1. LDMOS for RF Power Amplifiers
David Fernandez

2. Outline Power Amplifier Critical Factors for Performance
LDMOS Device Technology
LDMOS Power Amplifier performance
Future trends and challenges for LDMOS
References

3. RF Power Amplifier

4. Power Amplifier Critical Factors
Linearity – Critical when signal contains both amplitude and phase modulation
Power Efficiency – Defined as Pout/Pdc
Break Down Voltage – Cellular Base Station application have supply voltages of near 30V.
High Frequency – parasitic capacitances should be minimal.
Gain
Cost
Integrated

5. LDMOS Device Technology
Channel formed by difference in lateral extension of P-base and N+ source regions
Both regions self-aligned to left-hand side during ion-implantation
P-sinker, highly doped, connects source to substrate creating source connected ground plane

6. LDMOS Device Technology
Shield between gate and drain to reduce feedback capacitance and combat threshold ‘drift’.
LDD region formed by light N-type dopant
Doping of the LDD region strongly correlated with breakdown voltage

7. RF LDMOS Power Amplifier
Better Linearity as a result of shielding
High electric field at gate edge in LD-Mosfet results in electron injection into gate oxide leading to vthreshold drift which deteriorates linearity. Shielding mitigates.
Reduction of feedback capacitance improves linearity

8. RF LDMOS Power Amplifier
Better Gain and Cost accomplished through directly grounding source.
With direct source grounding as compared to other power mosfets, no inductive bond-wires needed to connect source to package ground terminal. Source inductance deteriorates gain at high frequencies.
No complex and costly packaging needed to keep drain insulated from ground terminal – drain and ground terminal are on opposite sides of wafer.

9. RF LDMOS Power Amplifier
Lateral expansion – smaller channel length, resulting in higher frequency potential :
Higher Break down voltage (75 – 80 V) as a result of proper doping of LDD region:

10. RF LDMOS Power Amplifier
Power efficiency improved through lower output capacitance compared to other power mosfets.
Integrated – Gate and drain terminals are on the same side of wafer.

11. Challenges and future trends
Continued device innovations has led to 7th generation LDMOS Power Mosfets that provide improved RF performance and remain low-cost.
Thermal resistance as decreased and as a result the reliability of these devices is improved.
Linearity has improved through both circuit and device design approaches.
Currently compound semi-conductor devices (GaN) offer comparable if not improved efficiency and linearity, however cost and reliability issues make it difficult to displace LDMOS in the near future.

12. References “Silicon RF Power Mosfets,” B Jayant Baliga
“RF Power Amplifiers for Wireless Communications,” Steve Cripps
“Challenges and Opportunities for Compound semiconductor devices in Next Generation Wireless Base Stations Power Amplifiers,” Lawrence Lawson IEEE 2005
“Status and Trends of silicon LDMOS base station PA technologies to go beyond 2.5 GHz applications,” F. Van Rijs IEEE 2008
“A LDMOS Technology Compatible with CMOS and Passive Components for Integrated for RF Power Amplifiers,” Jun Cai et.al, IEEE 2000