Static information storage and retrieval – Read/write circuit – Having fuse element
Reexamination Certificate
2001-01-26
2001-12-04
Nelms, David (Department: 2818)
Static information storage and retrieval
Read/write circuit
Having fuse element
C365S230030, C365S230060, C365S189011
Reexamination Certificate
active
06327211
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inverter, in particular, to an inverter which has a variable threshold potential.
2. Description of the Background Art
Conventionally, in a semiconductor integrated circuit, such as a flash memory, many inverters have been used in order to carry out a signal level detection, a signal transmission, a signal delay and the like. An inverter outputs an “H” level in case that an input signal level is lower than a threshold potential VTH, and outputs an “L” level in case that the input signal level is higher than the threshold potential VTH. However, for various reasons, there are some cases that the threshold potential VTH of a manufactured inverter diverges from its design value.
In the prior art, as shown in
FIG. 22
, a plurality (three in
FIG. 22
) of P-channel MOS transistors
121
to
123
and an N-channel MOS transistor
124
are formed so as to constitute an inverter. First, the P-channel MOS transistor
121
and the N-channel MOS transistor
124
configure an inverter, which functions to measure a threshold potential VTH of the inverter. On the basis of the measurement result, any one of a plurality of masks is selected and, if necessary, the P-channel MOS transistor
122
and/or
123
has been connected in parallel to the P-channel MOS transistor
121
in order that a threshold potential VTH of the inverter is adjusted.
However, in the prior art, a plurality of masks have to be prepared in advance, which results in taking significant amount of time and labor. Moreover, a wafer manufacturing process is temporally interrupted to measure a threshold potential VTH of a manufactured inverter, and a mask is selected and set based on the measurement result, which results in taking significant amount of time and labor.
SUMMARY OF THE INVENTION
The present invention has been made taking the aforesaid problem in the prior art into consideration. It is, therefore, a principal object of the present invention to provide an inverter which its threshold potential can be changed in a simple manner at a low cost.
According to one aspect of the present invention, a first transistor of a first conductive type is connected between a line of a first source potential and an output node, a second transistor of a second conductive type is connected between a line of a second source potential and the output node, a third transistor of a second conductive type and a fuse are connected between a line of a second source potential and the output node, and each input electrode of the first to third transistors is connected to an input node. Therefore, the fuse is blown out, and thereby, a threshold potential of the inverter is changed, so that the threshold potential of the inverter can be changed in a simpler manner at a low cost as compared with the conventional case where the threshold potential of the inverter has been changed by exchanging masks.
Preferably, the third transistor and the fuse are provided in plural sets, and current driving forces of a plurality of the third transistors are set to values different from each other. In this case, it is possible to change the threshold potential of the inverter into more variety of steps.
According to another aspect of the present invention, a first transistor of a first conductive type is connected between a line of a first source potential and an output node, a second transistor of a second conductive type and a first variable resistor circuit are connected in series between the output node and a line of a second source potential, and the inverter is provided with a first threshold setting circuit including at least one first fuse, and, by setting a resistance value of the first variable resistor circuit on the basis of the result whether or not the first fuse is blown out, setting a threshold potential of the inverter. Therefore, by blowing out the fuse, a threshold potential of the inverter can be changed, so that the threshold potential of the inverter can be more simply changed at a lower cost as compared with the conventional case where the threshold potential of the inverter has been changed by exchanging masks.
Preferably, the first variable resistor circuit include a plurality of the third transistors of the second conductive type which are connected in parallel between one electrode of the second transistor and the line of the second source potential, and the first threshold setting circuit make each of the third transistors conductive or nonconductive on the basis of the result whether or not the first fuse is blown out. Therefore, in this case, it is possible to readily constitute the first variable resistor circuit, and to easily set the resistance value of the first variable resistor circuit.
Preferably, current driving forces of a plurality of the third transistors are set to values different from each other. Therefore, in this case, it is possible to change the threshold potential of the inverter into more variety of steps.
Further, preferably, the first fuse is provided so as to correspond to each of the third transistors, and the first threshold setting circuit make the corresponding third transistors conductive or nonconductive in accordance with the result that the first fuse is blown out. Therefore, in this case, it is possible to readily constitute the first threshold setting circuit.
Further, preferably, the first fuse is provided fewer than a number of the third transistors, the first threshold setting circuit include a first decoder which outputs a plurality of first control signals of the same number as the third transistors on the basis of the result whether or not the first fuse is blown out, and a plurality of the third transistors become conductive or nonconductive in response to a plurality of the first control signals. Therefore, in this case, it is possible to change the threshold potential of the inverter in a multi-step by fewer first fuses.
Further preferably, the inverter further includes: a second variable resistor circuit which is interposed between one electrode of the first transistor and the line of the first source potential; and at least one second fuse, and a second threshold setting circuit are provided, which set a threshold potential of the inverter by setting the resistance value by the second variable resistor circuit on the basis of the result whether or not the second fuse is blown out. Therefore, in this case, it is possible to make the threshold potential of the inverter higher or lower.
Further preferably, the second variable resistor circuit include a plurality of fourth transistors of first conductive type which are connected in parallel between one electrode of the first transistor and the line of the first source potential, and the second threshold setting circuit make each of the fourth transistors conductive or nonconductive on the basis of the result whether or not the second fuse is blown out. Therefore, in this case, it is possible to readily constitute the second variable resistor circuit, and to easily set the resistance value of the second variable resistor circuit.
Further preferably, current driving forces of a plurality of the fourth transistors are set to values different from each other. Therefore, in this case, it is possible to change the threshold potential of the inverter into even more variety of steps.
Further preferably, the second fuse is provided so as to correspond to each of the fourth transistors, and the second threshold setting circuit make the corresponding fourth transistors conductive or nonconductive in accordance with the result that the first fuse is blown out. Therefore, in this case, it is possible to readily constitute the second threshold setting circuit.
Further preferably, the second fuse is provided fewer than a number of the fourth transistors, the second threshold setting circuit include a second decoder which outputs a plurality of second control signals of the same number as the fourth transistors on the basis of the result whether or not the second fuse is
Kai Yoshihide
Kobayashi Kazuo
Lam David
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
Nelms David
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