Abstract:
An oscillator circuit is provided which employs a single active element. In particular, the circuit utilizes a novel four-terminal semiconductor device which is able to act as a transistor with three of its terminals and, in case of use as the transistor, has the emitter-grounded current amplification factor h FE  controllable with a voltage at its fourth terminal. When the novel four-terminal semiconductor device is connected so as to operate as an emitter-grounded transistor, voltage changes at its output terminal, which corresponds to the collector of the transistor, is the same in phase as voltage changes at its fourth terminal. This fact is utilized in the present invention to cause oscillation by the generative feedback from the output terminal to the fourth terminal.

Description:
BRIEF STATEMENT OF THE INVENTION 
     An oscillator circuit is provided which employs only a single element. This active element is a semiconductor device comprising a first semiconductor region of one conductivity type, a second semiconductor region of the opposite conductivity type adjacent said first region with a first semiconductor junction therebetween, a third semiconductor region of the same conductivity type as said first region adjacent said second region with a second semiconductor junction therebetween, first, second and third terminals coupled to said first, second and third regions, respectively, a fourth terminal connected to said semiconductor device and having at least a portion thereof located adjacent to said first region at a position which is spaced from the said first terminal, and an insulating layer separating said fourth terminal from said first region. 
     This semiconductor device is able to act as a transistor with three of its terminals, and, in case of use as the transistor, has the emitter-grounded current amplification factor h FE  controllable with a voltage at its fourth terminal. When the novel four-terminal semiconductor device is connected so as to operate as an emitter-grounded transistor, voltage changes at its output terminal, which corresponds to the collector of the transistor, is the same in phase as voltage changes at its fourth terminal. This fact is utilized in the present invention to cause oscillation by the generative feedback from the output terminal to the fourth terminal. 
     The oscillator circuit according to the present invention is extremely simple in circuit construction due to use of the novel four-terminal semiconductor device. 
     CROSS REFERENCE TO RELATED CASES 
     Cross reference is to applications Ser. Nos. 427,648 and 427,647, which are assigned to the same assignee as the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are, respectively, cross sectional views of a three-terminal semiconductor device used for explaining the four-terminal semiconductor device of the present invention; 
     FIG. 3 is a cross-sectional view of a preferred embodiment of the novel four-terminal semiconductor device of the present invention; 
     FIG. 4 is a circuit diagram of a circuit used for measuring the characteristics of a novel four-terminal device shown in FIG. 3; 
     FIG. 5 is a graph showing the characteristics of the four-terminal semiconductor device of FIG. 3, wherein the collector current in micro amperes is plotted against the voltage between the gate and the emitter of the semiconductor device; 
     FIG. 6 is a circuit diagram of an oscillator employing the semiconductor device shown in FIG. 3; 
     FIG. 7 is a graph showing the relation of I C  to V G  and the relation of the output voltage V O  to V G  ; and 
     FIGS. 8 and 9 show specific circuit examples of impedances Z 1  and Z 2  of FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The novel oscillator circuit of the present invention employs a novel four-terminal device. In order to better appreciate the nature and characteristics of the four-terminal device, a description will first be made to two novel semiconductor devices which, as indicated, are disclosed in application Ser. Nos. 427,648 and 427,647, and which are assigned to the same assignee as the present invention. Referring to the semiconductor devices illustrated in FIGS. 1 and 2, the emitter-grounded current amplification factor h FE  of a transistor, which is one of parameters evaluating the characteristics of the bipolar transistor, can be expressed by the following equation (1), if the base-grounded current amplification factor of the transistor is taken as α. ##EQU1## 
     The factor is expressed as follows: 
     
         α = α* β γ                          (2) 
    
     where α* represents the collector amplification factor, β the base transfer efficiency and γ the emitter injection efficiency, respectively. 
     Now, if the emitter injection efficiency γ of an NPN-type transistor is taken into consideration, γ is given by the following expression (3). ##EQU2## where J n  represents the current density of electrons injected from the emitter to the base of the transistor and J p  the current density of holes injected from the base to the emitter of the transistor, respectively. 
     Since J n  and J p  are expressed by the following equations (4) and (5), respectively, ##EQU3## the ratio δ of J n  and J p  is expressed as follows: ##EQU4## where L n  represents the diffusion distance of the minority carriers in the base of the transistor; L p  the diffusion distance of the minority carriers in the emitter of the transistor; D n  the diffusion constant of the minority carriers in the base; D p  the diffusion constant of the minority carriers in the emitter; n p  the concentration of the minority carriers in the base under the equilibrium state; p n  the concentration of the minority carriers in the emitter under the equilibrium state; V a voltage applied to the emitter junction of the transistor; k the Boltzmann&#39;s constant; T temperature; and q the absolute value of electron charge. 
     If it is assumed that the impurity concentration in the emitter of the transistor is taken as N D  and that in the base of the transistor taken as N A , the term p n  /n p  can be replaced by the term N A  /N D . Further, since L n  is restricted by the base width W and L n  = W, the ratio δ is expressed as follows: ##EQU5## 
     The diffusion constants D n  and D p  are functions of transfer of the carrier and temperature and in this case they are assumed constant substantially. 
     As may be obvious from the above respective equations, in order to increase the current amplification factor h FE  of a transistor, it is sufficient to make the ratio δ small. 
     Therefore, in any ordinary transistor, the impurity concentration N D  of its emitter is selected high enough so as to make the ratio δ small. 
     However, if the impurity concentration of the emitter is selected sufficiently high, for example, more than 10 19  atoms/cm 3 , lattice defects and dislocation occur in the crystal of the semiconductor body of the transistor to deteriorate the crystal. Further, due to the fact that the impurity concentration of the emitter itself is high, a life time τ p  of the minority carriers injected to the emitter from the base becomes short. 
     Since the diffusion distance L p  is expressed by the following equation (8) 
     
         L.sub.p = √D.sub.p τ.sub.p                      (8) 
    
     the diffusion distance L p  of the minority carriers or holes becomes short. Therefore, as may be apparent from the equation (7), δ can not be made small so much and hence the injection efficiency γ can not be made high over a certain value. As a result, the current amplification factor h FE  can not be made high so much in the ordinary transistor. 
     The novel three-terminal semiconductor device is free from the defects mentioned just above inherent to the prior art transistor. As the semiconductor device used in this invention, an NPN-type one and a PNP-type one could be considered as in the case of the prior art transistor, but an NPN-type semiconductor device will be now described with reference to FIGS. 1 and 2, by way of example. 
     As shown in FIG. 1, the NPN-type three-terminal semiconductor device consists of a first semiconductor region 1 of N -  type conductivity formed in a semiconductor substrate S of N +  type conductivity, a second semiconductor region 2 of P type conductivity formed in the semiconductor substrate S adjacent the first region 1, and a third semiconductor region 3 of N -  type conductivity formed in the substrate S adjacent the second region 2 to form a first PN-junction J E  between the first and second regions 1 and 2 and a second PN-junction J c  between the second and third regions 2 and 3, respectively. 
     With the semiconductor device as shown in FIG. 1, at the position facing the first junction J E  and apart from it by a distance smaller than the diffusion distance L p  of the minority carriers or holes injected from the second region 2 to the first region 1, a potential barrier having energy higher than that of the minority carriers or holes, or at least heat energy is formed in the first region 1. In the example of FIG. 1, the impurity concentration in the first region 1 is selected low sufficiently such as in the order of 10 15  atoms/cm 3  and region 1a of N +  type conductivity or the impurity concentration of about 10 19  atoms/cm 3  is formed in the first region 1 to form an LH-junction and hence to form the barrier. 
     The impurity concentration in the second region 2 is selected in the order of 10 15  to 10 17  atoms/cm 3  and that in the third region 3 is selected sufficiently low, such as in the order of 10 15  atoms/cm 3 . 
     In the semiconductor substrate S adjacent to the third region 3 but apart from the second junction J c , there is formed a region 3a of N +  type conductivity and with the impurity concentration of about 10 19  atoms/cm 3 . 
     A first electrode 4E is formed on the high impurity concentration region 1a in the region 1 in ohmic contact therewith; a second electrode 4B is formed on the second region 2 in ohmic contact therewith; and a third electrode 4C on the high impurity concentration region 3a adjacent the third region 3 in ohmic contact therewith, respectively. From these electrodes 4E, 4B and 4C, there are led out first, second and third terminals E, B and C, respectively. In FIG. 1, reference numeral 5 indicates an insulating layer made of, for example, SiO 2  and formed on the surface of the substrate S. 
     The semiconductor device shown in FIG. 1 can be used as a transistor. In such a case, the first region 1 serves as an emitter region; the second region 2 as a base region; and the third region 3 as a collector region, respectively, a forward bias is applied to the emitter junction J E  and a reverse bias is applied to the collector junction J C . 
     Thus, the holes injected from the base or the second region 2 to the emitter or first region 1 have a long life period of time due to the fact that the emitter region 1 has the low impurity concentration and good crystal property, and hence the diffusion distance L p  of the holes in the emitter region 1 becomes long. As a result, as may be apparent from the equations (6) and (3), the emitter injection efficiency γ can be made high. However, in the case that the diffusion distance L p  is made long, if the injected holes into the emitter region 1 may arrive at the surface of the substrate S and may be recombined with electrons on the surface in practice, the diffusion distance L p  could not be made substantially long. With the semiconductor device shown in FIG. 1, since the potential barrier is formed in the emitter region 1, which potential barrier faces the emitter junction J E , at the position with a distance smaller than the diffusion distance L p  of the minority carrier, the amount of the surface recombination is reduced and the diffusion distance L p  can be taken long sufficiently. 
     Due to the fact that the potential barrier is formed as described above in the example shown in FIG. 1, there is performed such an effect that the current density or component J p  of the holes injected from the base region 2 to the emitter region 1 is reduced. That is, on the LH-junction J H  in the emitter region 1, there is caused a false Fermi level difference or built-in electric field which acts to suppress the diffusion of the holes or the minority carrier. Therefore, if the Fermi level is sufficiently high, the diffusion current caused by the concentration gradient of holes and the drift current caused by the built-in electric field are cancelled on the LH-junction with each other to reduce the hole current J p  injected from the base 2 through the emitter region 1 of low impurity concentration. By this effect, the ratio of electron current arriving at the collector region 3 relative to current component passing through the emitter junction J E  is increased and hence the emitter injection efficiency γ is increased as apparent from the equation (3) to make the current amplification factor h FE  high. 
     The above level difference (the height of the potential barrier) must be more than the energy of holes or at least the heat energy. The heat energy can be approximated as kT, but the above level difference is desired to be more than 0.1 eV. Within the transition region of the potential, the diffusion distance L p  of the holes must be not terminated within the transition region, or it is required that the diffusion distance L p  of the hole must be greater than the width of the transition region. 
     In the case that the LH-junction J H  is formed as shown in FIG. 1, the potential barrier of 0.2 eV can be formed by suitably selecting the amount of impurity and gradient of the high impurity concentration region 1a. 
     FIG. 2 shows another example of the three-terminal semiconductor device in which reference numerals and letters which are the same as those used in FIG. 1 indicate the same parts, so that their description will be omitted. 
     In the example of FIG. 2, in order to form a PN-junction J S  facing the first or emitter junction J E , an additional region 6 of P type conductivity is formed in the first region 1. In the example of FIG. 2, the distance between the junctions J S  and J E  is selected smaller than the diffusion distance L p  of the minority carrier in the first region 1. The other construction of the example shown in FIG. 2 is substantially the same as that of the example shown in FIG. 1. 
     With the example of FIG. 2, since the diffusion distance L p  of the hole injected to the first region 1 is long as described above, the hole arrives at the additional region 6 effectively and then is absorbed thereby. When the additional region 6 is floated from an electrical point of view, its potential is increased as the number of holes arriving at the additional region 6 is increased. Thus, the PN junction J S  formed between the regions 6 and 1 is biased forwardly to its rising-up voltage substantially, and then holes will be re-injected to the first region 1 from the additional region 6. Thus, the concentration of holes in the first region 1 near the additional region 6 will be increased, and accordingly, the concentration distribution of holes between the junctions J E  and J S  in the first region 1 is made uniform and the gradient thereof becomes gradual to reduce the diffusion current J p  from the second region 2 to the first region 1. 
     If, in the semiconductor devices described above, a fourth region or control region is formed in the first region 1 thereof and a fourth electrode or control electrode (gate) is connected thereto so as to form a novel four-terminal semiconductor device, its current amplification factor may be varied by applying a control voltage to the control electrode (gate). 
     A description will now be given of the novel four-terminal semiconductor device which is employed in the present invention. With reference to FIG. 3, a control electrode (gate) is formed on a surface of the first semiconductor region 1 (emitter region) of the semiconductor device of the three-terminal type shown in FIG. 1 through an insulating layer. 
     In the embodiment of FIG. 3, a control electrode 4G, such as a metal layer made of, for example, aluminum and having a predetermined area is formed on a part of the first semiconductor region (emitter region) 1 of the device shown in FIG. 1 through an insulating layer (gate insulating layer) 7 which has a predetermined thickness, for example, 100 A (angstrom), and is made of, for example, SiO 2  similar to the insulating layer 5, and corresponds to the gate insulating layer of a MOS FET. A gate terminal G is led out from the control electrode 4G as a fourth terminal. A part 8 in the first region 1 opposing the control electrode 4G is the semiconductor control region. 
     If a gate bias voltage is applied between the gate and emitter of the four-terminal semiconductor device or the gate terminal G and the emitter terminal E, the current amplification factor or emitter-grounded current amplification factor h FE  thereof is varied in response to the gate bias voltage along a curve which is convex in the downward direction and substantially symmetrical with respect to its minimum value. In other words, if a bias voltage which is negative relative to the emitter terminal E in the device of FIG. 3, within the positive range from the threshold voltage of the bias voltage, a storage layer CG having the function similar to the LH-junction J H  as the potential barrier in FIG. 1 is formed in a part of the first region (emitter region) 1 as the voltage approaches the positive direction. Thus, the current density J P  of the holes of the diffusion current from the second region (base region) 2 to the first region (emitter region) 1 decreases, and consequently the factor h FE  increases. 
     While, within the negative region from the threshold voltage of the bias voltage, an inverse layer IN is formed in a part of the emitter region 1 or control region 8 as the voltage approaches the negative direction and, similar to the case where the additional region 6 in FIG. 2 is floated in the electrical point of view, holes are re-injected from the inverse layer IN to the emitter region 1. Thus, the current density J P  of the holes of the diffusion current from the base region 2 to the emitter region 1 decreases, and consequently the factor h FE  increases. 
     FIG. 5 is a graph showing the characteristics of the novel four-terminal semiconductor device shown in FIG. 3 which is measured by a measuring circuit shown in FIG. 4. 
     In FIG. 4, reference letter Q indicates the novel four-terminal semiconductor device shown in FIG. 3 as a symbol, in which a short line is added to the symbol of the prior art bipolar transistor in parallel to its emitter as the gate of the novel four-terminal semiconductor device Q. In FIG. 4, the novel four-terminal semiconductor device Q is shown as an emitter-grounded type. In the figure, reference letter R L  indicates a collector load resistor of the device Q, V CC  its collector voltage source, I C  its collector current, I B  its base current (constant), and V GE  its gate-emitter voltage, respectively. 
     When its collector-emitter voltage V CE  is 3V (volts), and the base current I B  is 1 μ A (micro-ampere), the characteristics of the gate-emitter voltage (gate bias voltage) V GE  (V) - collector current I C  (μ A) and emitter-grounded current amplification factor h FE  are shown in the graph of FIG. 5. 
     According to the characteristic curve of FIG. 5, it may be understood that the current amplification factor h FE  is varied in response to the variation of the gate bias voltage along a curve which is convex in the downward direction and approximately symmetrical with respect to its minimum value (where the gate-emitter voltage is at the above threshold voltage). 
     When the thickness of the emitter region 1 of the semiconductor device shown in FIG. 3 is selected smaller than the diffusion distance L p  of the holes (injected carriers), the surface recombination, where the gate-emitter voltage V GE  is substantially equal to the threshold voltage, is greatly influenced. Thus, the life time period of the injected carrier (minority carrier) becomes short, and accordingly, the minimum value of the factor h FE  can be made smaller. 
     The embodiment shown in FIG. 3 is an NPN type element, but it may, of course, be possible that the semiconductor device be made as a PNP type as in the case of a bipolar transistor. 
     FIG. 6 shows one embodiment of an oscillator circuit according to the present invention. In this embodiment, a novel four-terminal semiconductor device Q having a first or emitter electrode E, a second or base electrode B, a third or collector electrode C and a fourth or gate electrode G is connected substantially in the emitter-grounded configuration. The collector electrode C is provided with a load resistor R C  and the base electrode B is connected to a bias source 11 through a resistor R B  to be supplied with a base current I B . The gate electrode G is connected to a negative bias voltage source 13 through an impedance element Z 2 . The negative bias voltage source 13 supplies the gate electrode G with a negative bias voltage V A  lower than a voltage V T  by which a gate-emitter voltage V GE  is given such a value as to make the current amplification factor h FE  of the device Q take minimum value at the low point on the characteristic curve shown in FIG. 5. The gate electrode G is also connected to the collector electrode C through an impedance element Z 1  to form a feedback path. 
     In this circuit, ##EQU6## where ΔV G  is the variation in the gate potential V G  ; and ΔI c  is the variation in the collector current I c . 
     In accordance with the characteristic curve of h FE  as shown in FIG. 5, the relation between the collector current I c  and the gate potential V G  is shown as a curve 15 in FIG. 7. So g m  becomes positive over the range where V G  is lower than V T . 
     If A represents the gain of the device Q without feedback, β represents a feedback ratio, V I  represents a base input voltage and V O  represents a collector output voltage, ##EQU7## Therefore, ##EQU8## 
     Accordingly, when g m  is positive, degenerative feedback is established and when g m  is negative, generative feedback is established. Curve 16 in FIG. 7 shows the relation between the collector output voltage V O  and the gate potential V G . Further, if g m  is given a negative value to satisfy 1 + g m  .sup.. R C  .sup.. β = 0, the gain of the device Q with the feedback becomes infinity and oscillation is caused. Such a value of g m  is obtained by selecting the gate bias voltage V A  at a suitable value lower than the potential V T . 
     FIGS. 8 and 9 show examples of FIG. 6 in greater detail. In the example of FIG. 8, a capacitor 17 and a resistor 18 are used as the impedance elements Z 1  and Z 2  of FIG. 6, respectively, thereby to determine an oscillating frequency. In the example of FIG. 9, a capacitor 19 and an inductor 20 are used as the impedance elements Z 1  and Z 2  in FIG. 6, respectively, thereby to determine an oscillation frequency. 
     From the above description, it will be apparent that a very simple oscillator circuit is provided which employs only a single active element. 
     It will be apparent to those skilled in the art that many modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.