1. Field of the Invention
The present invention is directed to a semiconductor device, especially to a device comprising a dc voltage circuit for generating an internal supply voltage, a device comprising a memory cell array, and a device comprising a plurality of transistors having a SOI structure.
2. Background of the Invention
In order to perform various processes with only one semiconductor chip, there has been a semiconductor device comprising a semiconductor chip on which a plurality of devices are integrated and arranged. Since each of the devices may require a different supply voltage, it becomes necessary to generate an internal supply voltage which is different from an external supply voltage supplied from the outside.
FIG. 12 shows an example of the dc voltage circuit for generating one of the internal supply voltages inside the semiconductor chip. To nodes N1 and N3, different external supply voltages are applied, respectively. The external supply voltage applied to the node N3 includes a ground voltage. A transistor 3 connected to the node N1 supplies the internal supply voltage to an internal circuit 2 formed in a semiconductor chip 1. The output of the transistor 3 is controlled by a differential amplifier 4 so that the internal supply voltage corresponds to a reference voltage Vref. For that reason, in the dc voltage circuit comprised of the transistor 3 and the differential amplifier 4, the reference voltage Vref and the internal supply voltage are applied to an inverting input terminal and a non-inverting input terminal of the differential amplifier 4, respectively, and then the output of the differential amplifier 4 obtained by amplifying the difference in voltage between those terminals is transmitted to the gate of the transistor 3.
Further, for a large distance between the internal circuit 2 and a node N2 to which the internal supply voltage is applied from the dc voltage circuit, parasitic resistance between them causes a voltage drop, making the value of the internal supply voltage to be applied to the internal circuit 2 different from that of the reference voltage Vref. If the dc voltage circuit is formed in the vicinity of the internal circuit 2 to reduce the voltage drop, supply of the reference voltage Vref becomes difficult.
In order to improve processing efficiency, there has been a semiconductor device in which a logic and a memory are integrated on one semiconductor chip. In such a semiconductor device, an external bus would neither affect the processing nor limit the processing speed. A DRAM is one of the most frequently employed memories in such a semiconductor device. Though achieving high integration and low cost, the DRAM requires a boost voltage to be applied to a selected gate of a memory cell. A gate oxide film of a transistor in the memory cell, to which the boost voltage is applied, requires a certain degree of thickness.
FIG. 13 is a graph showing correlation among the external supply voltage supplied to the semiconductor chip from the outside, the boost voltage and the internal supply voltage both generated inside the semiconductor chip. In FIG. 13, the solid line with the reference numeral 31 indicates the internal supply voltage; the solid line with the reference numeral 32 indicates the boost voltage (VPP); the dotted line with the reference numeral 34 indicates a voltage two times the external supply voltage; and the dotted line with the reference numeral 35 indicates a voltage three times the external supply voltage. The internal supply voltage 31 has constantly been increased to obtain the boost voltage VPP.
As described above, in the semiconductor device comprising the conventional dc voltage circuit, the differential amplifier directly drives a gate voltage of the transistor for supplying the internal supply voltage. This increases an output load capacity of the differential amplifier and prevents a high-speed operation of the dc voltage circuit. Thus, when the internal circuit operates at high frequency, the internal supply voltage cannot be stable.
Further, parasitic resistance or the like prevents supply of the desired internal supply voltage to the internal circuit formed in any location in the semiconductor chip.
Moreover, besides the current necessary for charge or discharge of the boost voltage, another current is necessary to generate the boost voltage inside the semiconductor chip, which increases the consumption of current in the whole semiconductor chip.