Abstract:
A high-frequency semiconductor device includes a microwave monolithic integrated circuit having first and second passive element sections each having at least one passive element as well as an FET. The FET has a gate connected to the first passive element section and a drain connected to the second passive element section. A bed configured as a plate of a conductive member is provided for mounting thereon the microwave monolithic integrated circuit. The bed has at least one opening or hole formed therethrough. The hole of the bed is provided at a specified position of the bed so as to underlie either one the first or second passive element section. With such an arrangement, it becomes possible to obtain enhanced performance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to high-frequency semiconductor devices and radio transmitter/receiver (transceiver) devices. 
     2. Description of the Related Art 
     Conventional high-frequency amplifiers for amplification of high-frequency signals at frequencies higher than or equal to 1 gigahertz (GHz) may include a transmitter amplifier adaptable for use in portable and mobile digital cellular radiotelephone terminals based on a currently available personal handy phone (PHP) system—in Japan the acronym “PHS” is more popular, so this acronym will be used hereinafter. An exemplary configuration of such a PHS terminal will be explained as follows. 
     In such high-frequency amplifiers, a source-grounded or “common-source” amplifier using GaAs metal semiconductor field effect transistors (MESFETs) has been typically employed today. The power gain per stage is approximately 10 decibels (dB), and the use of a serial combination of multiple—two to four—stages of such amplifiers permits the resultant circuitry to have an increased power gain ranging from 20 dB to 40 dB, more or less. This high-frequency amplifier is commercially available for use as a microwave monolithic integrated circuit (MMIC) in a growing electronics market. For reduction of cost penalties, MMICs are typically mounted or “embedded” in plastic packages, which are low-cost housings. 
     One prior art high-frequency semiconductor device designed for use as a MMIC is shown in FIG.  2 . FIG. 2 is a diagram showing a plan view of a MMIC  10  having a high-frequency amplifier circuitry architecture in a four-stage configuration using four MESFETs with sources grounded, where the MMIC  10  is mounted to the frame of a plastic package. The MMIC  10  is put on a metallic plate  2 , called a “bed” among those skilled in the art to which the invention pertains. 
     The MMIC  10  is configured including FETs  12   1 , . . . ,  12   4 , matching circuits MC 1  to MC 4 , each of which consists of a capacitor and an inductor, and internal connection pads  14   a  to  14   1 ,  14   n.    
     The semiconductor device also includes “external” pins  4   a-   4   n  along with bonding wires  20   a-   20   n.  Pins  4   g,    41 ,  4   m  are connected to the bed  2 . 
     One matching circuit MC 1  has a capacitor MC 1   a  and an inductor MC 1   b  and is connected to pads  14   a,    14   h.  Another matching circuit MC 2  having a capacitor MC 2   a  and an inductor MC 2   b  is connected to a pad  14   i.  Another matching circuit MC 3  is formed of a capacitor MC 3   a  and inductor MC 3   b  and is coupled to a pad  14   j.  The remaining matching circuit MC 4  with a capacitor MC 4   a  and an inductor MC 4   b  is coupled to a pad  14   k.    
     On the other hand, an FET  12   1  at the initial stage (first-stage FET) has its gate connected to the matching circuit MC 1  and its drain connected to the matching circuit MC 2  with a source connected to the pad  14   b.  The second-stage FET  12   2  has a gate connected to the matching circuit MC 2 , a drain connected to the matching circuit MC 3 , and a source coupled to the pad  14   c.  The third-stage FET  12   3  has a gate connected to the matching circuit MC 3  with a drain coupled to the matching circuit MC 4  and with a source coupled to the pad  14   e.  The fourth-stage FET  12   4  has its gate connected to the matching circuit MC 4  and also coupled via a high resistance element to the pad  14   f,  a drain coupled to the pad  14   n,  and a source coupled to the pads  14   g,    141 . An output of the high-frequency semiconductor device is derived from the drain node of FET  124 , i.e. the pad  14   n.    
     The pads  14   a-   14   f  are connected by bonding wires  20   a-   20   f  to pins  4   a-   4   f,  respectively; pads  14   h-   14   k  are connected via bonding wires  20   h-   20   k  to pins  4   h-   4   k,  respectively. The pad  14   g  is connected to the bed  2  via three bonding wires  20   g,  pad  141  is coupled to bed  2  by four bonding wires  201 . As the bed is coupled to the grounded power supply in most cases, the pins  4   g,    41 ,  4   m  are provided as GND-pins. The pad  14   n  is tied via the bonding wire  20   n  to the output pin  4   n.    
     Accordingly, pads  14   g,    141 , which are connected to the source of the final-stage FET  12   4  that is in closest proximity with the source side and thus suffers most significantly from a parasitic inductance problem, are directly connected by bonding wires to the bed  2 . 
     While the bed  2  is typically connected to more than one GND-pin in the way stated above, an inductance does exist at the GND-pins  4   g,    41 ,  4   m  shown in FIG. 2, which results in the bed  2  not being set at the “ideal” GND in terms of high-frequency activities. Hereinafter, this GND which is potentially “floating” from the true GND at a certain impedance determinable in terms of high-frequencies will be referred to as a “virtual” GND. 
     An explanation will next be given of what kinds of problems can occur due to the presence of the virtual GND, rather than the ideal GND. While spiral inductors MC 1   b,  MC 2   b,  MC 3   b,  MC 4   b  are mounted on the MMIC  10  together with metal-insulator-metal (MIM) capacitors MC 1   a- MC 4   a,  their layout areas are significant to the extent that they occupy a major part of a limited chip area. Hence, a coupling capacitance is present between these elements and the bed with a semiconductor substrate laid or “sandwiched” therebetween. 
     FIG. 3 is a pictorial representation of the semiconductor device structure to demonstrate how such extra capacitive components reside therein. As shown, a coupling capacitance Cp 1  exists between a circuit element  31  in MMIC  10  and its bed  2  whereas another coupling capacitance Cp 2  is between an element  32  and bed  2 . Because the bed  2  is potentially “floating” in terms of high frequencies as stated previously, the element  31  and element  32  are electrically coupled together via the capacitances Cp 1 , Cp 2 . Now, if it is assumed that the bed  2  is completely floating in potential from the true GND in the worst case, the element  31  and element  32  could be coupled together via a series-connected capacitor that is formed by the capacitance components Cp 1  and Cp 2 . 
     Such an undesirable capacitive coupling between or among certain elements on the MMIC  10  can result in a variety of kinds of malfunctions and operational failures. Especially, in multi-stage amplifier circuitry with a series combination or “cascade” connection of multiple FETs, the element-to-element capacitive coupling can result in serious problems, such as oscillation. An evaluation was done by the present inventors, described herein, which revealed that the most problematic issue lies in capacitive coupling between an input-side matching circuit of the N-th stage FET and an output-side matching circuit of its neighboring, (N+1) th stage FET (where, N is a natural number). 
     Suppose that an amplifier includes a serial combination of four stages of FETs as shown in FIG.  4 . As shown in FIG. 4, four matching circuits MC 1 , MC 2 , MC 3 , MC 4  are present between an input stage and output stage of such amplifier. Respective of the matching circuits is configured from a spiral inductor and an MIM capacitor, which can be capacitively coupled together through the bed  2  laid therebetween. For clarity purposes, consider the coupling between the matching circuits MC 1 , MC 3  only, which will be represented as a coupling capacitance Cf. 
     FIG. 5 shows the stability of the circuitry in FIG. 4, as obtained by simulation. An amplifier tested is of the class with a gain of 40 dB in the 1.9 GHz band. Supposing that the frequency in question as to the stability falls within a range of from 0.1 GHz to 10 GHz; then, it has been investigated how a minimal value (Kmin) of a stability factor K varies depending upon the feedback capacitance Cf in this frequency range. The result is shown in FIG.  5 . As is apparent from FIG. 5, when Cf goes beyond 13 femto-farads (fF), the value Kmin becomes less than 1 (i.e., Kmin&lt;1) resulting in dissatisfaction of the absolute stability criteria involved. When Cf further increases to exceed 17 fF, the value Kmin is less than zero (Kmin&lt;0), which results in oscillation in a system of 50-Ohm (Ω) input/output impedance. Incidentally, those inductors and capacitors for use in the MMIC  10  inherently have a capacitance relative to the bed, which capacitance is at least in the order of magnitude of several tens of fF. Thus, there must apparently exist the possibility that the Cf value increases up to about 18 fF or greater, which in turn leads to the creation of a problem that the circuitry is hardly stable in operation. Prior art approaches to achievement of the intended stability are merely to reduce the gain of amplifier circuitry, per se. 
     Another problem faced with the prior art is that in the case where the MMIC is an oscillator, any desired oscillation frequency is by no means obtainable even in view of the element-to-element or “interelement” capacitance via the bed. 
     It has been described that prior art high-frequency semiconductor devices are encountered with difficulties in achieving a desired circuit operation unless the inter-element capacitance via the bed is taken into careful consideration, and sometimes faced with a more serious problem as to the inability to attain any desired operation even after consideration of such inter-element capacitance. 
     The present invention has been made in view of the foregoing technical background, and its primary objective is to provide a new and improved high-frequency semiconductor device with an MMIC capable of achieving increased performance. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to address the above described and other shortcomings of conventional devices. As there are various facets of the present invention, only selected features of the present invention are discussed below in this section of the document. However, a more complete discussion of the present invention is provided in subsequent sections. 
     In order to achieve the above-described and other objects, a high-frequency semiconductor device is provided with the following features. A microwave monolithic integrated circuit is included and has a circuit element having first and second passive element sections each having at least one passive element, and a transistor with a gate connected to the first passive element section and a drain connected to the second passive element section. A bed is mounted on the microwave monolithic integrated circuit, and is made of a conductive plate having at least one opening. The opening is provided at a position of the bed underlying at least one of the first passive element section and the second passive element section. 
     It is desirable that the first passive element section be connected to an input side of the transistor whereas the second passive element section be connected to an output side of the transistor. 
     It is also desirable that the opening be formed at a position of the bed underlying one of the first and second passive element sections which is placed on an input side of the transistor. 
     It is desirable that the opening be provided at a position of the bed selectively underlying at least one of the first passive element section and the second passive element section. 
     In addition, a high-frequency semiconductor device in accordance with this invention includes the following features. A microwave monolithic integrated circuit is included that has an amplifier circuit with a cascade connection of a plurality of amplifying elements, each having a matching circuit and a transistor with a gate connected to the matching circuit. A bed is mounted to the microwave monolithic integrated circuit and is made of a conductive plate having at least one opening. It is also desirable that the gate of the transistor be connected to the matching circuit on an input side with a drain of the transistor coupled to the matching circuit on an output side. It is desirable that the opening be formed at a position of the bed underlying at least one of the matching circuit on the input side of one of the amplifying elements with a maximal gain and the matching circuit on the output side thereof. It is desirable that the opening be formed at a position of the bed underlying the matching circuit on the input side of an initial stage one of the amplifying elements. It is desirable that each matching circuit have at least one passive element. Also desirably, the bed selectively has at least one opening under at least one of the matching circuits. 
     In the respective embodiments stated above, it is desirable that the passive element be at least one of a capacitor and an inductor. It is also desirable that the bed with the microwave monolithic integrated circuit mounted thereon be sealed by a resin material. Further, it is desirable that the microwave monolithic integrated circuit be securely fixed to the bed by use of adhesive made of nonconductive materials. 
     Furthermore, a radio transmitter/receiver device in accordance with the present invention includes an antenna unit, a high-frequency switch unit for switching between transmission and reception of the antenna unit, a first amplifier unit for amplifying a reception signal sent from the high-frequency switch unit, a receiver circuit unit for processing the amplified reception signal passed from the first amplifier unit, a transmitter circuit unit for sending forth a transmission signal, and a second amplifier unit for amplifying the transmission signal transferred from the transmitter circuit unit and for sending forth the amplified transmission signal toward the high-frequency switch unit. 
     The above-noted high-frequency semiconductor device is used in at least one of the first amplifier unit and second amplifier unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a plan view of a structure of a high-frequency semiconductor device in accordance with one preferred embodiment of the present invention, 
     FIG. 1B shows a cross-sectional view of the semiconductor device shown in FIG.  1 A. 
     FIG. 2 illustrates a plan view of one typical prior art high-frequency semiconductor device. 
     FIG. 3 shows a perspective view of a semiconductor device structure for explanation of capacitive coupling between adjacent elements with a bed laid therebetween. 
     FIG. 4 shows equivalent circuitry of the high-frequency semiconductor device shown in FIG.  2 . 
     FIG. 5 is a graph showing a relation of a minimal value of stability factor versus a feedback capacitance in the device shown in FIG.  4 . 
     FIG. 6 illustrates a configuration of a radio transmitter and receiver (transceiver) device also embodying the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1A and 1B, there is shown a configuration of a high-frequency semiconductor device in accordance with one preferred embodiment of the present invention. FIG. 1B illustrates a cross-sectional structure of the semiconductor device along the line AA that passes through an opening or hole  6  of a “bed” plate  2  shown in FIG.  1 A. The high-frequency semiconductor device embodying the present invention is a four-stage amplifier, which is configured including the bed plate  2  made of a metallic material along with pins  4   a,    4   b , . . .  4   n  and a microwave monolithic integrated circuit (MMIC)  10  as well as bonding wires  20   a-   20   n.  The four-stage amplifier is encased or “embedded” in a plastic package  11 . In the illustrative embodiment, the bed  2  is provided with an opening or hole  6 . 
     The MMIC  10  is mounted on the bed  2  and has field-effect transistors (FETS)  12   1 - 12   4 , matching circuits MC 1 -MC 4  each including a capacitor and an inductor, and pads  14   a-   141 ,  14   n.    
     One matching circuit MC 1  that has a capacitor MCIA and an inductor MC 1   b  is connected to associative pads  14   a,    14   h.  Another matching circuit MC 2  having a capacitor MC 2   a  and inductor MC 2   b  is connected to a pad  14   i.  Another matching circuit MC 3  including a capacitor MC 3   a  and inductor MC 3   b  is connected to a pad  14   j.  The remaining matching circuit, MC 4 , is formed of a capacitor MC 4   a  and inductor MC 4   b  and is connected to a pad  14   k.    
     On the other hand, the first stage FET  12   1  has a gate connected to the matching circuit MC 1 , a drain connected to the matching circuit MC 2 , and a source connected to the pad  14   b.  The second stage FET  12   2  has a gate connected to the matching circuit MC 2 , a drain connected to the matching circuit MC 3 , and a source coupled to a pad  14   c.  The third stage FET  12   3  has a gate connected to the matching circuit MC 3 , a drain coupled to the matching circuit MC 4 , and a source coupled to a pad  14   e.  The fourth stage FET  12   4  has a gate connected to the matching circuit MC 4  and also coupled to a pad  14   f  via a high-resistance element, a drain connected to pad  14   n,  and a source connected to pads  14   g,    141 . An output of the high-frequency semiconductor device is derived from the drain node of FET  12   4 , that is, the pad  14   n.    
     The pads  14   a-   14   f  are connected via the bonding wires  20   a-   20   f  to pins  4   a-   4   f,  respectively; the pads  14   h-   14   k  are connected via bonding wires  20   h-   20   k  to pins  4   h-   4   k,  respectively. The pad  14   g  is connected to the bed  2  via three bonding wires  20   g;  the pad  141  is coupled via four bonding wires  201  to the bed  2 . As the bed  2  may typically be coupled to the ground power supply, those pins  4   g,    41 ,  4   m  coupled to this bed  2  are provided as GND pins. Additionally, the pad  14   n  is coupled by a bonding wire  20   n  to an output pin  4   n.    
     Accordingly, the pad  14   g  which is connected to the source of the final-stage FET  12   4  that is nearest the source side and thus suffers from a problem of “parasitic” inductance—is directly connected to the bed  2  by the bonding wire  20   g,  whereas the pad  141  is directly coupled to the bed  2  via bonding wires  201 ,  20   m.    
     In the illustrative embodiment, the hole  6  provided in the bed  2  is disposed just beneath the matching circuit MCI to have a rectangular planar shape; simultaneously, hole  6  is sized so that it covers a specified area or region which immediately underlies the matching circuit MC 1 . It would be preferable that the size of this hole  6  be greater than a certain value equivalent to the dimension d of the element (in this embodiment, the matching circuit MC 1 ) added with a value equal to twice of the thickness ds of a substrate constituting this MMIC  10 —i.e. the hole size is greater than d+2ds. 
     With the hole  6  provided in the bed  2  of an element, the resultant capacitance between the bed  2  and the element of interest (matching circuit MC 1  in this embodiment) becomes a fringing capacitance only, which makes it possible to greatly reduce the capacitance between the bed  2  and the element when compared to prior art devices. This in turn enables achievement of well stabilized operations as the amplifier circuitry without reducing the gain thereof. 
     It should be noted that it is preferable to employ electrically nonconductive or dielectric materials as the mounting material (adhesive) for rigidly securing the MMIC  10  to the bed  2 . This can be said because even when the mount material behaves poorly, for example to seep and diffuse into the hole  6 , the inside of hole  6  may remain electrically nonconductive due to the absence of any electrical objects therein. 
     Also while the illustrative hole is of almost the rectangular shape, the length of its shorter sides is substantially equal to the inductor MC 1   b&#39; s one side length plus the value 2·ds whereas the length of its longer sides is set at the matching circuit MC 1 &#39;s length in the longitudinal direction plus 2·ds. With such length setup, the resultant coupling capacitance between an input matching circuit and the bed  2  is reduced to the extend that it is approximately half of that obtained when no holes are present therein. This makes it possible to attain sufficient stability thereby enabling achievement of a gain nearly as high as the maximum gain that the device should inherently offer. 
     An attempt was made to measure the gain by simulation with the high-frequency semiconductor device embodying the present invention used as a transmitter-side amplifier for use in portable and mobile digital cellular radiotelephone handsets based on a personal handy phone (PHP) system, also known as “PHS” in Japan, operable in 1.9 gigahertz (GHz) band, which demonstrated that the resulting gain was 42 dB. In contrast, an amplifier with the prior art amplifier configuration shown in FIG. 2 was as low as 36 dB in gain. This may encourage those skilled in the art to believe that the amplifier embodying the present invention offers higher gain than the prior art devices. 
     An explanation will next be given of a case where the high-frequency semiconductor device embodying the present invention is applied to a radio transmitter/receiver (transceiver) device. One typical configuration of this device is shown in FIG.  6 . The radio transmitter/receiver device  100  includes a high-frequency switch device  109 , an antenna  101 , a low-noise amplifier  102 , a receiver circuit  104 , a transmitter circuit  106 , and a power amplifier  108 . 
     At least one of the low-noise amplifier  102  and power amplifier  108  is designed employing the high-frequency semiconductor device of this invention. The high-frequency switch device  109  is operable to switch between reception and transmission of the antenna  101 . A reception signal received at the antenna  101  passes through this high-frequency switch device  109  to reach the low-noise amplifier  102 . This reception signal transferred is amplified at the low-noise amplifier  102 , and is then sent forth toward the receiver circuit  106 . At the receiver circuit  106  the reception signal transferred is processed thus obtaining desired reception. 
     A transmission operation of the radio transceiver  100  of FIG. 6 is as follows. The transmitter circuit  106  operates to produce a desired transmit signal, which is then sent to the power amplifier  108 . At power amplifier  108  the transmit signal sent thereto is amplified to provide an amplified signal, which is passed via the high-frequency switch device  109  to the antenna  101 . The transmit signal thus transferred is radiated over-the-air from the antenna  101  to the outside. 
     It must be noted that in the illustrative embodiment the bed  2  is provided with only one hole  6 , the bed may alternatively be modified to have a plurality of holes where necessary. When this is done, these holes are provided in those regions underlying the matching circuits of the bed. Also while the above embodiment is arranged so that the hole  6  is provided covering the underlying region of one specific matching circuit on the input side of the initial-stage FET  121 , similar effects and advantages are obtainable in cases where the hole is disposed in the underlying region of either a matching circuit on the input side of an FET of significant gain or a matching circuit on the output side thereof. 
     Although the foregoing description is devoted to one exemplary multi-stage amplifier, the present invention may also be effectively applicable to other similar device including, but not limited to, any type of MMIC with on-chip inductors and capacitors of large layout areas. In other words, providing the bed with one or more openings at specified portions underlying certain MMIC&#39;s on-chip elements that suffer from a capacitance coupling problem may avoid any undesired capacitive coupling through the bed, which in turn makes it possible to achieve the intended performance as high as possible. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.