JPH0421214B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a reference voltage device that can be built into a semiconductor integrated circuit (IC).

[Conventional technology]

Conventional IC internal reference voltage devices have mainly been constructed from Zener diodes. In this case, there are many variations in the Zener voltage and the temperature characteristics are poor, so external adjustment terminals and compensation elements are always required.
Furthermore, Japanese Patent Laid-Open No. 53-47953 describes an embodiment in which the difference in threshold voltage due to channel doping is used as a reference voltage device.

[Problem to be solved by the invention]

According to the method of JP-A No. 53-47953, it is not possible to obtain the desired reference voltage completely without adjustment because the variation in the dose amount for the channel and the variation in the gate film thickness affects the variation in the reference voltage. It was difficult, required some sort of external adjustment function, and was extremely troublesome to use.

The purpose of the present invention is to eliminate such drawbacks, and in order to minimize variations due to the manufacturing process, different threshold voltages are set based on the difference in work function between the gate electrode of the MOS transistor and the silicon substrate. The object of the present invention is to provide a reference voltage device which generates the difference between the threshold voltages as a reference voltage, completely requires no adjustment, and is suitable for mass production.

[Means to solve the problem]

The reference voltage device of the present invention includes a first MOS transistor having a first gate electrode made of polysilicon into which a first conductivity type impurity is introduced, a second MOS transistor having a first conductivity type impurity and a relatively high impurity concentration. a second gate electrode made of polysilicon into which conductivity type impurities are introduced;
A second MOS transistor with the same polarity as the MOS transistor of
A reference voltage is generated based on a difference between different threshold voltages of the first and second MOS transistors.

〔Example〕

FIG. 1 shows an example in which a reference voltage device is used in a battery voltage detection circuit for an electronic watch. The reference voltage device composed of MOS transistors 1, 2, 3, and 4 is described in detail in the aforementioned Japanese Patent Laid-Open No. 53-47953, and MOS transistors 3 and 4 are considered as a pair with different threshold voltages. The difference between the threshold voltages is output to point A. MOS transistor 5 is switched by clock φ and performs a sampling operation. Resistor 6 and Resistor 7
is designed so that the potential at point B when a desired power supply voltage is applied is the same as the potential at point A, which is the reference voltage output, by dividing the power supply voltage. Therefore, the output of the comparator 11 is at level "1" because the potential at point B is higher than the potential at point A at the initial power supply voltage. Also, when the power supply voltage decreases, A
The potential at point becomes higher than point B, and comparator 11
The output becomes level "0". The output of this comparator is stored in latch 12 by clock φ.

The problem with this device is the MOS transistor 3 and MOS transistor 3 for generating the reference voltage.
This is a structure for varying the threshold voltage of the transistor 4.

FIG. 2 is a diagram showing the basic configuration of a MOS transistor of the reference voltage device of the present invention. In the present invention, the difference in threshold voltage is obtained by the difference in work function between the material of the gate electrode and the silicon substrate. M.O.S.
The threshold voltage V _th of the transistor is determined by the following formula.

V _th =φ _G −φ _S +2φ _F +Q _D /C _O +Q _SS /C _O where φ _G is the work function of the gate electrode, φ _S is the work function of the silicon substrate, φ _F is the fermi level of the silicon surface, Q _D is the amount of charge on the silicon surface, Q _SS
is the interface level, and C _O is the capacitance per unit area of the gate. This φ _G is uniquely determined by the material of the gate, and in the case of a silicon gate structure, φ _G can be arbitrarily determined depending on the amount and type of impurity doped into the gate electrode. Furthermore, φ _S and φ _F on the silicon side are also uniquely determined if the impurity distribution is constant.

Figure 2 shows an N-channel with a silicon gate structure.
A pair of MOS transistors is shown. Figure 2
The MOS transistor 32 corresponds to that shown in FIG. 1, and the MOS transistor 33 corresponds to that shown in FIG. 1, respectively. A P ^- well 25 is formed in the N ^- substrate 26,
21 to 24 are diffusion layers serving as sources and drains. 27 is an insulating layer of _SiO2 , and 28 to 31 are Al for electrodes. The gate electrodes are 34 and 35, and a conductive channel is formed below these through a gate oxide film. The gate electrode 34 of the MOS transistor 32 is doped with the same N ⁺ as the source and drain through a normal process. On the other hand, the gate electrode 35 of the MOS transistor 33 is doped with P ⁺ of the opposite type to the source and drain. In this case, φ _G of the gate electrode 35 is +0.3 to +0.5 V based on the intrinsic Fermi, and φ _G of the gate electrode 34 is −0.3 to +0.5 V.
It becomes -0.5V.

Therefore, even if there are large variations in φ _S , 2φ _F , Q _D /C _O , and Q _SS /C _O between processes, it will affect the MOS transistors manufactured simultaneously on the same substrate, so the threshold voltage By taking the difference between the two, a reference voltage of about 0.6 to 1.0 V can be generated depending on the amount of doping to the gate electrode. Normally, the doping amount can be controlled fairly stably, and even if there is some variation, it is within ±10 mV.

FIG. 3 is an embodiment showing the structure of a MOS transistor in which the gate electrode is doped with impurities of the opposite type to the source and drain in the reference voltage device of the present invention. The MOS transistor in Fig. 3 is an example of a P-channel MOS transistor.
A MOS in which the gate electrode is doped with impurities of the opposite type to the source and drain, similar to 33 in Figure 2.
It has a transistor structure. Figure 3a,
b and c indicate the order of steps. a First, an oxide film 52 is applied to a substrate 53, and then polysilicon 51 is deposited. At this time, the polysilicon is heavily doped with an N type impurity only in the necessary region 58 to form N ⁺⁺ . b The polysilicon is then etched to form a gate electrode 54. c After that, unnecessary parts of the oxide film 52 are removed to form a gate oxide film 55, and the entire area is doped with P ⁺ to form the source/drain 5.
6,57 is formed. The gate electrode is preliminarily heavily doped with an impurity of the opposite type to the source/drain to become N ⁺⁺ , so it does not change even if it is doped with P ⁺ . Therefore, this structure was made
A reference voltage device can be constructed that generates a reference voltage based on the difference in threshold voltage between a MOS transistor and a MOS transistor whose gate electrode is doped with impurities of the same type as the source and drain.

Furthermore, according to the configuration shown in FIG. 3, since impurities are introduced into the gate electrode and at the same time into the adjacent silicon substrate, the sources and drains 42 and 43 can be formed in a self-aligned manner using the gate electrode as a mask.

〔Effect of the invention〕

The present invention utilizes a stable gate work function to form two MOS transistors with different threshold voltages, and generates a stable reference voltage based on the difference between the different threshold voltages. According to the present invention, all factors under the gate that cause variations in the manufacturing process are removed, so a fairly stable reference voltage can be obtained, and an adjustment terminal for adjusting the reference voltage can be eliminated. . Furthermore, since the structure is such that the impurity doped into the polysilicon of the gate electrode is the same type of impurity used for source/drain diffusion in the normal process, it is easy to cause differences in threshold voltages in the normal process.
A reference voltage device that can be configured as a MOS transistor and is suitable for mass production can be provided.

Further, since the threshold voltage is set based on the type and amount of impurity doped into the gate electrode, it is easy to set the generated reference voltage to a desired value.

Furthermore, since the source and drain are formed using the gate electrode as a mask, it is possible to miniaturize the MOS transistor.

For example, when the present invention is used as a reference voltage for a battery voltage detection circuit for an electronic watch, it requires no adjustment and
Since it can be easily incorporated into an IC, it completely eliminates the hassle of using it, making a major contribution to miniaturization, process reduction, and mass production.

[Brief explanation of drawings]

FIG. 1 is a diagram of a battery voltage detection circuit for an electronic watch using a reference voltage device. FIG. 2 is a structural diagram of a pair of transistors with different threshold voltages, which is the basic configuration of the present invention. 3a to 3c are diagrams showing an embodiment of the manufacturing process of a silicon gate transistor having a high threshold voltage in a reference voltage device of the present invention. 3...MOS transistor with high threshold value,
4... Normal high value MOS transistor, 3
4...N ⁺ doped gate electrode, 35...
P ⁺ doped gate electrode.

Claims (1)

[Scope of Claims] 1. A first MOS transistor having a first gate electrode made of polysilicon into which impurities of a first conductivity type are introduced; an impurity of the first conductivity type and a second conductivity transistor having a relatively high impurity concentration; a second gate electrode made of polysilicon into which type impurities are introduced;
A second MOS transistor with the same polarity as the MOS transistor of
A reference voltage device comprising a transistor, and generating a reference voltage based on a difference between different threshold voltages of the first and second MOS transistors. 2. The first MOS transistor has a source/drain region formed by introducing impurities of the first conductivity type into the substrate, and the second MOS transistor has the first conductivity type impurity introduced into the second gate electrode. Claim 1 characterized in that the source/drain region is formed by introducing an impurity of the first conductivity type into the substrate adjacent to the second gate electrode at the same time as introducing an impurity of the first conductivity type. Reference voltage device as described.