional method of fabricating the heterojunction bipolar transistor, the dielectric insulating layer  121  forms interfaces with the side of the emitter mesa, the side of the base-collector mesa, the exposed portion of the surface of the emitter layer, the exposed portion of the surface of the base layer, and the exposed portion of the surface of the sub-collector layer. The dielectric insulating layer  121  is formed of SiO2, Si3N4, or SiOxNy. However, recombination sites are formed at the interface of the compound semiconductor and the dielectric insulating layer because bonding coherence or smooth transition between the compound semiconductor and the dielectric insulating layer is difficult to form at the interface. This reduces a current gain of the bipolar transistor.  
         [0008]      FIGS. 2   a  and  2   b  respectively show DC current gains before and after deposition of a SiNx dielectric insulating layer of a heterojunction bipolar transistor having an InP emitter layer and an InGaAs base layer.  FIGS. 2   a  and  2   b  show a collector current Ic, a base current Ib, and a current gain according to a variation in a base-emitter voltage Vbe. The current gain corresponds to a value obtained by dividing the collector current Ic by the base current Ib. As shown in  FIGS. 2   a  and  2   b,  the current gain after deposition of the SiNx dielectric insulating layer is considerably reduced as the base current Ib is increased, compared to the current gain before deposition of the SiNx insulating layer. This is because of a strong interaction of the insulating layer and an extrinsic base surface having a large surface recombination rate due to a high doping concentration (10 19  to 10 20  cm −3 ), such as at the side of the base-collector mesa and the exposed portion of the surface of the base layer. The surface recombination effect becomes stronger as the size of the transistor is decreased in order to improve the performance of the transistor.  
         [0009]     To reduce the interface effect, a conventional technique has employed interface control layers. Specifically, a method of forming an interface layer between a compound semiconductor and an insulating layer was used. The interface layer is formed of one of the following combination layers.  
         [0010]     1) Oxide or insulator interface control layers such as Al 2 O 3 /In(PO 3 ) 3 /InP, SiO 2 /ECR oxide/InP, SiNx/anodic oxide/InP, and SiNx/PN/sulfur-treated InP.  
         [0011]     2) Interface control layers including sulfur such as SiO 2 /S/InP, SiO 2 /SiS 2 /InP, Si 3 N 4 /polysulfide/InP, and SiNx/InS/InP.  
         [0012]     3) Interface control layers using an ultra thin Si layer such as SiO 2 /Si/InGaAs, Si 3 N 4 /Si/InGaAs, SiO 2 /Si 3 N 4 /Si/InGaAs, SiO 2 /Si/InP, and Si 3 N 4 SiNx/Si/InP.  
         [0013]     Although the conventional interface control layer forming technique appropriately compensates a difference between the compound semiconductor and the insulating layer to prevent interface characteristic from being abruptly varied, the conventional technique is not an appropriate method for solving the aforementioned problem. That is, the problem caused by the surface recombination effect is not solved because of non-continuity between the compound semiconductor and interface control layer and between the interface control layer and the insulating layer.  
       SUMMARY OF THE INVENTION  
       [0014]     It is an object of the present invention to provide a heterojunction bipolar transistor using a compound semiconductor, and a method of fabricating the same.  
         [0015]     In one aspect of the present invention, a method of fabricating a heterojunction bipolar transistor comprises sequentially forming a sub-collector layer, a collector layer, a base layer, an emitter layer, and an emitter cap layer on a substrate; forming an emitter electrode on a first region of the emitter cap layer; selectively etching the emitter cap layer and the emitter layer to expose a portion of the base layer; forming a base electrode on a second region of the exposed portion of the base layer; selectively etching the base layer and the collector layer to expose a portion of the sub-collector layer; forming a collector electrode on a third region of the exposed portion of the sub-collector layer; forming a first dielectric layer on the overall surface of the substrate; selectively etching the first dielectric layer and the sub-collector layer to define an isolation region; forming a second dielectric layer on the overall surface of the substrate; selectively etching the second dielectric layer to form via holes on the emitter, base, and collector electrodes, respectively; and etching the first dielectric layer through the via holes.  
         [0016]     In another aspect of the present invention, a heterojunction bipolar transistor comprises a sub-collector layer formed on a substrate; a collector layer formed on a first region of the sub-collector layer; a collector electrode formed on a second region of the sub-collector layer; a base layer formed on the collector layer; an emitter layer formed on a third region of the base layer; a base electrode formed on a fourth region of the base layer; an emitter cap layer formed on the emitter layer; an emitter electrode formed on a fifth region of the emitter cap layer; and a first dielectric layer formed on the substrate, the first dielectric layer having via holes that expose at least portions of the collector, base, and emitter electrodes. There is a predetermined gap between the first dielectric layer and at least a portion of the surface of the sub-collector layer, the surface of the emitter layer, the side of the collector layer, the side of the base layer, the side of the emitter layer, and the side of the emitter cap layer.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention:  
         [0018]      FIG. 1  is a cross-sectional view of a conventional heterojunction bipolar transistor;  
         [0019]      FIGS. 2   a  and  2   b  respectively show DC current gains before and after deposition of a dielectric insulating layer of a heterojunction bipolar transistor; and  
         [0020]      FIGS. 3 through 14  are cross-sectional views showing a process of fabricating a heterojunction bipolar transistor according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor(s) of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.  
         [0022]     In the drawings, thicknesses of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Like reference numerals in the drawings denote like elements, and thus their description will be omitted.  
         [0023]     A heterojunction bipolar transistor and a method of fabricating the same according to an embodiment of the present invention are explained below.  
         [0024]      FIGS. 3 through 14  are cross-sectional views showing a process of fabricating the heterojunction bipolar transistor according to the embodiment of the present invention.  
         [0025]     Referring to  FIG. 3 , a sub-collector layer  20 , a collector layer  30 , a base layer  40 , an emitter layer  50 , and an emitter cap layer  60  are sequentially formed on a semiconductor substrate  10 . These layers  20 ,  30 ,  40 ,  50 , and  60  are grown on the semiconductor substrate  10  using an epitaxial growth such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD). The emitter cap layer  60  is formed for the purpose of reducing a contact resistance between the emitter layer  50  and an emitter electrode that will be formed later. Thus, the emitter cap layer  60  can be omitted.  
         [0026]     The substrate  10  is an electrically semi-insulating compound semiconductor substrate. A GaAs or InP compound semiconductor substrate can be used as the substrate  10 . Each of the sub-collector layer  20 , collector layer  30 , base layer  40 , emitter layer  50 , and emitter cap layer  60  can be formed of a combination of various III-V compound semiconductors, such as GaAs, InP, InAlAs, InGaAs, InGaP, AlGaAs, and so on, to construct the heterojunction bipolar transistor on the substrate  10 .  
         [0027]     For example, in the case of a GaAs substrate  10 , an n+ GaAs sub-collector layer  20 , an n-type GaAs collector layer  30 , and a p+ GaAs base layer  40  are sequentially grown on the GaAs substrate  10 . Subsequently, an n-type AlGaAs emitter layer  50  and an n+ InGaAs emitter cap layer  60  are grown on the emitter layer  50 . In the case of a InP substrate, the sub-collector layer  20 , collector layer  30 , and base layer are formed of AlGaAs on the InP substrate, and an InP emitter layer  50  and an InGaAs emitter cap layer  60  are sequentially grown thereon.  
         [0028]     Referring to  FIG. 4 , an emitter electrode  62  is formed on a predetermined region of the emitter cap layer  60 . Specifically, the emitter electrode  62  is formed in such a manner that a conventional electrode material is deposited and patterned through photolithography and lift-off processes. The emitter electrode  62  can be formed of Ti/Pt/Au, Au—Ge, Pd/In, Al/Ni/Ge, Ni/Au—Ge, or Pd/Au—Ge.  
         [0029]     Referring to  FIG. 5 , the emitter cap layer  60  and the emitter layer  50  are selectively etched using the emitter electrode  62  or a predetermined photoresist pattern (not shown) formed by photolithography as a mask, to form an emitter mesa, that is, an intrinsic base region.  
         [0030]     Referring to  FIG. 6 , base electrodes  42  are formed of a conventional electrode material on predetermined regions of the base layer  40 . As shown in  FIG. 7 , predetermined portions of the base layer  40  and collector layer  30  are selectively etched to form a base-collector mesa  200 .  
         [0031]     Referring to  FIG. 8 , collector electrodes  22  are formed of a conventional electrode material on predetermined regions of the sub-collector layer  20 . Referring to  FIG. 9 , a dielectric layer  70  is coated by tens of nm on the overall surface of the substrate  10 . The dielectric layer  70  is formed of an oxide or a nitride.  
         [0032]     Referring to  FIG. 10 , a photoresist pattern  72  is formed as an etch mask on the dielectric layer  70  formed on the emitter electrode  62 , base electrode  42 , and collector electrode  22  in order to define an isolation region. Then, exposed portions of the dielectric layer  70  and sub-collector layer  20  are sequentially etched using the photoresist pattern  72  as a mask. The dielectric layer  70  can be etched through dry etching such as reactive ion etching (RIE) or wet etching using a buffered oxide etchant (BOE). The sub-collector  20  can be also etched using dry etching or wet etching.  
         [0033]     Referring to  FIG. 11 , the photoresist pattern is removed, and then a dielectric layer  80  is coated on the dielectric layer  70 . The dielectric layer  80  is formed of a material different from the dielectric layer  70  through a deposition method and conditions different from those of the dielectric layer  70 , such that the dielectric layer  80  is harder to etch than the dielectric layer  70  or an etch rate of the dielectric layer  80  is lower than that of the dielectric layer  70 . For instance, if the dielectric layer  70  is formed of SiOx rapidly deposited at the normal temperature, the dielectric layer  80  can be formed of Al 2 O 3  slowly deposited at a high temperature.  
         [0034]     Referring to  FIG. 12 , via holes  82  that connect the emitter, base, and collector electrodes  62 ,  42 , and  22  to other elements of the bipolar transistor are formed. Specifically, a photoresist pattern  82  is formed as a mask on the dielectric layer  80  through photolithography, and the dielectric layer  80  is etched using the photoresist pattern  84  as a mask such that the etched dielectric layer  80  has no undercut, to form the via holes  82 . It is preferable that the via holes  82  have vertical cross sections. For this, inductively coupled plasma (ICP) using a high induced bias or RIE dry etching are used to form the via holes. Here, the portions of the dielectric layer  80  in which the via holes  82  are formed must be removed. On the other hand, the portions of the dielectric layer  70  formed under the dielectric layer  80 , in which the via holes are formed, can be partially or completely removed.  
         [0035]     Referring to  FIG. 13 , the dielectric layer  70  and the photoresist pattern are removed through the via holes  82 . The dielectric layer  70  is etched through wet etching using a BOE or dry etching having isotropic etch characteristics. Then, the dielectric layer  80  is etched at an etch rate considerably lower than the etch rate of the dielectric layer  70 . Here, when the dielectric layer  70  is etched, a specific gap is formed between the dielectric layer  80  and the side of the emitter mesa  100 , the side of the base-collector mesa  200 , the exposed portion of the surface of the base layer  40 , and the exposed portion of the surface of the sub-collector layer  20 . If the dielectric layer  70  is not completely etched, the dielectric layer  70  is partially left in the gap.  
         [0036]     Referring to  FIG. 14 , transmission lines  90  are connected to the emitter, base, and collector electrodes  62 ,  42 , and  22  to achieve the heterojunction bipolar transistor according to the present invention.  
         [0037]     As described above, the present invention can reduce a surface recombination current generated at the interface of the surface of a compound semiconductor and an insulating layer so as to improve a DC current gain of a heterojunction bipolar transistor. Accordingly, the performance of the heterojunction bipolar transistor can be maximized.  
         [0038]     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.