Efficiency Improvement of LDMOS Transistors for Base Stations

Efficiency Improvement of LDMOS Transistors for Base Stations
Efficiency Improvement of LDMOS
Transistors for Base Stations:
Towards the Theoretical Limit
F. van Rijs and S.J.C.H. Theeuwen
Ampleon, Halfgeleiderweg 8, 6534 AV, Nijmegen, The Netherlands
Email: [email protected]
Original publication: International Electron Devices Meeting, IEDM2006, pp. 205-208 (2006)
ABSTRACT
ACHIEVED PERFORMANCE TRENDS
We present the evolution in LDMOST technology of the last
decade, leading to a present 32 percent efficiency value for
a two carrier W-CDMA signal with -37 dBc IM3 and discuss
future prospects to increase the performance even further.
The achieved reduction of parasitic elements currently opens
the way for system concepts such as high efficiency classes
and Doherty type concepts.
In Figure 1 the realized increase in drain efficiency with a
2 carrier W-CDMA signal at -37 dBc IM3 is shown for the
LDMOST technology generations with the feature size
(gate-length) in brackets. The improvement rate is almost
2 percent point per year. The increase in efficiency has
been accompanied by an improvement in other relevant
parameters, as shown in Table 1 with the higher power
gain as most important one. Figure 2 shows the W-CDMA
performance at 2 GHz of the latest devices. The hot carrier
degradation as characterized by Idq-degradation extrapolated
to 20 years has been decreased significantly from year 2000
on and kept well under control after that [4] (Figure 3). The
power density has been increased with 60 % to a value of 0.66
W/mm. For smaller devices the value approaches 1.0 W/mm.
INTRODUCTION
In base stations for personal communication systems (GSM,
EDGE, W-CDMA), RF power amplifiers are key components. For
these power amplifiers, RF Laterally Diffused MOS (LDMOS)
transistors are the standard choice of technology because of
their excellent power capabilities, gain, linearity and reliability.
To meet the demands imposed by new communication
standards, the performance of LDMOS is subject to
continuous improvements [1,2,3]. Wideband CDMA (W-CDMA)
requires linear operation of the PA, which means operating
the amplifier sufficiently far in back off reducing at the same
time the efficiency of the PA. Nowadays much attention is paid
to improve this trade-off between linearity and efficiency on
device and system level.
In this paper, we present the evolution obtained in LDMOST
technology over the last 6 years; the continuous improvements
made in LDMOS technology which led to a present 32 percent
W-CDMA efficiency world-record value, and the future
prospects to increase the performance even further. Attention
will be paid to the maximum achievable theoretical limit.
1
Figure 1: Realized W-CDMA efficiency at 2.14 GHz versus time for
100 W LDMOS-transistors. The used gate-length is shown in brackets.
Amplify the future | Efficiency Improvement of LDMOS Transistors for Base Stations: Towards the Theoretical Limit
2000
2004
2005
2006
90 W
100 W
100 W
100 W
3 x 72 mm
2 x 90 mm
3 x 50 mm
3 x 50 mm
0.416
0.55
0.66
0.66
12.5 dB
13.5 dB
18 dB
18.5 dB
Peak PAE (%)
45 %
50 %
60 %
60 %
Eff (-37dBc 2c-WCDMA)
22 %
26 %
30 %
32 %
Idq degr (%)
25 %
5%
3%
2%
0.75
0.5
0.5
Pout (W) @P-1dB
Wg (mm)
Power density (W/mm)
Gain @ 2GHz (dB)
Rth (K/W)
Table 1: Achieved improvement of relevant device parameters.
LDMOS PROCESS AND EFFICIENCY ANALYSIS
The performance boost has been primarily accomplished by a
rigorous reduction of output losses of the LDMOST attaining
the device architecture of Figures 4 and 5. Figure 6 shows a
TEM cross-section of the LDMOST showing the gate and
shield in more detail. The last generations are processed in
a 8-inch CMOS-fab capable of lithography down to 0.14 um
where the LDMOST process is derived from C075 CMOS
(0.35um gate) process with LOCOS isolation.
Figure 3: Idq degradation versus time showing that the reliability is
well under control.
S
e
i
te
-
N+
P-
ie
e i te i
N+
SN
P-well
P-type epi
P-substrate
P-substrate
Figure 4: Schematic cross-section of state of the art LDMOST.
Figure 2: Latest 2c-WCDMA performance.
Additions to this process are the source plug to the substrate,
CoSi2 gate silicidation, tungsten shield, thick 2.8 μm fourth
AlCu metallization layer and p-well and drain-extension
implantation optimization.
2
Figure 7 shows the two dominant loss mechanisms for the
LDMOST. The first one is due to the on-resistance, which is
determined by the drain-extension and is frequency
independent. The second one, which is frequency dependent,
is due to loss in the output capacitance where the resistive
part is a combination of resistance of the drain-extension and
substrate resistance. Analysis of large signal performance at
various frequencies and devices optimized at several supply
voltages shows, see Figure 8, that above 10 V the peak
efficiency becomes frequency dependent. Hence, parallel
output losses are the dominant mechanism for the base
station transistors with a supply voltage of 28 V. At 1 GHz
almost the theoretical class-AB efficiency of 78.5 % is
reached.
Amplify the future | Efficiency Improvement of LDMOS Transistors for Base Stations: Towards the Theoretical Limit
Figure 8: Peak efficiency versus supply voltage at three frequencies
showing the dominance of parallel losses above 10 V supply voltage.
Markers are experimental results and solid lines are theoretical
curves. The LVLDMOST data with a supply voltage of 2.4, 3.0 and 4.8 V
where taken from reference [5].
Figure 5: SEM cross-section of state of the art LDMOST.
Figure 6: TEM cross-section of the gate region.
For low voltage the efficiency is dominated by series losses
due to the on-resistance only. The gain as function of supply
voltage and frequency is shown in Figure 9. The performance
at 28 V and 3.5 GHz has been improved to such an extent that
LDMOS is nowadays the common choice for WiMAX
applications.
Figure 9: Gain versus supply voltage at three frequencies.
Markers are experimental results and solid lines are theoretical
curves.
Despite the fact that the cut-off frequency increases with
decreasing supply voltage the gain rapidly diminishes. This
has to do with the reduction of the load resistance for lower
supply voltages.
TOWARDS HIGH EFFICIENCY
1
⋅
1
Figure 7: Two loss mechanisms in LDMOST denoted by series losses
(top picture) and parallel losses (bottom picture).
3
Having identified the main loss-mechanisms, a continuous
effort has been undertaken to reduce the capacitance and
onresistance significantly. Figure 10 shows the output
capacitance and on-resistance per unit of gate periphery
(mmWg) as function of technology generation. The onresistance has been reduced by optimizing the drain
extension, where the introduction of the shield (in 2004)
made it possible to improve the trade-off between hot-carrier
degradation and on-resistance. The lower output-capacitance
was made possible, among other measures, by a rigorous
reduction of the drain-junction area without sacrificing
electromigration performance [4] using the back-end process
depicted in Figure 4 and 5.
Amplify the future | Efficiency Improvement of LDMOS Transistors for Base Stations: Towards the Theoretical Limit
Figure 10: Improvement in output capacitance and on-resistance.
Table 2 shows the values for a given output power. Due to
the increased power-density the total on-resistance for a 100
Watt transistor did not change but the output capacitance has
been reduced with more than 40 % leading to the achieved
improved efficiency.
2000
2004
2005
2006
90 W
100 W
100 W
100 W
3 x 72 mm
2 x 90 mm
3 x 50 mm
3 x 50 mm
Power density (W/mm)
0.416
0.55
0.66
0.66
gm (S)
6.66
8.65
10.83
10.8
0.094
0.090
0.097
0.097
75.0
57.0
42.5
42.5
Pout (W) @P-1dB
Wg (mm)
Ron (Ohm)
Cout (pF)
Figure 11: Efficiency curves (solid lines) for various output losses and
ideal class-AB efficiency without losses (dashed line) versus back-off
level. Markers are the results of the technology generations showing
the path taken towards higher efficiency.
CONCLUSIONS
To conclude, the efficiency improvement as shown in the last
years can still be continued until the theoretical limit of 39 %
has been reached. Although the improvement via loss
reduction has almost come to an end, the improvement via
linearity has just been started. Moreover, because the
LDMOST becomes more and more an ideal transistor not
hampered by its parasitics, system concepts such as high
efficiency class operation, Doherty type concepts and DPD
start to work better. So, higher and faster efficiency
improvements are expected and already obtained (see Figure
12) using these concepts.
Table 2: On-resistance and output capacitance for devices with similar
output power.
Increasing the efficiency via reduction of the losses is not the
only way. In Figure 11, the W-CDMA efficiency is plotted
against the back-off power necessary to achieve the desired
linearity (markers). Lines are 1-tone efficiency curves for
several degrees of losses and the ideal class-AB curve
without losses (dashed line). The higher efficiency of the
2006 generation has been reached by lowering the back-off
level instead of by going to a different efficiency line with
fewer losses. Making the LDMOS more linear has made this
possible. Optimizing the harmonic impedances has also great
influence on the linearity [6,7]. Figure 11 also shows a point
at 39 % that is considered to be the highest efficiency possible
for a conventional class-AB power amplifier. Here the device
has such a linearity that the distortion is due to hard clipping
only. A back-off level of -6dB is than required to meet the
EVM specification.
4
Figure 12: As figure 1 but complemented with efficiency results of
Doherty PA’s using the same technology.
Amplify the future | Efficiency Improvement of LDMOS Transistors for Base Stations: Towards the Theoretical Limit
ACKNOWLEDGEMENTS
The authors wish to acknowledge the LDMOST technology
team of MST RF power base stations, T. Bakker for the
W-CDMA results and J. Gajadharsing for Doherty results.
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© Ampleon Netherlands B.V. 2017
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Date of release: May 2017
Document identifier: AMP PP 2017 0502
Printed in the Netherlands
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