This invention relates to semiconductor devices, for example, semiconductor devices for thermal heads of printers.

According to one aspect of the present invention there is provided a semiconductor device comprising a drive circuit and a logic circuit connected in series characterised by the drive circuit and the logic circuit comprising complementary metal oxide semiconductors.

The drive circuit may consist of a P-channel metal oxide semiconductor or an N-channel metal oxide semiconductor.

Preferably means are provided for driving the drive circuit and the logic circuit with different voltages.

The drive circuit may be arranged to supply a constant current.

According to a further aspect of the present invention there is provided a thermal head for a printer including a semiconductor device according to the present invention.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which:-

• Figures la and 1b are plan and side views respectively of a thermal head for a printer;
• Figure 2 is a diagram showing the basic arrangement of a heating element and drive circuit of the thermal head of Figure 1;
• Figure 3 is a block diagram of a drive circuit and logic control circuit of a conventional thermal head;
• Figure 4 is a diagram of one embodiment of a semiconductor device according to the present invention; and
• Figure 5 illustrates a semiconductor device according to the present invention acting as a driver circuit for a thermal head of a printer.

As shown in Figures la and lb, a thermal head 1 for a printer consists of a heating resistor 3, a drive circuit 4, and a logic control circuit 5, these components being mounted on a ceramic substrate 2.

Since the heating resistor 3 consumes a relatively large current, a bipolar element is conventionally used in the drive circuit 4 for driving the thermal head. Figure 2 shows the basic arrangement of the heating resistor 3 and the drive circuit 4 which are connected in series with a power source 8. As the temperature rises, the leakage and the amplification factor of the bipolar element increases, so that the current consumed by the thermal head increases. Heat is generated by this increase in current and hence an even larger current flows. This phenomenon is sometimes referred to as "thermal runaway". If the bipolar element is mounted on the same substrate as the heating resistor, conditions worsen as a result of the heat generated and, if the substrate 2 has poor heat radiating efficiency, the bipolar element undergoes thermal breakdown. Thermal runaway and erroneous operation caused by heat raise serious problems regarding the reliability of conventional thermal heads.

Thermal heads mainly are of the linear type or the serial type. In recent years, linear thermal heads have been put into practice, and attention has been given to the generation of heat and consumption of large current by bipolar elements of such thermal heads. In a linear thermal head, 3 to 16 heat generator dots are provided per millimetre, and a driver circuit is provided for each. For printing a line on an A-4 page, about 1700 dots are required if 8 dots are provided per millimetre in the standard manner. Each dot must be supplied with a current of between 20 mA and 80 mA to generate heat, which requires a power source with a relatively large capacity. It therefore becomes necessary to reduce the current flowing in the logic control circuit to be as small as possible to minimise the load on the power source.

Figure 3 shows a conventional thermal head employing a bipolar element. The logic control circuit 5 and the drive circuit 4 are supplied with a voltage of 4 to 6 volts from a power source 7, to supply heating elements with a relatively high voltage of, for example, 15 to 30 volts.

Referring now to Figure 4, there is shown one embodiment of a semiconductor device according to the present invention incorporated into a thermal head for a printer. The drive circuit 4 is constituted by an open drain N-channel metal oxide semiconductor (MOS) to cope satisfactorily the voltage and current requirements, and the logic control circuit 5 is constituted by a complementary MOS to reduce current consumption. The development of a latch-up phenomenon by a high voltage and heavy current is prevented by using the N-channel MOS for the drive circuit 4. This same effect can be achieved using a P-channel MOS for the drive circuit 4 provided it has an open drain. The same effect can also be obtained using a complementary MOS, but this depends upon the conditions. A silicon substrate on which a complementary MOS is formed is usually N-type. When an N-channel MOS is used for the drive circuit 4, however, the silicon substrate must be of P-type.

The reason why a metal-cxide semiconductor is not subject to thermal runaway will now be explained. If the drive circuit 4 comprises an N-channel MOS as shown in Figure 4, current I flowing in the N-channel MOS is given by:

where:

• I: current flowing in the N-channel MOS,
• W: channel width of the N-channel MOS,
• L: channel length of the N-channel MOS
• p: mobility
• C: gate capacity
• V_G: gate voltage of the N-channel MOS,
• V : threshold voltage of the N-channel MOS,
• V : drain-source voltage of the N-channel MOS.

In equation (1), the parameters which change with temperature are the mobility µ and the threshold voltage V . The mobility p has a negative temperature coefficient and, hence, acts to reduce the current I as the temperature rises. The threshold voltage V_T also has a negative temperature coefficient so that the expression (V_G - V_T) increases with temperature and this acts to increase the current I. In equation (1), the temperature coefficients of the mobility p and the expression (V_G - V_T) cancel and, hence, the current I is automatically prevented from increasing with any increase in temperature. Therefore, if thought is given as how to radiate the heat produced by the heating resistor or resistors of a thermal head, the heat produced by the drive circuit 4 and by the heating resistor or resistors will have no significant effect on reliability. Accordingly, if the thermal head is provided with a suitable radiator plate it can manufactured at reduced cost, and reliability can be enhanced.

The semiconductor device of Figure 4 consists of the logic control circuit 5, the drive circuit 4 which is arranged to supply constant current, and a.level shifter 6. Figure 5 illustrates the drive circuit 4 in detail connected in series with the heating resistor 3.

The logic control circuit 5 is driven by a relatively low voltage from the power source 7 to reduce its current consumption, and the drive circuit 4 is driven by a relatively high voltage from the power source 8 via the level shifter 6. The heating resistor 3 which conventionally is driven by a bipolar element is here driven by a MOS. However, a current equivalent to that of the bipolar element must be supplied through the _MOS, and thus a voltage as high 15 to 30 volts must be applied to the drive circuit 4 from the level shifter 6. However, the logic control circuit 5, which consists of a CMOS, consumes a large current if it is driven by a voltage of 15 to 30 volts, which offsets the advantages of using a CMOS. An operating voltage of 4 volts to 6 volts is sufficient to accomplish speed of operation of less than 10 MHz required by the CMOS. For reasons of current consumption and stability of operation, therefore, a voltage of 4 volts to 6 volts is applied to the logic control circuit 5.

Up until now, a bipolar element has been used in the logic control circuit because a relatively large drive current was required, which resulted in thermal runaway and eventual thermal breakdown. According to the present invention, on the other hand, the relatively large drive current is obtained by a MOS. Namely, the drive circuit is operated at a relatively high voltage to correct the defects of the MOS, and the occurrence of thermal breakdown is prevented by utilising the saturation of the driving current according to temperature of the MOS. The consumption of electric current, moreover, is reduced by the use of a CMOS. That is, high-speed operation comparable to that of a bipolar element is accomplished with a current which is less than one- thirtieth of the current needed by a bipolar element.

The semiconductor device according to the present invention and described above when used in a thermal head has the advantages of small current consumption, high speed, and increased reliability and with any future increase in dot density of thermal heads should be able to operate at faster speeds.

A semiconductor device according to the present invention can be used not only with thermal heads but also with plasma display devices, fluorescent display tubes, and other devices which require a high voltage and large current. Further, although the drive circuit 4 has been described as being constituted by an N-channel MOS it may be constituted by a P-channel MOS or a CMOS.