BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to variable capacitance components formed in a semiconductor substrate. 
   2. Discussion of the Related Art 
   A component having a capacitance varying in accordance with a voltage reference is used, for example, in a voltage-controlled oscillator (VCO). 
     FIG. 1  partially and schematically illustrates a perspective view of a first electrode of a variable capacitance formed in a lightly-doped N-type silicon substrate  1 . Substrate  1  comprises a series of identical regularly spaced ribs  2  each having a rectangular planar base. 
     FIG. 2  illustrates, in a partial simplified lateral cross-section view, a Schottky diode using the first electrode of FIG.  1 . The silicon surface of the first electrode may be completed by a heavily-doped N-type surface region  3 . Region  3  is continuous and extends in the upper planar surface of ribs  2 , along their vertical walls and in the planar surfaces of substrate  1  separating the bases of neighboring ribs. A layer  4  of a conductive material capable of forming a Schottky barrier, such as a metal silicide, covers the entire structure. The Schottky diode thus comprises a first electrode (cathode) formed by substrate  1  comprising ribs  2  and a second electrode (anode) formed by silicide  4 . The diode is generally completed by metallizations (not shown) contacting, at their rear surface, substrate  1  and, at their front surface, silicide  4 . 
   In a reverse biasing (positive voltage applied on the cathode), the diode of  FIG. 2  behaves as a capacitor, the inter-electrode insulator of which is the space charge area (or depleted area) which extends in region  3 , if present, and substrate  1 . 
     FIG. 3  is a graph partially and schematically illustrating variation C(V) of capacitance C of the diode of  FIG. 2  versus value V of the reverse voltage. Capacitance C drops when value V increases. From a given limiting value or pinch-off voltage Vp, capacitance C abruptly varies towards a low value Vth. Pinch-off voltage Vp is the value for which the extension of the depleted area in substrate  1  is such that the volume of ribs  2  is completely depleted by the carriers. Value Vth corresponds to the limiting value of extension of the depleted area into substrate  1 . 
   A disadvantage of a variable capacitance such as previously described is the discontinuity in the capacitance variation around pinch-off voltage Vp. Due to this discontinuity, the capacitance (frequency) range corresponding to a reverse biasing beyond pinch-off voltage Vp has to be, in practice, excluded from the operation, which does not enable sweeping a wide frequency range in an application to a VCO. 
   For a VCO to operate over a wide frequency range, several distinct devices must thus be used to enable frequency tunings on different frequency ranges. This goes against the desire to reduce the size of devices. 
   The problems previously discussed for a Schottky diode also arise if a reverse-biased PN junction is used as a variable capacitance. As compared to the structure of the Schottky diode of  FIG. 2 , a very heavily-doped P-type surface region is formed in heavily-doped N-type region  3 . The presence of the P-type region further increases the complexity of the setting of the doping of N-type region  3 , if present. It should be reminded that region  3  is optional; the heavily-doped P-type region may thus replace it. 
   The previously-discussed problems generally arise in any variable capacitance. 
   SUMMARY OF THE INVENTION 
   The present invention aims at providing a variable capacitance having a uniform variation according to voltage, with no abrupt variation. 
   The present invention aims at providing such a capacitance which enables sweeping an extended capacitance range. 
   To achieve these and other objects, the present invention provides a variable capacitance formed in a semiconductor substrate with a ribbed surface, having a first electrode formed of all the ribs protruding from the substrate, of portions of the substrate underlying the ribs, and of at least portions of the substrate separating the bases of two ribs, having a second electrode superposed to at least one portion of the first electrode. The ribs are irregular in terms of cross-section and/or planar base surface area. 
   According to an embodiment of the present invention, two successive ribs exhibit non-parallelepiped shaped trapezoidal bases of same surface areas. 
   According to an embodiment of the present invention, two successive ribs are arranged in quincunx. 
   According to an embodiment of the present invention, two successive ribs exhibit trapezoidal bases of different surface areas. 
   According to an embodiment of the present invention, the rib bases are rectangular. 
   The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial simplified perspective view of a conventional electrode of a variable capacitance; 
       FIG. 2  illustrates in a partial simplified transversal cross-section view of a Schottky diode using the first electrode of  FIG. 1 ; 
       FIG. 3  is a graph illustrating the variation of the capacitance versus the reverse bias voltage of the diode of  FIG. 2 ; 
       FIG. 4  illustrates, in a partial simplified perspective view, an embodiment of an electrode of a variable capacitance according to the present invention; 
       FIG. 5  is a graph illustrating the capacitance variation versus the voltage applied across a capacitor comprising an electrode having the structure of  FIG. 4 ; 
       FIG. 6  illustrates, in a partial simplified perspective view, another embodiment of an electrode of a variable capacitance according to the present invention; and 
       FIG. 7  is a graph illustrating the capacitance variation versus the voltage applied across a capacitance comprising an electrode having the structure of FIG.  6 . 
   


   DETAILED DESCRIPTION 
   For clarity, the same elements have been referred to with the same reference numerals in the different drawings and, further, the drawings are not to scale. 
   A feature of the present invention is to replace an electrode comprising ribs of regular geometry with an electrode comprising ribs of irregular geometry regarding their cross-section, or their surface area, or both. 
     FIG. 4  illustrates in a partial simplified perspective view an embodiment of an electrode  10  of a variable capacitance according to the present invention. 
   Electrode  10  is formed in a semiconductor substrate  11 , for example, silicon, lightly doped of a first conductivity type, for example N. It should be noted that substrate is used to designate as well a uniformly-doped single-crystal silicon wafer as epitaxial regions and/or regions specifically doped by diffusion/implantation formed on or in a solid substrate. Electrode  10  is more specifically formed of a set of ribs  12  which exhibit a trapezoidal base having its two parallel sides of different lengths, L 1  and L 2 . Ribs  12  are thus irregular in terms of cross-section. 
   According to an embodiment of the present invention, when ribs  12  are irregular in terms of cross-section, they are, as illustrated in  FIG. 4 , regular in terms of surface area, that is, all identical. 
   Preferably, as illustrated in  FIG. 4 , ribs  12  are arranged in quincunx in substrate  11 . Thus, the surface area taken up by two neighboring ribs  12  is reduced as compared to the surface area that they would take up if their coplanar parallel sides exhibited the same length L 1  or L 2 . 
   Such an electrode  10  is then usable to form a variable capacitance. Thus, in a Schottky diode, electrode  10  can be completed by a doped surface region of the same conductivity type as substrate  11 , but more heavily doped. The Schottky diode is completed by an anode formed by a conformally superposed layer capable of forming a Schottky barrier. 
     FIG. 5  is a graph which partially and schematically illustrates variation C(V) of capacitance C of such a Schottky diode according to the reverse voltage V applied thereacross. 
   As compared to homologous curve C(V) of  FIG. 3  comprising a conventional electrode illustrated in  FIG. 1 , variation C(V) obtained with an electrode  10  according to the present invention is more uniform. This improved uniformity especially translates as a suppression of the capacitance discontinuity around a pinch-off voltage. This improved uniformity is allowed by the fact that ribs  12  are progressively depleted for successive voltages and not abruptly around a threshold. 
   The possible variation range of the voltage reference of a VCO is then extended with respect to the case described in relation with  FIGS. 1 and 2 . It is thus possible to securely sweep a wider frequency range. The present inventor&#39;s studies have enabled showing that, for a small voltage range, for example, between 0 and 3V, the minimum capacitance is on the order of from one tenth to one twentieth of the maximum capacitance. Further, this ratio directly depends on the rib dimensions. 
     FIG. 6  illustrates, in a partial simplified perspective view, another embodiment of an electrode  20  of a variable capacitance according to the present invention. 
   Electrode  20 , formed in a substrate  21 , comprises ribs  22 ,  23 ,  24 , and  25  having parallelepiped-shaped trapezoidal bases, for example, rectangular, but of irregular widths L 3 , L 4 , L 5 , and L 6 . Ribs  22 ,  23 ,  24 , and  25  are thus regular in cross-section and irregular in surface area. 
   Then, as schematically and partially illustrated by the graph of  FIG. 7 , a curve of the capacitance variation according to voltage V which is uniform by steps is obtained for a variable capacitance comprising electrode  20  of FIG.  6 . Each step corresponds to the pinching, at the total depletion, of ribs  22 ,  23 ,  24 , and  25  having a given width L 3 , L 4 , L 5 , and L 6 . The variation at the end of each step is limited by the neighboring presence of non-depleted ribs. 
   According to an embodiment not shown, an electrode of a variable capacitance may comprise ribs simultaneously irregular in cross-section and in surface area. As it is irregular in cross-section, each rib has a trapezoidal base such that its parallel supports have different lengths. As they are irregular in surface area, the bases of different ribs exhibit different surface areas. 
   The present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, specific shapes and arrangements of the ribs have been considered in the description of the present invention. However, those skilled in the art will know that the shape of the trapezoidal ribs and their number may be adjusted according to constraints linked to the specific manufacturing process. 
   Further, a variable capacitance formed as a reverse-biased Schottky diode has been considered in the foregoing description of the present invention. However, the present invention also applies to any other forming of a variable capacitance in which the insulating area is formed by a space charge area. Thus, the variable capacitance may be formed as a reverse-biased PN junction. Similarly, the capacitance may be formed by the stacking of any one of the previously-described trapezoidal base electrodes, of a MOS-type insulator (oxide), and of a second conductive electrode. 
   The variable capacitance may be formed of the association of several variable capacitances. For example, capacitances are formed on the planar upper trapezoidal surfaces of the ribs of the first electrode from a junction, either of Schottky type, or of PN type, and MOS-type capacitances are formed on the lateral walls of the ribs and between two ribs. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.
A variable capacitance formed in a semiconductor substrate with a ribbed surface, having a first electrode formed of all the ribs protruding from the substrate, of portions of the substrate underlying the ribs, and of at least portions of the substrate separating the bases of two ribs, having a second electrode superposed to at least one portion of the first electrode. The ribs are irregular in terms of cross-section and/or planar base surface area.