1. Field
This disclosure relates generally to transistors, and more specifically, to a laterally diffused metal oxide semiconductor (LDMOS) transistor for a radio frequency power amplifier and method therefor.
2. Related Art
Laterally diffused metal oxide semiconductor (LDMOS) transistors are used in high power analog circuits. One application for LDMOS transistors is in radio frequency (RF) power amplifiers. Optimization characteristics such as power density and linearity often conflict. That is, it may be difficult to have good power density without compromising linearity. Conversely, it may be difficult to have good linearity without compromising power density and/or gain. Also, there are other trade-offs that can be important.
A conventional RF LDMOS transistor is constructed on a relatively thin P-type epitaxial silicon layer over a highly doped P-type silicon substrate. In addition to a standard MOSFET structure, one common LDMOS transistor structure includes a laterally diffused P-well, a sinker, and a lightly doped drain region. The lightly doped drain is formed by implanting an N-type doping through a screen oxide in the selected region after forming a gate electrode. A doping level of the lightly doped drain has a large impact on power density, efficiency, and hot carrier injection (HCI) reliability. A common way to improve power density is to increase the doping level of the lightly doped drain. However, a consequence of increased drain doping is an undesirable increase in the peak electric-field in the drain region that reduces HCI reliability, thus limiting the increased power density.
Another LDMOS transistor structure is the same as the above described structure except that the lightly doped drain region extends beyond an edge of the gate electrode and into the channel region underneath the gate electrode. The extended lightly doped drain is formed prior to gate electrode formation by diffusing an N-type layer across the silicon surface. This structure also improves power density; however, the increased power density causes a substantial degradation in linearity. In both of the above described LDMOS structures, improving power density can be difficult without compromising other performance related characteristics, such as linearity, efficiency, and HCI reliability.
Therefore, what is needed is an LDMOS transistor that solves the above problems.