A FIXTURE FOR MEASURING THE STATIC CHARACTERISTICS OF MICROWAVE 3-TERMINAL ACTIVE COMPONENTS

Abstract: The fixture for measuring the static characteristics of microwave active components comprises mechanical devices for supporting two dielectric substrates. Two terminals of the components under test are separately connected to electrical circuits constructed on said dielectric substrates. This fixture can be adapted to geometric sizes of component case and allows it to be submitted to thermal tests, without mechanical stresses due to expansion. The electrical circuit constructed on the dielectric substrates ensures the circuit stability avoiding spurious oscillations during static characteristic measurement.

Description: Description The present invention refers to measurements of the electrical characteristics of 3-terminal active-components for microwave circuits and more particularly it concerns a fixture for measuring such static characteristics. Measurable components can be gallium arsenide (GaAs) or silicon (Si), FET or bipolar transistors, for small or large signals, encapsulated in their case or as chips, etc. Measurements are aimed at obtaining output, input or mutual characteristic curves (drain current I D versus of drain voltage V D for different gate voltages V G , gate current I G versus gate voltage V G or drain current I D versus gate voltage V G for different drain voltages V D , in case of FETs), all under static conditions, i.e. in dc current. This type of measurement proves useful both to determine the electrical behaviour of the component, and to predict its behaviour from the reliability standpoint. In fact it allows bias current and voltage measurements for a particular quiescent operating point and their possible variations under extreme thermal conditions. A good execution of said measurements presents however some difficulties, chiefly due to the rise of spurious oscillations in the component under test, which not only alter the voltage and current values but can also cause component destruction. These spurious oscillations preferably take place when a high current flows through the component, since in these conditions transconductivity value increases and hence the whole measuring circuit gain increases also. Low intrinsic capacitances of the component and those between the terminals and connections to bias networks are sufficient to cause a positive reaction to a determined frequency. It is to be added that these components, meant to operate in the microwave domain, present very high cutoff frequencies (even 30 GHz) and hence extremely high gains at very low frequencies, in correspondence of which spurious oscillations can take place. Measurement circuit is then to be stabilized within a wide frequency range, practically starting from dc current, so as to obtain reliable measured values and avoid the risk of destroying the component. In case of FET, the destruction is generally due to the failure of the gate junction, namely a Schottky type junction, directly biased when wide amplitude oscillations are present. Different methods are presently used in laboratories to avoid spurious oscillations in the test circuit and hence to overcome the disadvantages above. A first method uses series resonant circuits and RC circuits, placed in parallel with the output circuit, which operate as loads at the spurious oscillation frequency. This method however is not of immediate use since these circuits have a frequency-selective behaviour; in addition, since they are implemented with a lumped elements, the previous knowledge of the frequency of the spurious oscillation is required in order to obtain the tuning of the above circuits for each type of active component. Another method consists of inserting a stabilizing resistance in series with the input circuit, connecting it as close as possible to the component itself in order to reduce as much as possible reactive parasitic parameters. This resistance reduces the stage gain in a very wide frequency band, avoiding spurious oscillations. Yet it allows neither direct voltage measurement at the input port, nor current measurement through the output port when a considerable current amount flows through the input port. In fact, in this case, voltage drop across the stabilizing resistance is considerable, that is why the voltage measured at the output terminal and the corresponding output current are considerable. A further method consists in surrounding the test component with electromagnetic wave absorbent materials, such as certain graphite-loaded spongy materials. It is clear that in this case encumbrance is considerable, especially if the number of test fixtures is high, and effectiveness is poor especially at the lowest frequencies, where the active component is more unstable. In the article "Handling Automation and Temperature Control Added to Transistor S-Parameter Measurement" by R.E. Markley in "26th Electronic Components Conference", San Francisco, 26-28 April, 1976, pages 55 to 60, the author describes a handling and contacting mechanism, which can be used both for RF and DC measurements, but only when the active component to be tested is already contained in its package. There is no provision for measuring chip components of different sizes. Moreover, no mention is made to any arrangement for suppressing low frequency oscillations. The above disadvantages are overcome by the fixture for measuring the static characteristics of microwave active components, provided by the present invention, which allows contemporaneous measurement of currents and voltages at input and output ports up to the maximum permitted values, without any danger of spurious oscillations, with reverse and direct bias. It does not use narrow-band components and hence its effectiveness extends from dc current to the maximum operation frequency of the component without needing any tuning. It has reduced size and is easy to make. The present invention provides a fixture for measuring the static characteristics of the microwave active components as described in claim 1. The foregoing and other characteristics of the present invention will be made clearer by the following description of a preferred way of embodiment thereof, given by way of non limiting example, and by the annexed drawings wherein: Fig. 1 is an exploded view of the supporting mechanical part; Fig. 2 shows one of the two dielectric substrates whereupon the electrical circuit is constructed; Fig. 3 is an equivalent electrical diagram of the measurement circuit. In the following example the active component under measurement is supposed to be a microwave FET, supplied with gate, source and drain terminals. The supporting mechanical part, shown in Fig. 1, supports two dielectric substrates whereupon electrical circuits are constructed and separately connected to the gate and drain of the FET under test. It can be adapted to the geometrical sizes of the component case and allows the FET to be submitted to thermal tests avoiding too great stresses due to expansion. It consists of the following parts: two metallic supporting blocks SU1 and SU2, which are respectively screwed to two flanges FL1 and FL2, provided with holes FO1 and FO2 for housing two electrical connectors with noise-suppressing filters for the application of bias voltages to the FET; a metallic intermediate block IN allowing the two supporting blocks to be connected and an intermediate groove to be obtained for housing with accuracy the FET case. To this aim the intermediate block is made every time with width and height suited to match the dimensions of the case of the FET under test. Namely the width will be more or less equal to the case width and the height will be such as to allow FET terminals to easily rely on the electrical circuit lines. The FET can be fixed on the intermediate block by screws screwed into holes FT′ and FT″, while holes FP′ and FP″ allow a pressing element PR, described hereinafter, to be fastened. This intermediate block is also designed to guarantee the electrical contact to FET source and the thermal contact to the whole component; two small plugs SP′ and SP″ to align the two supporting blocks and the intermediate block when they are blocked by screw V, passing through SU2 and IN and screwed into threaded hole FF of SU1; two plates PO1 and PO2, screwed to supporting blocks SU1 and SU2, support the already-mentioned dielectric substrates CI1 and CI2. CI1 and CI2 are fastened by adhesives conducting the electrical current to ensure an effective ground. Plates PO1 and PO2 are meant to adapt thermal expansions of supporting blocks SU1 and SU2 to thermal expansions of dielectric substrates CI1 and CI2, in order to avoid excessive mechanical stresses during heating cycles occurring while measuring the FET under test. The material PO1 and PO2 are made of is then to present a thermal expansion coefficient which is intermediate between that of the plates and that of the dielectric substrates: e.g. if the supporting blocks are made of brass and the substrates of alumina, PO1 and PO2 can be made of an iron-nickel-cobalt alloy, the so-called kovar; the already-mentioned pressing element PR is made of the same material as plates PO1 and PO2 and supplied with two insulating bars SE1 and SE2, adapted to press FET gate and drain terminals onto substrates CI1 and CI2 to ensure their electrical contact without requiring soldering. The electrical circuit constructed onto each one of substrates CI1 and CI2 can be seen in Fig. 2. On the dielectric substrate, made of quartz, alumina, beryllium oxide or the like, there are deposited on one side a ground conductive plane and on the other some conductive and resistive lines. More particularly, line PC is made of conductive material, e.g. of gold, and is equipped on one end with a pad for the connection to the power-supply connector. On the other end the gate or drain of the FET under test is placed in contact. Two resistive lines RP' and RP'', each equipped with two branches perpendicularly arranged, are placed in contact with both edges of line PC. Said lines can be made of suitable resistive substances, e.g. TaN or others. In symmetrical position with respect to line PC and parallel to it, there are two more conductive lines PL' and PL''. Said lines are also placed in contact with the ends of the branches perpendicular to the resistive lines RP' and RP''. Lines PL' and PL'' are in turn connected to capacitor terminals (not shown in the Figure), which are grounded through plates PO1 and PO2 (Fig. 1). The lateral ends of resistive lines RP' and RP'' result grounded in ac current and insulated in dc current. This circuit can be constructed with the thin film technique. Let's now examine the electrical operation of the fixture with reference to the equivalent electrical circuit shown in Fig. 3. The resistive lines parallel to the conductive lines are equivalent to sets of resistances A' and A'', the perpendicular lines to sets of resistances B' and B'', in addition there are sets of grounded capacitors C' and C''. Each set of capacitors consist of at least a high capacitance capacitor and one of low capacity, so as to reduce parasitic reactance both at low and high frequencies. The equivalent high-frequency resistances seen from FET gate or drain towards the ground can thus be conveniently set to about 50 ohm. This value has been obtained by two resistive parallel branches, but it could also be obtained by three or more branches of less width. This combination of conductive and resistive lines allows a great stability of the measuring circuit to be obtained by damping possible oscillations. In fact, a possible wave generated by the transistor is progressively attenuated along resistive lines and reflected wave formation is prevented since there are no considerable discontinuity points. These points would certainly be formed if lumped resistances were connected to FET gate and drain. Said resistances, therefore, could not ensure circuit stability. To evaluate the impedance matching between conductive lines and drain and gate terminals of the transistor under test, measurements have been made over a wide range of frequencies. More particularly, the reflection coefficient seen by transistor terminal in the most disadvantageous condition has been measured, i.e. the condition in which the connector with noise-suppressing filter does not result connected to the power-supply source: such a coefficient has resulted inferior to 0.1 (-20 dB) in range within 3 and 26 GHz. As known, reflection coefficients causing transistor instability present values in modulus which are generally much higher than the above-mentioned value. It is clear that what described has been given by way of non limiting example. Variations and modifications are possible without going out of the scope of the invention, as defined by the claims.

Claims: A fixture for measuring the static characteristics of microwave 3-terminal active components, comprising supporting mechanical devices for supporting two dielectric substrates (CI1, CI2) carrying electrical circuits connected separately to input and output terminals of the component under test and a pressing element (PR) for pressing the component input and output terminals onto the dielectric substrates (CI1, CI2) to ensure their electrical connection without soldering, characterized in that it comprises the following parts: two metallic supporting blocks (SU1, SU2) which are respectively screwed to two flanges (FL1, FL2) provided with holes (FO1, FO2) for housing two electrical connectors with noise-suppressing filters for the application of bias voltages to the component; a metallic intermediate block (IN) allowing the two supporting blocks to be connected and an intermediate groove to be obtained for housing, with accuracy, the component, fastened by screws, two holes (FP', FP'') in the intermediate block allowing the pressing element (PR) to be fastened, this intermediate block having also the task of ensuring the electrical contact to the common terminal of the component and the thermal contact to the whole component; two small plugs (SP', SP'') to align the two supporting blocks and the intermediate block when they are blocked by a screw (V), passing through a supporting block (SU2) and through the intermediate block (IN) and screwed into a threaded hole (FF) of the other supporting block (SU1); two plates (PO1, PO2), screwed to the supporting blocks (SU1, SU2), to support the dielectric substrates (CI1, CI2) and adapt the thermal expansions of the supporting blocks (SU1, SU2) to the expansions of the dielectric substrates (CI1, CI2); the pressing element (PR), equipped with two insulating bars (SE1, SE2) for pressing onto the component terminals; the two dielectric substrates (CI1, CI2), each one of them carrying on one side a ground conductive plane and on the other the following lines: a. a median line (PC), made of conductive material and equipped at one end with a pad for the connection to a power-supply connector; b. two resistive lines (RP', RP'') ajacent to the median line (PC), to which they are electrically connected along the edges, and supplied with resistive branches placed perpendicularly; c. two conductive lines (PL1, PL''), placed in symmetrical position with respect to the median line (PC) and parallel to it and in connection with the ends of the perpendicular branches of the resistive lines (RP', RP''). A fixture as in claim 1, characterized in that said conductive lines (PL', PL'') are in turn connected to the terminals of capacitors grounded through said plates (PO1, PO2). A fixture as in claim 1 or 2, characterized in that said plates (PO1, PO2) are made of a material whose thermal expansion coefficient is intermediate between that of the supporting blocks (SU1, SU2) and that of dielectric substrates (CI1, CI2). A fixture as in any of claim 1 to 3, characterized in that said metallic intermediate block (IN) has a width nearly equal to that of the component case and a height allowing an easy connection of the component input and output terminals to the median line (PC) of the dielectric substrates (CI1, CI2).

Inventors: ANGELUCCI, ANGELO, BUROCCO, ROBERTO, TITINET, GIANNI CLERICO
Applicants: CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1989
Main classification: G01R31
IPC codes: G01R31, G01R1