Micro Device Division, Hitachi, Ltd. developed the following two technologies for High voltage IC platform suitable for measuring instruments, medical equipments and in-vehicle equipments. One is high performance P-channel-LDMOS transistors on wide range rated voltage from 35 V to 200 V SOI LDMOS platform technology. The other is 300 V Field-MOS FETs for High-Voltage switching IC's. These technologies will enable to integrate a lot of discrete components and High Voltage Transistors on single chip. These technologies were developed to meet demands shown below.
Each transistor used in semiconductor integrated circuits has three terminals called gate, drain and source. A transistor breaks when overvoltage is applied. It is called breakdown voltage that the maximum voltage for a transistor to be applied without breakdown. In conventional technology, transistors each having its own breakdown voltage between source and drain from 35 V to 200 V are made by their own manufacturing process. It is difficult to integrate a lot of discrete components and such types of transistors on a chip. Enabling these components and transistors to be integrated on a chip is the key issue for miniaturization of instruments and equipments.
The technology that enables to integrate transistors on a chip with various values of breakdown voltages between source and drain from 35 V to 200 V has been developed. The technology uses transistors with various length in horizontal direction but only with the same structure in depth direction (see Figure 1). Then there is no need to use additional photo masks to make such kind of transistors.
Figure 1. Schematic structure
of P-channel LDMOS
Figure 2. Breakdown voltage and on-resistance
of 35 V to 200 V LDMOS transistors
Through the optimization of the manufacturing process for the transistors shown above, very low on-resistance was obtained with keeping high on/off-state breakdown voltages (see Figure 2).
Since the technology, mentioned above, enables components to be integrated on a chip, then it will be able to miniaturize instruments and equipments by using the chip, and improve reliability by reducing the number of components used in them.
In conventional technology, transistors with breakdown voltage over 200 V between source and drain have not so high breakdown voltage between gate and source. Therefore there must be the dedicated circuit for inputting high voltage signal into gate terminal of a transistor. To eliminate such kind of dedicated circuits and transistors is another key issue for minimizing chip-size and consumption power of chips.
The technology that enables to make transistors with breakdown voltage over 300 V between gate and source has been developed. The technology adopts the LOCOS type oxide film for gate oxide of High voltage transistors (see Figure 3). The processes for making such kind of transistors were optimized using computer simulation (see Figure 4). Finally, proto-type transistors have shown their breakdown voltage between gate and source terminals over 400 V in typical usage, that is to say, developed transistors have chances in various kinds of applications that require high reliability (see Figure 5).
Figure 3. Schematic structure
of N-channel MOS
Figure 5. On-state characteristics
of N-channel MOS
Figure 4. Device simulation of N-channel MOS
As a result, the dedicated circuit for inputting high voltage signal into gate terminal of a transistor can be eliminated, and the second demand was met.
Micro Device Division, Hitachi, Ltd. has developed technologies for several types of transistors for High voltage IC's to meet demands in application areas, for example, measuring instruments, medical equipments and in-vehicle equipments. By using these new technologies, Micro Device Division will be able to give chances many customers to develop their newly valuable products.
The technologies above were presented in ISPSD (International Symposium on Power Semiconductor Devices & IC's) 2011, held in San Diego, California, USA from May 23.
