Fujitsu Develops CMOS Logic-Based High-Voltage Transistor for Power Amplifiers

Fujitsu Microelectronics Asia announced that Fujitsu Laboratories Limited and Fujitsu Microelectronics Limited have co-developed a CMOS logic(1) process-based high-voltage transistor featuring high breakdown voltage, suitable for power amplifiers used in wireless devices. The new technology makes it possible for power amplifiers to be formed on the same die as CMOS logic control circuitry to achieve single-chip integration, thereby making high-performance, low-cost power amplifiers feasible.

Due to the fact that power amplifiers for wireless devices demand high power output at high frequencies, currently compound semiconductors such as gallium-arsenide (GaAs) are commonly used, mounted as a chip separate from control circuitry based on a general-purpose CMOS logic chip. If these chips' functions could be integrated onto a single chip, it would enable cost reduction of the overall module and likely speed adoption of wireless devices to be used with wireless communication standards such as WiMAX and LTE(3). Thus, there is a need for transistors that are compatible with CMOS logic process technology, and which can satisfy the requirements of power amplifiers necessary for WiMAX and other wireless communication standards.

Technological Challenges - The power output required of power amplifiers for use in high-frequency applications, such as WiMAX, exceeds the breakdown voltage of transistors used with standard CMOS logic processes. Overcoming this hurdle while remaining compatible with CMOS process technology requires an increase in the transistor's breakdown voltage, which is achieved with a structure that lowers the electric field around the drain, as electric fields can lead to transistor failure. Furthermore, structures with high breakdown voltages typically increase the transistor's on-resistance(4), making it difficult to obtain satisfactory performance at high frequencies. Therefore, any solution would need to both raise breakdown voltage and avoid increasing on-resistance.

Newly Developed Technology - To overcome the aforementioned issues, Fujitsu developed a new transistor structure with the following key characteristics:
1.    The transistor's drain is surrounded by a "lightly doped drain" (LDD) region, which overlaps with the gate. This lowers the electrical field extending horizontally to the drain, and the electrical field extending to the gate oxide layer, thereby raising the breakdown voltage.
2.    The dopant distribution in the transistor channel follows a lateral gradient. This lowers the density of dopant on the drain side of the channel, thus limiting the increase in drain resistance, which is the main part of on-resistance. It also lowers the electrical field extending horizontally to the drain, also raising the breakdown voltage.