Overview

Vertical power MOSFETs, fabricated using the double-diffusion technology, have been commercially available since the mid-1970s. The double-diffusion process allowed control of the channel length to the micron dimension without the need for expensive state-of-the-art lithography tools. The high input impedance of the MOS-gate structure simplified the drive circuit requirements when compared with bipolar transistors being used at that time. In addition, their superior switching speed opened new applications operating at 10 50 kHz. During the last three decades, the design and fabrication technology for power MOSFETs has been improved to enable extension of their applications to switching frequencies up to 1 MHz. The physics and design methodology for these devices has been treated in detail in several textbooks ^{[1] , [2]}.

The vertical double-diffused (VD) MOSFETs were also optimized for RF applications by replacing the traditional polysilicon gate with metal gate structures. Although the vertical architecture enables the design of high voltage devices with significant output power, their operating frequency was limited to below 500 MHz. With a focus on serving avionics and pulse-power applications, the VD-MOSFETs available in the market do not exhibit adequate linearity for cellular power amplifiers.

In this chapter, the basic operating principles of the VD-MOSFET structure will be discussed. The physics of operation will be elucidated by using the results of two-dimensional numerical simulations ^[3] of a typical cell design. Although the exact values for the doping profiles, the gate oxide thickness, and channel length may vary from manufacturer to manufacturer, the basic...