Abstract:
A semiconductor variable capacitance diode comprising an insulating layer interposed between a laminated body of a first conductivity type semiconductor layer and a second type conductivity semiconductor layer and an electrode. An application of forward bias voltage to the electrode induces a variable capacitance proportional to the applied forward bias voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor variable capacitance diode. 
     2. Description of Related Art 
     A MOS Varactor having a MOS structure and a P-N junction diode having a P-N junction structure have been known as a semiconductor variable capacitance diode. The present invention relates to the latter. 
     Said P-N junction diode comprises a laminated body consisting of a P-type semiconductor layer 1 and an N-type semiconductor layer 2 and electrodes 4, 5 provided on both sides of said laminated body, as shown in FIG. 1. The P-N junction diode is used with a backward voltage applied, as shown in the drawing. A junction capacitance of this P-N junction diode is changed depending upon said applied voltage, as shown in FIG. 2. Said capacitance is inversely proportional to a square root of the voltage. It is the reason why the capacitance is changed that an extension of a depletion layer of P-N junction is dependent upon the applied voltage. Since said extension of said depletion layer resulting from a change of the applied voltage is limited, a ratio of a change of the capacitance to said change of the voltage can not be increased. In order to solve this problem, for example, a device, in which a gradient of a distribution of concentration of impurities in said P-type semiconductor layer 1 and said N-type semiconductor layer 2 is steepened (density the junction side) has been adopted but it has not been sufficient. 
     In addition, though it is desirable for a variable capacitance diode to have a large performance index Q, Q can not be increased because the application of backward voltage naturally increases its series resistance. 
     Furthermore, the application of backward voltage naturally produces a small leakage current. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an ideal semiconductor variable capacitance diode showing an increased ratio of a change of a capacitance to a change of a voltage, an increased performance index Q and a reduced leakage current and a semiconductor device using the same. 
     It is another object of the present invention to provide a semiconductor variable capacitance diode capable of increasing capacitance with an increase of an applied voltage and a semiconductor device using the same. 
     The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic sectional view showing the conventional semiconductor variable capacitance diode; 
     FIG. 2 is a capacitance-voltage characteristic diagram of the conventional variable capacitance diode; 
     FIG. 3 is a schematic sectional view showing a first preferred embodiment of a semiconductor variable capacitance diode according to the present invention; 
     FIG. 4 is a schematic sectional view showing a connecting condition of said semiconductor variable capacitance diode according to the present invention with a power source; 
     FIG. 5 is an energy band diagram under the condition shown in FIG. 4; 
     FIG. 6 is a capacitance-voltage characteristic diagram of the diode according to the present invention; 
     FIG. 7 is a schematic sectional view showing a second preferred embodiment of a semiconductor variable capacitance diode according to the present invention; and 
     FIG. 8 is a schematic sectional view showing a connecting condition of said semiconductor variable capacitance diode according to said second preferred embodiment of the present invention with a power source. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 is a schematic sectional view showing a semiconductor variable capacitance diode. Said semiconductor variable capacitance diode comprises a laminated body consisting of a P-type semiconductor layer 1, a N-type semiconductor layer 2 and a dielectric layer 3 and electrodes 4, 5 provided on both sides of said laminated body. 
     The semiconductor variable capacitance diode according to the present invention is connected with a power source 6 forwardly, as shown in FIG. 4. Holes 7 are injected from said P-type semiconductor layer 1 into said N-type semiconductor layer 2 in the known manner by connecting the semiconductor variable capacitance diode with said power source 6 forwardly. FIG. 5 shows an energy band structure of the diode according to the present invention in the case where a forward voltage is applied, as shown in FIG. 4. Said holes 7 injected from the P-type semiconductor layer 1 into the N-type semiconductor layer 2 arrive at a boundary surface of the N-type semiconductor layer 2 and said dielectric layer 3. Accordingly, a capacitor comprising the holes 7, the dielectric layer 3 and said electrode 4 is formed. A forward electric current of a P-N junction diode consisting of a P-type semiconductor layer and an N-type semiconductor layer is exponentially increased with an increase of said applied voltage, as known. According to the construction of the present invention, a diode current does not flow because the dielectric layer is disposed between the electrodes 4, 5, but the quantity of the holes 7 is exponentially increased with said increase of the applied voltage similarly. Since the number of the holes 7 corresponds to an area of a capacitor electrode on one side of the dielectric layer 3, said number of the holes 7 is nearly directly proportional to a value of capacitance. Accordingly, as shown in FIG. 6, said capacitance is exponentially changed with a change of the voltage. This shows that the ratio of a change of capacitance is remarkably increased in comparison with the conventional variable capacitance diode in which the capacitance is inversely proportional to the square root of the voltage. 
     And, the semiconductor variable capacitance diode according to the present invention shows characteristics that the value of capacitance is increased with an increase of the applied voltage which is of opposite polarity to that of the conventional variable capacitor diode. Moreover, in principle, the value of capacitance can be increased up to the quantity desired with an increase of the holes 7, in short an increase of the voltage. 
     Next, as to the series resistance of such diodes, since the forward voltage is applied, said series resistance is merely a forward resistance. Accordingly, the value of resistance is remarkably reduced in comparison with the conventional variable capacitance diode in which the backward voltage is applied. Thus, said performance index Q is greatly increased in comparison with the conventional one. 
     Next, as to a leakage current, since the dielectric layer 3 is disposed between the electrodes 4, 5, no current other than a current for charging the capacitor flows between the electrodes 4, 5. 
     The semiconductor variable capacitance diode according to the present invention or a semiconductor device including one connected with the power source 6 in the form as shown in FIG. 4 has highly superior characteristics as described above. 
     A method of producing such the semiconductor variable capacitance diode will be below described. 
     A silicon P-type semiconductor substrate having thickness of about 100 μm and a specific resistance of 1 to 6Ω.cm, preferably 2 to 6Ω.cm, is used as the P-type semiconductor layer 1 and a silicon N-type semiconductor having a specific resistance of 1 to 6Ω.cm, preferably 1 to 5Ω.cm, is formed with a thickness of 10 μm to be used as the N-type semiconductor layer 2. 
     Then, a SiO 2  layer with a thickness of 2,000 Å is formed on the N-type semiconductor layer 2 by the thermal oxidation method to be used as the dielectric layer 3. The electrode 4 is formed on an upper surface of the dielectric layer 3 by the vapor coating method while the electrode 5 is formed on a surface of the P-type semiconductor substrate by the vapor coating method. 
     The characteristics shown in FIG. 6 are the results of measurement in the above described preferred embodiment in which said area of the electrodes 4, 5 is 0.25 cm 2 . 
     FIG. 7 shows another preferred embodiment in which relation of the P-type semiconductor layer 1 and the N-type semiconductor layer 2 to the dielectric layer 3 is opposite to that in the preferred embodiment shown in FIG. 3. That is to say, the electrode 5, the N-type semiconductor layer 2, the P-type semiconductor layer 1, the dielectric layer 3 and the electrode 4 are laminated in the described order. In this case, the power source 6 is connected so that the electrode 4 may be positive and the electrode 5 may be negative to apply the forward voltage, as shown in FIG. 8. 
     Electrons, which have been injected from the side of the N-type semiconductor 2, are collected on a boundary surface of the dielectric layer 3 and the P-type semiconductor layer 1 to form a capacitor with the dielectric layer 3 between said boundary surface of the dielectric layer 3 and the P-type semiconductor layer 1 and the electrode 4. The characteristics are quite the same as those in the preferred embodiment shown in FIGS. 3, 4 and the increased ratio of a change of capacitance, the increased performance index Q and the leakage current close to 0 can be obtained. 
     It is sufficient that the semiconductor variable capacitance diode according to the present invention has such construction that the application of the forward voltage leads to the formation of the capacitor by the holes or electrons collected on one side of the dielectric layer as described above. 
     The material of the dielectric layer 3 is not limited to SiO 2 , and other dielectric materials such as Al 2  O 3 , Si 3  N 4  can be used. A laminated body consisting of them can be also used. 
     As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within the metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.