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.
