JPH07302876A - Protective circuit against electrostatic breakdown - Google Patents

Abstract

PURPOSE:To improve the resistance to the electrostatic breakdown of an inner circuit by the excessive voltage pulse based on the electrostatic charge collecting in an input potential terminal. CONSTITUTION:Excessive voltage pulses are directed to the earth by inserting unidirectional Zener diodes 3 and 23, which are integrated in a semiconductor substrate, and bidirectional Zener diodes 4, which consist of polycrystalline silicon layers where p regions and n regions adjoin one another alternately through insulating films on the semiconductor substrate and are connected in series, in parallel between the circuit leading to an inner circuit potential terminal 1 from an input potential terminal 2 and the earth.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrostatic breakdown protection circuit which functions to protect an internal circuit from breakdown due to an overvoltage pulse generated by accumulation of electrostatic charges.

[0002]

2. Description of the Related Art In a MOS integrated circuit such as MOSLSI or MOSVSI, a large-scale protection circuit cannot be provided in order to avoid deterioration of the driving capability of transistors. Therefore, it is necessary to provide a detour path that serves as an escape route for an excessive voltage pulse that exceeds the rated voltage of the transistor due to accumulated electrostatic charges. FIG. 2 shows a protection circuit for satisfying such a situation. That is, the resistor 21 is inserted between the internal potential terminal 1 and the input potential terminal 2 of the internal circuit 10 including the MOS transistors 11 and 12, and the excessive voltage pulse that has entered the input potential terminal 2 is applied to the internal circuit 1 by this resistor. Delay the propagation. In the meantime, charge M
It escapes through the OS transistor 22. Further, the MOS transistor 23 allows the pulse which could not be absorbed by the transistor 22 to escape, thereby protecting the internal circuit from electrostatic breakdown. The MOS transistor 22 has a function of making the current flowing from the input terminal 2 to the internal potential terminal 1 a constant current, in addition to the function of releasing an overvoltage due to electrostatic charge. Instead of the MOS transistor 22 and the MOS transistor 23, a Zener diode having the same Zener voltage integrated in the semiconductor substrate may be used to release an overvoltage pulse exceeding the Zener voltage.

[0003]

However, even if the protection circuit as shown in FIG. 2 is provided, electrostatic breakdown may still occur. An object of the present invention is to provide an electrostatic breakdown protection circuit which can solve this problem and further improve the electrostatic breakdown withstand capability, but does not increase the size of the chip.

[0004]

In order to achieve the above object, an electrostatic breakdown protection circuit of the present invention comprises a semiconductor substrate between a circuit connecting an input potential terminal and an internal circuit potential terminal and a ground. It is assumed that an overvoltage absorption circuit including a Zener diode integrated in the body and an overvoltage absorption circuit including a Zener diode formed on a semiconductor substrate via an insulating film are connected in parallel. It is effective that the zener diode in the semiconductor substrate is a zener diode in only one direction, and the zener diode on the semiconductor substrate is a plurality of bidirectional zener diodes alternately connected in series. It is preferable that the Zener diode on the semiconductor substrate has alternating p and n regions of polycrystalline silicon formed on the substrate via an insulating film.

[0005]

FIG. 1 shows a protection circuit of the present invention, and the same parts as those in FIG. 2 are designated by the same reference numerals. A circuit for connecting the internal potential terminal 1 and the input potential terminal 2 is provided with a circuit for escaping an excessive voltage pulse due to electrostatic charge to the ground potential across the resistor 21 like the conventional protection circuit. Is provided in parallel with a circuit including a Zener diode 3 formed by diffusion on a semiconductor substrate and a circuit in which a plurality of bidirectional Zener diodes 4 formed on the semiconductor substrate via an insulating film are connected in series. There is. By connecting the bidirectional Zener diodes in series, a voltage drop occurs in the forward Zener diode when a high reverse voltage is applied to the Zener diode in one direction, so that the Zener diode is not easily destroyed. Since the two overvoltage pulse absorption circuits are connected in parallel in this manner, the charge is divided and absorbed, and even if one of the absorption circuits is broken, the other works, so that the reliability is increased. By forming one on the base and the other in the base, the size of the base is not increased.

[0006]

FIG. 3 shows a protection circuit according to an embodiment of the present invention, that is, a circuit using an electrostatic breakdown withstand voltage improving circuit 5. The same parts as those in FIGS. 1 and 2 are designated by the same reference numerals. ing.
The Zener diode 3 and the Zener diode 23 are formed by introducing impurities into a silicon chip by diffusion and have a Zener voltage of 40 V. The Zener diode 4 is a polycrystal formed on a silicon chip via an oxide film. A plurality of bidirectional Zener diodes are alternately connected in series by introducing two kinds of impurities into a silicon layer to alternately form p regions and n regions in a plane parallel to the substrate surface. Is. Since the polycrystalline silicon layer for the Zener diode is formed under the pad portion on the substrate, the chip area is not increased. This circuit 5 is connected to the power supply voltage terminal 6 and is a depletion type N channel M
By connecting to the connection point of the constant current circuit composed of the OSFET 7 and the filter circuit composed of the resistor 8 and the capacitor 9, the input potential terminal 2 and the internal potential terminal 1 are connected via the filter circuit. As a result, the input potential terminal 2
An excessive voltage pulse that is generated due to the accumulation of static electricity in the circuit delays the propagation to the internal circuit due to the action of the resistor 21,
During that time, the electric charge is divided into the diffusion Zener diode 3 and the polycrystalline silicon Zener diode 4 to maintain a high withstand voltage. The pulse which is not absorbed by these Zener diodes is escaped to the ground by the diffusion Zener diode 23.

[0007]

According to the present invention, the electrostatic breakdown withstand capability is improved by incorporating an overvoltage absorption circuit composed of a zener diode in the substrate and an overvoltage absorption circuit composed of a zener diode on the substrate in parallel. Does not increase the area.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a basic configuration of an electrostatic breakdown protection circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional electrostatic breakdown protection circuit.

FIG. 3 is a circuit diagram showing a usage example of an electrostatic breakdown protection circuit according to an embodiment of the present invention.

[Explanation of symbols]

 1 Internal potential terminal 2 Input potential terminal 3 In-board Zener diode 4 On-board Zener diode 5 Electrostatic breakdown withstanding circuit

Claims (3)

[Claims] 1. An overvoltage absorption circuit including a Zener diode integrated in a semiconductor substrate between a circuit connecting an input potential terminal and an internal circuit potential terminal and a ground, and an insulating film on the semiconductor substrate. An electrostatic breakdown protection circuit characterized in that an overvoltage absorption circuit including a formed Zener diode is connected in parallel. 2. The zener diode in the semiconductor substrate is a zener diode in only one direction, and the zener diode on the semiconductor substrate is formed by alternately connecting a plurality of bidirectional zener diodes in series. The electrostatic discharge protection circuit described. 3. The electrostatic breakdown protection circuit according to claim 2, wherein the Zener diode on the semiconductor substrate comprises alternating p and n regions of polycrystalline silicon formed on the substrate via an insulating film. .