JPH061832B2 - Input/Output Protection Device - Google Patents

Description

Detailed Description of the Invention The present invention relates to an insulated gate field effect transistor (hereinafter referred to as IGFE
This concerns improvements to protective devices for the EV-100 (called T).

To improve the electrical performance of an IGFET, such as its threshold voltage and voltage gain, it is better to make the gate insulating film thinner. However, making the gate insulating film thinner reduces the dielectric strength, and for example, when a silicon oxide film of about 500 Å is used as the gate insulating film, it will be permanently destroyed by a voltage of about 60 V. To protect the gate insulating film from such destruction, a protection device such as that shown in Fig. 1 has been used in the past.

First, the operation of the conventional example shown in FIG.
It will be described as an N-channel input protection device.

In FIG. 1, the gate of the protected IGFET 101 is connected to the protective I
A protective IGFET 102 is connected to the source of the GFET 102.
The drain of the protective IGFET 102 is connected to the input terminal 11 via a resistor 111.
Manufactured using the same manufacturing method as 01, the gate insulating film thickness is IG
The gate of the protection IGFET 102 is connected to the constant power supply voltage supply line 13 at a connection point 12. In the protection device of the conventional example shown in FIG.
When the voltage applied to the input terminal 11 is smaller than the voltage obtained by subtracting the threshold voltage of the protection IGFET 102 from the potential of the constant power supply voltage supply wiring 13, the source-drain of the protection IGFET 102 becomes conductive and the protected IGFET 101 operates normally. Conversely, when the voltage applied to the input terminal 11 is larger than the voltage obtained by subtracting the threshold voltage of the protection IGFET 102 from the potential of the constant power supply voltage supply wiring 13, the potential difference between the gate and source of the protection IGFET 102 decreases and approaches a non-conductive state, so that the gate of the protected IGFET 101 is connected to the constant power supply voltage supply wiring 13.
Therefore, even if an excessive voltage is applied to the input terminal, the protected IGFET 101 is protected from gate film destruction.
For protection, the voltage applied between the drain and gate of IGFET 102 is determined by the voltage applied to the input terminal and the constant power supply voltage supply line 1.
As a result, the breakdown voltage of the conventional protection circuit of Fig. 1 is higher by a potential of 13 than when a voltage is applied directly to the gate of IGFET 101. However, the conventional protection device of Fig. 1 has a drawback in that the gate insulating film of protection IGFET 102 is broken down near the drain when a spike-like pulse voltage is applied. For example, in the conventional protection device of Fig. 1 using a silicon oxide film with a gate oxide film of 500 Å, the resistance value of resistor 111 is set to 1 kΩ, and
It was found that when a high voltage was charged to a capacitance of 0.00 pF and this voltage was applied using a mercury contact relay, a spike voltage of about 150 V destroyed the protective IGFET.

An object of the present invention is to provide a protection device which can withstand even higher spike-like overvoltage inputs than the conventional protection devices by providing a means for preventing breakdown of the protection IGFET 102 in the above-mentioned conventional example.

The protection device for an insulated gate field effect transistor according to the present invention is characterized by comprising, for example, a first protective insulated gate field effect transistor having a source electrically connected to the gate or drain of a protected insulated gate field effect transistor and a drain connected to an input or output terminal via a resistor, and a source connecting the gate of the protective insulated gate field effect transistor to a voltage source via a resistive element.

The configuration and effects of the present invention when applied to an input protection device will be described in detail below with reference to FIG. 2 which shows one embodiment of the present invention.

In FIG. 2, the gate of the protected IGFET 201 is connected to the source of the first protective IGFET 202.
The drain of the first protection IGFET 202 is connected to the input terminal 21 via a resistor 211. The gate of the first protection IGFET 202 is connected to the source of the second protection depletion-mode IGFET 203, and the gate and drain of the second protection depletion-mode IGFET 203 are connected to the power supply voltage supply wiring 23 at a connection point 22.

Next, the operation of the input protection device of the embodiment shown in Fig. 2 will be described. When the voltage applied to the input terminal 21 is a DC voltage, the first protection IGFET 202 is in a conductive state, and operates in the same manner as the conventional protection device of Fig. 1, and the protection capability is also approximately equal to that of the protection device of Fig. 1. However, when a spike-like pulse voltage that leads to electrostatic breakdown is applied in an actual natural state, the protection device of Fig. 2 according to the embodiment of the present invention has a significantly greater protection capability than the conventional protection device of Fig. 1. That is, in the conventional protection device of Fig. 1, when a spike-like high voltage is applied to the input terminal 11, the high voltage is applied almost as it is to the drain of the protection IGFET 102, and the protection IGFET
The voltage of the gate of T102 is the same potential as the constant power supply voltage supply wiring 13, and the potential of the constant power supply voltage supply wiring 13 is about several volts when the semiconductor integrated circuit is in operation and is approximately the ground potential when it is not in operation, so the gate insulating film of the protective IGFET 102 is destroyed near the drain.
3, a second protective depletion-mode IGFET 20
The source of the second protection depletion-type IGFET 203 is inserted on the gate side of the first protection IGFET 202, and the gate and drain of the second protection depletion-type IGFET 203 are inserted on the connection point 22 side with the power supply voltage supply wiring 23. Therefore, when a spike-like high voltage is applied to the input terminal 21, the drain potential of the protection IGFET 202 becomes high and the gate potential of the protection IGFET 202 becomes low.
The gate potential of the protective IGFET 202 also increases at the same time due to the capacitance between the drains.
Since the second protective depletion-type IGFET 203 is inserted between the gates of the IGFET 202, this second protective IGFET 203 acts as a resistor and limits the charging current of the gate capacitance of the protective IGFET 202.
The gate potential of the protection IGFET 202 does not drop instantly immediately after the application of the high voltage. That is, the gate potential of the protection IGFET 202 drops with a time constant determined by the resistance component of the protection depletion-mode IGFET 203 and the gate capacitance of the protection IGFET 202, and eventually becomes the same as the power supply voltage of the wiring 23. However, before the gate potential of the protection IGFET 202 becomes the power supply voltage of the wiring 23, the protection IGFET 202
The PN junction between the drain and the substrate of the protection IGFET 2 breaks down.
Since the drain voltage of the protective IGFET 202 is approximately equal to the substrate potential, the potential difference between the drain and gate of the protective IGFET 202 is equal to the potential difference between the substrate and the wiring 23, which is smaller than that of the conventional protective device shown in FIG. 1, and the gate insulating film of the protective IGFET 202 itself can be protected from destruction.
If the resistor 111 is 1 kΩ at 0.00 Å,
While gate oxide film breakdown occurs at a spike-like pulse voltage of about 300 V, the protective effect is observed even at a spike-like pulse voltage of 300 V or more, and the superiority of the protective device of the present invention over conventional protective devices is clear.

In addition, even when the protection device of the present invention is applied to an output circuit,
In terms of protective effect, a breakdown voltage more than twice as high as that of the prior art was obtained, similar to the embodiment of FIG. 2 applied to the input circuit.

Although the present invention has been described above by taking an N-channel IGFET as an example, it goes without saying that the protection device of the present invention is also effective for a P-channel IGFET.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing the configuration of a conventional protection device, and FIG. 2 is a circuit diagram showing the configuration of an embodiment of the present invention. In the figure, 11 ... input terminal, 13 ... constant power supply voltage supply wiring, 101 ... protected IGFET, 102
...Protection IGFET, 111...Resistor, 21...Input terminal, 23...Wiring for constant power supply voltage supply, 201...IGFET to be protected, 202...Protection IGFET, 203
...Protective depletion-type IGFET, 211...
A resistor.

Claims (1)

[Claims] [Claim 1] An input/output protection device which has a source-drain current path connected between a circuit node to which an insulated gate field effect transistor to be protected is connected and an input terminal or an output terminal, and which has a protective insulated gate field effect transistor with its gate connected to a constant current voltage supply wiring, and which protects the insulated gate field effect transistor to be protected by clamping the voltage of the circuit node to a voltage which differs from the constant voltage by the threshold voltage of the protective insulated gate field effect transistor when a voltage higher than the constant voltage of the constant power supply voltage supply wiring is applied to the input terminal or output terminal, wherein a resistive element is inserted between the protective insulated gate field effect transistor and the constant power supply voltage supply wiring.