Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-10-17
2002-06-11
Abraham, Fetsum (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S296000, C257S298000, C257S300000, C257S357000, C257S360000
Reexamination Certificate
active
06404017
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a protection circuit for a semiconductor integrated circuit equipped with a semiconductor non-volatile memory device, more specifically, to a protection circuit provided on a semiconductor integrated circuit to protect an internal circuit from static electricity intruding at a terminal to which a program voltage for writing into the memory device is applied.
2. Description of the Related Art
In a semiconductor integrated circuit equipped with a semiconductor non-volatile memory device (hereinafter, referred to as “memory device”), information is written into the memory device by a voltage higher than a power supply voltage. Especially, a memory device of a junction breakdown-type or a fuse element breakdown-type requiring a current for writing is supplied with a program voltage for writing from the outside, and a terminal (hereinafter, referred to as “program voltage terminal”) for supplying the program voltage is provided.
Due to the existence of such a terminal, there is inevitably a possibility that static electricity intrudes at the terminal, and thus a protection circuit for protecting an internal circuit from the static electricity is provided.
The protection circuit, if composed of a diode or a transistor generally used as a typical circuit element, can not discriminate between static electricity and a program voltage which intrude at the program voltage terminal, resulting in operation even when the program voltage is supplied.
Further, the protection circuit is provided also in the case of the program voltage terminal being an input/output terminal of the internal circuit. In that case, the protection circuit is used in the state in which its input/output terminal is not supplied with a voltage higher than the power supply voltage from a power supply voltage terminal, and when a voltage higher than the power supply voltage is applied, the protection circuit operates accepting all the cases as static electricity mixing.
For example, it is assumed that the protection circuit is composed of a positive voltage protection diode for protecting the internal circuit from a static electricity of positive voltage and a negative voltage protection diode for protecting the internal circuit from a static electricity of negative voltage. It is conceivable that either static electricity of positive voltage or static electricity of negative voltage intrudes at the input/output terminal. When the static electricity of positive voltage intrudes therein, the protection circuit operates to pass the electricity to a GND terminal due to the forward characteristic of the positive voltage protection diode, and when the electricity of negative voltage intrudes therein, the protection circuit operates to pass the electricity to the power supply voltage terminal due to the forward characteristic of the negative voltage protection diode.
The configuration and operation of the conventional protection circuit as described above will be explained concretely here.
FIG. 9
is a circuit diagram showing the configuration of a semiconductor integrated circuit provided with the conventional protection circuit. This protection circuit is a circuit for protecting an internal circuit
9
from static electricity which intrudes at a program voltage terminal
4
and comprises a first circuit portion
1
and a second circuit portion
11
.
The first circuit portion
1
comprises a positive voltage protection diode
14
connected between a GND line
8
and a program voltage line
10
in a forward direction as viewed from the program voltage terminal
4
, and a negative voltage protection diode
16
connected between the program voltage line
10
and a power supply voltage line
12
in a backward direction as viewed from the program voltage terminal
4
.
The second circuit portion
11
comprises a negative voltage protection transistor
18
connected between the GND line
8
and the program voltage line
10
in parallel with the positive voltage protection diode
14
, and a positive voltage protection transistor
20
connected between the program voltage line
10
and the power supply voltage line
12
in parallel with the negative voltage protection diode
16
.
A GND terminal
2
at the ground potential and an input/output terminal
3
of the internal circuit
9
are connected to the GND line
8
. The program voltage terminal
4
which is a pad for supplying a program voltage and a memory device
50
are connected to the program voltage line
10
. A power supply voltage terminal
6
for supplying a power supply voltage and an input/output terminal
7
of the internal circuit
9
are connected to the power supply voltage line
12
.
The positive voltage protection diode
14
and the positive voltage protection transistor
20
are connected each other to protect the internal circuit
9
from static electricity of positive voltage which intrudes at the program voltage terminal
4
. Further, the negative voltage protection diode
16
and the negative voltage protection transistor
18
are connected each other to protect the internal circuit
9
from static electricity of negative voltage which intrudes at the program voltage terminal
4
. The memory device
50
is a memory device of a junction breakdown-type which requires a current for writing or a fuse element breakdown-type which is a type of being fused by a large current and connected between the GND line
8
and the program voltage line
10
.
Meanwhile, a transistor has a PN junction as in a diode, and thus the transistor can be regarded as one diode. However, because of the existence of a gate terminal, the transistor (FET) is low in withstand voltage of the reverse junction to the PN junction as compared with the diode due to its configuration. The withstand voltage can further decrease in accordance with the voltage applied to the gate terminal.
More specifically, assuming that there is no second circuit portion
11
composed of the negative voltage protection circuit
18
and the positive voltage protection transistor
20
and thus the protection circuit composed of only the first circuit portion
1
as shown in
FIG. 10
, the transistor of the internal circuit
9
is often broken first. Assuming such a case, it is necessary to provide the second circuit portion
11
as in the protection circuit shown in FIG.
9
.
Regardless of whether or not each of these protection circuits has the second circuit portion
11
, it operates normally if the magnitude of the voltage applied to the program voltage terminal
4
is that of either the power supply voltage or the ground voltage.
However, a protection circuit comprising a typical diode or transistor as in the above-described conventional protection circuit can not discriminate between the static electricity and the program voltage which intrude at the program voltage terminal. Therefore, there is a problem that either the positive voltage protection diode
14
or the negative voltage protection diode
16
operates even when the program voltage greater than the power supply voltage is applied to the program voltage terminal
4
, making it impossible to write into the memory device
50
.
Further, in the case in which both of the power supply voltage and the program voltage are at negative voltage in the protection circuit shown in
FIG. 10
, when the program voltage is supplied to the program voltage terminal
4
, a drawn current
53
flows in from the power supply voltage terminal
6
because the magnitude of the program voltage is greater than that of the power supply voltage. For this reason, there is also a problem that a current required for writing into the memory device
50
can not be obtained and, more than that, the internal circuit
9
malfunctions due to the change in power supply voltage.
If the protection circuit is configured to cope with a program voltage which can be applied thereto and operate only when static electricity with a different polarity (for example, positive voltage) from that of the program voltage in
Imai Toshio
Takizawa Toru
Abraham Fetsum
Armstrong Westerman & Hattori, LLP
Citizen Watch Co. Ltd.
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