LDMOS for RF Power Amplifiers

David Fernandez

Outline

Power Amplifier Critical Factors for Performance

LDMOS Device Technology LDMOS Power Amplifier performance Future trends and challenges for LDMOS References

RF Power Amplifier

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

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

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

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

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.

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:

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5.34*10 ^13 4( ) ^ 3DN BV

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.

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.

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