Patent Publication Number: US-6670674-B2

Title: Transistor and power amplifier with improved bandwidth

Description:
TECHNICAL FIELD 
     The present invention relates to a Laterally Diffused Metal Oxide Semiconductor (LDMOS) transistor which can be connected to other devices in such a way that the resulting total device has a high bandwidth and high output power with a small surface area. 
     BACKGROUND ART 
     When connecting LDMOS transistors to other circuits, it is of great importance that the transistor can be matched to the impedance of the circuits to which it is connected, for example in order to achieve high power, high efficiency and high linearity. LDMOS transistors are currently constructed as discrete chips that can be matched to the surrounding circuits by means of, for example, matching networks on circuit boards. The connection of the LDMOS transistor to the surrounding circuits is carried out by the use of so-called bonding wires, which severely restricts the bandwidth in the resulting total circuit, depending upon the length of the bonding wires that is required by known technology. 
     DISCLOSURE OF INVENTION 
     There is thus a need to be able to match an LDMOS transistor to the impedance of other circuits in a way that does not restrict the bandwidth to the same extent as in previously known solutions. 
     This need is met by the present invention by providing an LDMOS transistor comprising gate, source and drain, and an earth plane located under these, where the transistor&#39;s earth plane has been given such an extent in the lateral direction of the transistor that an additional component of chip type, for example a matching network or a power amplifier, can be mounted on it, arranged in such a way that the LDMOS transistor and the additional component of chip type have a shared earth plane, while at the same time the LDMOS transistor and the other component(s) are so close to each other that the length of the connections does not restrict the bandwidth in the same way as the connections in previously known solutions. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will be described in greater detail below, with reference to the following drawings, in which 
     FIG. 1 shows a first embodiment of the invention, and 
     FIG. 2 shows an equivalent circuit diagram for the device in FIG. 1, and 
     FIG. 3 shows a second embodiment of the invention, and 
     FIG. 4 shows an equivalent circuit diagram for the device in FIG.  3 . 
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     FIG. 1 shows a first way of realizing the invention: An LDMOS transistor  110  is realized in a known way by doping a silicon substrate  120 . The LDMOS transistor  110  will comprise source, drain and gate, in addition to an earth plane located below these, which earth plane consists of a part of the silicon substrate in question that has been doped so that it has become conductive and can thereby constitute an earth plane. The part of the substrate that is to constitute the earth plane is suitably but not necessarily doped with boron such that the substrate has a resistance that lies suitably in the range 8-30 mohmcm. 
     As shown in FIG. 1, the part of the substrate  120  on which the LDMOS transistor has been arranged only comprises a part of the total substrate. According to the invention, the part of the substrate that has been doped in order to constitute the earth plane in the transistor  110  extends beyond what is required for the transistor alone, whereby a space has been created, enabling an additional component  130  to be placed on this extended earth plane. The additional component  130  may be thus mounted on the earth plane created, whenever suitable. 
     The additional component  130  can already have its own earth plane before it is placed on the substrate  120 , which is shown in FIG. 1 by the designation  160 , or else the earth plane consisting of the extended substrate  120  can constitute the only earth plane for the component  130 . 
     The component  130  can be attached to the extended earth plane of the substrate in a large number of ways, including gluing with silver epoxy or eutectic bonding. After being attached to the substrate  120 , the component  130  and the LDMOS transistor will have a common earth in the earth plane on the substrate, which is advantageous when the component and the transistor are to work together. The additional component  130  is connected to the LDMOS transistor by means of bonding wires  140 ,  150 , but due to the invention, the bonding wires (which can, of course, be more or fewer than two in number, the number shown in FIG. 1 being only an example) can be made considerably shorter than what has previously been possible, which greatly increases the available bandwidth for the resulting combination of transistor and component. 
     The additional component  130  can of course, in principle, be of any type, but can, for example, be a power amplifier of the Gallium Arsenide (GaAs) type that is suitably implemented in Monolithic Microwave Integrated Circuit (MMIC) technology, or a matching network, or a combination of these. 
     FIG. 2 shows an equivalent circuit diagram for the LDMOS transistor  110  and the additional component  130  from FIG. 1, in which the component  130  is shown as an amplifier combined with a matching network. In this case, the additional component is suitably a chip that comprises all the components that are to be connected to the transistor  110 . Thus the additional component may be referred to as a component of chip type. 
     A further variant within the frame of the present invention is that the part of the earth plane of the transistor  110  extending beyond what is actually required for the transistor is given such an extent that it can be used for more than one component in addition to the transistor. Such an embodiment is shown in FIG. 3, where parts that are also included in the embodiment in FIG. 1 have been given the same reference numbers as corresponding parts in FIG.  1 . Unlike the embodiment in FIG. 1, however, the “earth plane part” of the substrate  120 , in other words the conductive part of the substrate, has been extended so that not only the component  130  can use it as earth plane together with the LDMOS transistor  110 , but also an additional second component  330  can utilize the part of the substrate  120  that has been doped to become conductive in order to constitute an earth plane. In other words, the LDMOS transistor  110 , the first additional component  130  and the second additional component  330  have thereby a shared earth plane such that they can also be placed close to each other with short connection distances, being suitably connected by bonding wires in accordance with what has been described in connection with FIG.  1 . The component  330  is connected to the LDMOS transistor  110  by means of bonding wires  340 ,  350 , which number of bonding wires is only to be regarded as an example. The second component  330  may be thus mounted on the earth plane created, whenever suitable. 
     As with the first additional component  130 , the second additional component  330  can, in principle, be of any type, but can, for example, be a matching network. FIG. 4 shows an equivalent circuit diagram for the embodiment in FIG. 3, where the second additional circuit  330  has been exemplified by a matching network. In this way, by means of the invention, an LDMOS transistor  110  can be connected to an amplifier  130  via a matching network  130  and a matching network  330  on the output of the transistor, the whole of the resulting circuit  130 ,  110 ,  330  being able to be realized with a shared earth plane and short connections, whereby a device is obtained that is compact and has a broadband function, fulfilling the aim of the invention. 
     Both the first and second additional component may be referred to as components of chip type.