LDMOS for RF Power Amplifiers

Power Amplifiers
David Fernandez
Power Amplifier Critical Factors for
 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.
LDMOS Device Technology
Channel formed by difference in lateral extension of Pbase and N+ source regions
Both regions self-aligned to left-hand side during ionimplantation
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 :
wt 
Higher Break down voltage (75 – 80 V) as a
result of proper doping of LDD region:
5.34*10 ^13 4
ND  (
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.
“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