1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device wherein an integrated circuit including MOSFETs allows a potential of its substrate having the device to be varied.
2. Description of the Related Art
An integrated circuit includes many MOSFETs. ON-OFF switching characteristics of a transistor depend on a threshold voltage of each MOSFET. The threshold voltage depends on limitations of the integrated circuit such as the speed, the standby current, etc. i.e. the current drivability of the MOSFET or the leakage current of the MOSFET when the gate voltage is 0 V.
The threshold voltage of the MOSFET is generally determined by the thickness of a gate oxide film or the impurity concentration in the Si substrate under the gate oxide film. In general, in order to increase the threshold voltage, it is merely necessary to increase the thickness of the gate oxide film or the impurity concentration in the Si substrate under the gate oxide film. On the other hand, in order to lower the threshold voltage, it is necessary to reduce the thickness of the gate oxide film and the impurity concentration in the Si substrate under the gate oxide film. However, if the threshold voltage is increased, the current drivability of the MOSFET fails while the leakage current is restricted. On the other hand, if the threshold voltage is decreased, the current drivability of the MOSFET increases and at the same time the leakage current increases.
As explained above, when the threshold voltage of the MOSFET is set, the leakage current and the current drivability are set by themselves. If the MOSFET is scaled down, the thickness of the gate oxide thickness must be reduced small to prevent the punch-through and the short channel effect. In this case, a preferable threshold voltage may not be obtained unless the impurity concentration is excessively increased.
There is a manner proposed to solve this problem, such as supplying a substrate bias to a portion of the integrated circuit or all the portions thereof, and this is accomplished mainly in a DRAM. Since the substrate bias causes the threshold voltage of the MOSFET to be increased, the leakage current can be lowered even at the time when the impurity concentration is slightly low. Then, it has been proposed and accomplished to vary the impurity concentration in the Si substrate under the gate oxide film of the MOSFET in the integrated circuit in accordance with an area of the substrate, to set the threshold voltage of the MOSFET to be small for the purpose of increasing the current drivability, or to set the threshold voltage of the MOSFET to be great for the purpose of increasing the leakage current.
This improvement manner is effective when the concentration is low or when the operating voltage is under 5V. However, if the degree of the integration of MOSFET is increased, difficulty in the processes is also increased, since preparation manners for the high-speed operation is not consistent with that for the low standby. If the operating voltage is lowered, a rate of the threshold voltage to the operating voltage is increased to keep off-leakage current, and thus the difficulty is further increased.
It is analytically known that the threshold voltage should be under 0.3 V to maintain the high-speed operation, i.e. the threshold voltage should be approximately under 20% of the operating voltage, for example when the operating voltage is 1.5 V. On the other hand, in order to make the standby current of an integrated circuit having more than 300,000 logic gates approximately under 10 .mu.A, the threshold voltage .should be higher than 0.6 V. If operating voltage is different, since the threshold voltage for keeping high-speed operation is different, for example, the threshold voltage of 0.6 V is enough for high-speed operation when an operating voltage is 3 V, however the threshold voltage than 0.3 V is required when an operating voltage is 1.5 V. Therefore it is very difficult to set both a proper threshold voltage and a low standby current in the conventional manner.
As described above, in the semiconductor device having the MOSFETs, only one threshold voltage value can be set by one MOSFET in the integrated circuit. Setting both the high-speed operation of the integrated circuit and the low standby current or determining the optimum threshold voltage at which the operating voltages are different is difficult in prior art.