Mis variable capacitor and temperature-compensated...

Details Mis variable capacitor and temperature-compensated... Mis variable capacitor and temperature-compensated...

1999-02-23

2001-05-29

Mis, David (Department: 2817)

Oscillators

With device responsive to external physical condition

Temperature or light responsive

C331S1160FE, C331S158000, C331S17700V

Reexamination Certificate

active

06239662

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a MIS variable capacitor formed on a semiconductor substrate and a temperature-compensated oscillator using the same.
2. Description of the Related Art
MIS variable capacitors vary in capacitance in accordance with an applied direct current voltage, thus being used for controlling the oscillation frequency of a voltage control oscillator. Moreover, since its structure resembles that of a MOS integrated circuit, it is quite easy that the MIS variable capacitor is integrated with the MOS integrated circuit, so that it is also used as a capacitor element which is provided on the semiconductor substrate forming the MOS integrated circuit.
Furthermore, the MIS variable capacitor has an advantage of obtaining a wide range of adjustment of the oscillation frequency even at low voltages, since a large capacitance variation can be obtained in a small voltage range at low voltages, compared with a variable capacitance diode (varicap) utilizing voltage dependence of a depletion layer capacitance in a PN junction of a semiconductor.
An example of conventional MIS variable capacitors will be explained briefly with reference to FIG.
12
. In the MIS variable capacitor, an insulating film
89
and a conducting film
87
are formed in that order on an N type semiconductor substrate
91
, and a heavily doped N region
83
with an impurity concentration heavier than that of the semiconductor substrate
91
is provided to contact a covered region
91
a
of the semiconductor substrate
91
which is covered with the insulating film
89
.
In the MIS variable capacitor, a terminal
85
of an electrode on the conducting film side and a terminal
81
of an electrode on the semiconductor side are respectively conducted from the conducting film
87
and from the heavily doped N region
83
, to form a variable capacitor composed of conducting film—insulating film—semiconductor. The capacitance value of the MIS variable capacitor varies by the voltage Vi applied between the terminal
81
and the terminal
85
.
For example, as shown in
FIG. 14
, when the voltage Vi which is applied between the terminals
81
and
85
increases from the voltage value Va (negative voltage on the terminal
85
side in relation to the terminal
81
) to the voltage value Vb (positive voltage on the terminal
85
side in relation to the terminal
81
), changing form minus to plus at the voltage value Vc in between, the capacitance value of the MIS variable capacitor shown in
FIG. 12
increases as shown by a curved line
80
. In
FIG. 14
, the horizontal axis indicates voltage values of the voltage vi applied between the terminals
81
and
85
and the vertical axis indicates capacitance values. The variation in capacitance value is caused by the actions, described hereinafter, in the covered region
91
a
of the semiconductor substrate
91
shown in FIG.
12
.
At low voltage Va, as shown in
FIG. 12
, holes of minority carriers are induced in the vicinity of the surface of the covered region
91
a to form an inversion layer
92
and a depletion layer
94
thereunder. When the voltage value rises from that state, the thickness of the depletion layer
94
reduces responsively. The inversion layer
92
becomes conductive and the depletion layer
94
becomes an insulating layer.
In this example, since the depletion layer
94
having insulating properties exists in the covered region
91
a
, the MIS variable capacitor is configured such that a capacitor composed of the conducting film
87
and the inversion layer
92
sandwiching the insulating film
89
and a capacitor composed of the inversion layer
92
and a portion of the semiconductor substrate
91
under the depletion layer
94
sandwiching the depletion layer
94
are connected in series. Accordingly, its capacitance value is a serial combined capacitance value of both capacitors.
When the thickness of the depletion layer
94
reduces as the applied voltage Vi increases, the capacitance of the capacitor which is formed across the depletion layer
94
increases, so that the capacitance of the MIS variable capacitor also comes to increase as shown in FIG.
14
.
However, when the applied voltage Vi further rises to reach the voltage value Vb shown in
FIG. 14
, the depletion layer
94
disappears as shown in FIG.
13
and an accumulation layer
95
in which electrons are induced on the surface of the covered region
91
a
is then formed. Since the accumulation layer
95
is conductive, when the depletion layer
94
having insulating properties disappears, the capacitance value of the MIS variable capacitor becomes equal to the capacitance value of the capacitor which is formed across the insulating film
89
of which the film thickness does not change, thus being a fixed capacitance value at voltages exceeding Vb.
As described above, the MIS variable capacitor has the property of varying in capacity in relation to the voltage change in a certain range of the applied voltage value, but the capacitance variation poorly responds the voltage change. Therefore, there is a disadvantage that, when the voltage applied between the terminals
81
and
85
is momentary changed in the reducing direction from the voltage value Vb to the voltage value Va in
FIG. 14
, the variation of the capacitance value can not catch up the voltage change and the capacitance value slowly varies after the voltage changes.
This is presumed to be because, when the applied voltage is reduced, holes are induced in the vicinity of the surface of the covered region
91
a
to form the inversion layer
92
, but the holes are minority carriers in the N type semiconductor substrate
91
and so the holes are supplied slowly, therefore time is needed for the concentration of minority carriers to reach the thermal equilibrium. Until the concentration of minority carriers reaches the thermal equilibrium, the capacitance of the MIS variable capacitor varies since the thickness of the depletion layer
94
changes.
Accordingly, there is a disadvantage that when the above conventional MIS variable capacitor is used for frequency control of the oscillation circuit in the voltage control oscillator, since the capacitance value does not respond the change in control voltage, the oscillation frequency changes with delay in relation to the change in control voltage, which is a serious trouble in response of the frequency of the voltage control oscillator.
Moreover, it is also a disadvantage that since the MIS variable capacitor reduces greatly in capacitance value because the depletion layer
94
is provided after forming of the inversion layer
92
, when the MIS variable capacitor is used as a standard capacitor for phase compensation of an amplifier or the like, the capacitor function weakens in a range of low voltages where the inversion layer is formed and the required capacitance can not be obtained, therefore a range of usable voltage is limited to a range above a predetermined value.
SUMMARY OF THE INVENTION
This invention is accomplished to solve the above disadvantages in an MIS variable capacitor and the first object of the present invention is to improve response of the capacitance variation in relation to the change in applied voltage.
The second object of the present invention is to reduce the degree of a drop in capacitance value in the voltage range where an inversion layer is formed, so as to enlarge the voltage range where the capacitor can be effectively used.
The third object of the present invention is to provide a temperature-compensated oscillator which can perform compensation control of an oscillation frequency in relation to temperature changes with a good response.
To achieve the above described first and second objects, this invention provides an MIS variable capacitor having a capacitor structure of conducting film—insulating film—semiconductor which is composed of a semiconductor of a first conduction type, an insulating film formed on the semiconductor of the first conduction type, and a conducting film formed on the

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