Abstract:
A power amplifier includes a substrate, a heat sink for dissipating heat, and a heterojunction bipolar transistor (HBT) disposed on the substrate. The HBT includes a collector, a base, and at least an emitter. The power amplifier further includes an emitter electrode directly connecting the heat sink and the emitter of the HBT. The emitter electrode is a flip-chip bump, and the heat sink is a metal layer that sandwiches the HBT with the substrate. Alternatively, the emitter electrode is a backside via that penetrates the substrate, and the heat sink is a metal layer, disposed on the substrate opposite the HBT.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a division of application Ser. No. 10/904,027, filed Oct. 19, 2004, from which the specification and drawings are carried forward without amendment. Additionally, application Ser. No. 10/904,027, filed Oct. 19, 2004 is itself a continuation-in-part of application Ser. No. 10/064,514, filed Jul. 23, 2002, which is included herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a power amplifier, and more specifically, to a heterojunction bipolar transistor (HBT) power amplifier integrated circuit. 
   2. Description of the Prior Art 
   Power amplifier integrated circuits using HBTs are suitable for a wide range of applications and are particularly well adapted for use as high power microwave amplifiers such as those used in mobile phones. 
   A frequent and often serious problem with HBT power amplifiers is excessive heat buildup. Power amplifier integrated circuits operate at high current density, and hence high power density, and thus, heat generated by devices of the HBT elevates junction temperature significantly above ambient temperature. High junction temperature degrades the device reliability and limits the maximum power density of the device. Additionally, operating at higher power density risks thermal runaway of the power amplifier, in which the power amplifier suffers catastrophic device failure. Furthermore, operating at a higher junction temperature reduces device mean time to failure (MTTF). Typically, for a given application, larger devices are required to overcome this problem, leading to increased cost and inefficient use of space. 
   Adlerstein et al. in U.S. Pat. No. 5,986,324, which is incorporated herein by reference, describes in detail an HBT structure and operation thereof. Miura et al. in U.S. Pat. No. 5,793,067, which is also incorporated herein by reference, teaches how a transistor structure can be made with widened leads to reduce thermal resistance. However, both Adlerstein et al. and Miura et al. teach the use of an emitter air-bridge that is costly and causes undue fabrication complexity. 
   The prior art heat dissipation in power amplifier transistors, such as HBTs, is inadequate. Moreover, prior art solutions providing heat dissipation are difficult and costly to manufacture, impacting yield and reliability. Such insufficient heat dissipation prevents prior art power amplifiers from operating at high current densities or high power, and dictates a larger power amplifier for a given application. Finally, the cost associated with using the air-bridge for cooling is also much higher compared to a die without the air-bridge. 
   SUMMARY OF THE INVENTION 
   It is therefore a primary objective of the present invention to provide a power amplifier integrated circuit having high heat dissipation to solve the problems of the prior art. 
   Briefly summarized, the present invention includes a substrate, a heat sink for dissipating heat, a transistor disposed on the substrate including a collector, a base, and at least an emitter. The present invention further includes an emitter electrode directly connecting the heat sink and the emitter. 
   According to one preferred embodiment of the present invention, the transistor is a heterojunction bipolar transistor (HBT). 
   According to one preferred embodiment of the present invention, the emitter electrode is a flip-chip bump and the heat sink is a metal layer, and the heat sink and the substrate sandwich the transistor. 
   According to another preferred embodiment of the present invention, the emitter electrode is a backside via penetrating the substrate and the heat sink is a metal layer, and the heat sink and the transistor sandwich the substrate. 
   It is an advantage of the present invention that heat accumulated in the transistor is readily dissipated through the emitter electrode and the heat sink, such that the transistor can operate at a substantially high power. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of an HBT power amplifier according to the preferred embodiment of the present invention. 
       FIG. 2  is a schematic diagram of the HBT power amplifier of  FIG. 1  dissipating heat. 
       FIG. 3  is a cross-sectional view of the HBT power amplifier of  FIG. 1 . 
       FIG. 4  is a schematic diagram of an HBT power amplifier according to a second embodiment of the present invention. 
       FIG. 5  is a cross-sectional view of the HBT power amplifier of  FIG. 4 . 
   

   DETAILED DESCRIPTION 
   The present invention will be described in two embodiments. Both embodiments comprise a heterojunction bipolar transistor (HBT) disposed on a GaAs substrate. However, the present invention is not limited by such and can be utilized with other types of transistors on different substrates. Furthermore, the present invention should not be construed as limited to application as a power amplifier. 
   Please refer to  FIG. 1 .  FIG. 1  shows a plan view of an HBT power amplifier  10  according to the preferred embodiment of the present invention. Typically, HBTs comprise pluralities of bases, emitters, collectors, and other functional devices, however, the HBT power amplifier  10  is shown as simplified for explanatory purposes. The HBT  10  comprises a collector  12 , an emitter  14   a , an enlarged emitter  14   b , and a base  16  all disposed on a substrate  50 . The substrate  50  is a GaAs substrate, but may be another suitable substrate material. The emitter  14   a  and the emitter  14   b  are formed by a metallization process that is well known in the art. Both emitters  14   a ,  14   b  are electrically and thermally connected together by another metallization layer  18 . The enlarged emitter  14   b  is connected to a heat sink (item  22 ,  FIG. 3 ) by an emitter electrode  20 . In the preferred embodiment, the emitter electrode  20  is a flip-chip bump that connects the enlarged emitter  14   b  to the heat sink  22  for heat dissipation and electrical grounding. The electrical operation of the HBT  10  is well known in the art and will not be described in detail in this description. 
     FIG. 2  shows the HBT power amplifier  10  during operation. Heat is generated in proportion to operating current density or power of the HBT  10  and is accumulated at the emitters  14   a ,  14   b . Heat as represented by arrow  40  flows from the emitters  14   a ,  14   b  to the flip-chip bump  20  and finally to the heat sink  22 . While some heat is radiated to surrounding components and materials, a large portion of heat, as represented by the arrow  40 , is thermally conducted through the efficient path provided by the flip-chip bump  20  and the heat sink  22 . In this way, the present invention provides enhanced cooling to functional devices of the HBT such as the emitters  14   a ,  14   b.    
   Please refer to  FIG. 3 .  FIG. 3  shows a cross-sectional view of the HBT power amplifier  10  of  FIG. 1  along a section line  3 - 3  shown in  FIG. 1 . In  FIG. 3 , the heat sink  22  is a metal layer. The flip-chip bump  20  is shown connecting the emitter  14   b  and the metal layer  22 . Heat represented by arrows  42  can be seen flowing from the emitters  14   a ,  14   b  through the flip-chip bump  20  and finally to the metal layer  22 . The enhanced thermal conduction as provided by the preferred embodiment of the present invention allows the HBT power amplifier  10  to operate at a substantially high power. 
     FIG. 4  shows a schematic diagram of an HBT power amplifier  10 ′ according to a second embodiment of the present invention. The HBT power amplifier  10 ′ is similar to the HBT power amplifier  10  except that the HBT  10 ′ comprises a second enlarged emitter  14   b  rather than the emitter  14   a . The HBT power amplifier  10 ′ further differs in that emitter electrodes  20 ′ are backside vias provided to both enlarged emitters  14   b . As a result, heat dissipation from the emitters  14   b  is evenly distributed between the emitters  14   b  as represented by an arrow  44 . Similar to the preferred embodiment, the backside vias  20 ′ conduct the heat  44  to a heat sink (item  30 ,  FIG. 5 ). The electrical operation of the HBT  10 ′ is substantially the same as that of HBT  10 . Furthermore, electrical grounding of the emitters  14   b  provided by the backside vias  20 ′ is essentially identical to the electrical grounding provided by the flip-chip bumps  20  in the preferred embodiment. A further difference of the second embodiment as shown in  FIG. 4  is that the metallization layer  18  is optional as emitters  14   b  are both thermally connected and electrically grounded to the heat sink  30 . With this structure, the present invention provides enhanced cooling to functional devices of the HBT such as the emitters  14   b.    
   Please refer to  FIG. 5 .  FIG. 5  is a cross-sectional view of the HBT power amplifier  10 ′ of  FIG. 4  along a section line  5 - 5  shown in  FIG. 4 . The heat sink  30  is a backside metal layer. As shown in  FIG. 5 , backside vias  20 ′ penetrate the substrate  50 . Heat is conducted from the emitters  14   b  through the backside vias  20 ′ and to the metal layer  30  as represented by arrows  46 . The enhanced thermal conduction as provided by the second embodiment of the present invention allows the HBT power amplifier  10 ′ to operate at a substantially high power. 
   Naturally, the present invention as described in the preferred embodiment and the second embodiment, can be applied to an HBT power amplifier having arrays of bases, emitters, collectors, and other functional devices. Fabrication of the present invention HBT power amplifiers  10 ,  10 ′ can be accomplished by currently available semiconductor manufacturing technologies. 
   Generally, the emitter areas are enlarged, as illustrated by emitters  14   b , so that the flip-chip bump  20  or the backside via  20 ′ can be placed depending on the specific application of the HBT amplifier  10 ,  10 ′. An increased amount of flip-chip bumps  20  or backside vias  20 ′ tends to increase thermal efficiency at the expense of device area. Thus, a specific layout to maximize thermal efficiency while minimizing device area is a design choice. In one embodiment, the area of the enlarged emitter  14   b  is greater than the area of the emitter  14   a , and the enlarged emitter  14   b  has a center of area located laterally away from the collector  12  and the base  16 . 
   In contrast to the prior art, the present invention provides an efficient thermal path to the heat sink in the close proximity of the source of heat generation, which is the emitter of the transistor. The emitter electrode provides thermal conduction and electrical grounding to the emitter. The emitter electrode provided can be a flip-chip bump or a backside via. The heat sink can be a metal layer that can be directly disposed on a substrate. For these reasons, the present invention provides a heat conduction path that is more thermally efficient and more cost effective than that provided by the prior art. Accordingly, the present invention power amplifier can operate at a higher current density and associated higher power than a comparable prior art power amplifier. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.