Power HBT Technology offers today the best compromise for high power – high efficiency amplifiers up to Ku band. Many improvements have been published in the past to offer a better behaviour in terms of thermal heating and microwave performances. Since the reliability limiting factors have been solved, significant improvements could be proposed to get more power. In this paper, we report on the proposal of new elementary cells used in multi-finger transistors. Based on these, compact very high power amplifiers could be considered

Caillas, N.

Chartier, E.

Delage, S. L.

Floriot, D.

Jacquet, JC

Piotrowicz, S.

English

AMS Acta - Alm@DL - Università di Bologna

 1 
InGaP Power HBTs : Basic power cells for  
High Power Transistors  
 
D. Floriot*, E. Chartier, N. Caillas, S. Delage, JC Jacquet, S. Piotrowicz 
Thales Research and Technology, Domaine de Corbeville, 91404 Orsay France 
         * didier.floriot@thalesgroup.com 
 
Abstract –Power HBT Technology offers today 
the best compromise for high power – high 
efficiency amplifiers up to Ku band. Many 
improvements have been published in the past to 
offer a better behaviour in terms of thermal 
heating and microwave performances. Since the 
reliability limiting factors have been solved, 
significant improvements could be proposed to 
get more power. In this paper, we report on the 
proposal of new elementary cells used in multi-
finger transistors. Based on these, compact very 
high power amplifiers could be considered.  
 
I – Introduction 
 
GaAs based power HBT technology offers today 
the best compromise in terms of efficiency / power 
/ linearity and cost depending on the application 
addressed. A large part of the manufactured devices 
or opened foundry services are proposed for 
communications based applications in which 
medium power and high linearity are required. The 
power handling capabilities and maximum 
frequency of use is thus limited to oriented mobile 
phone applications. For high power applications 
(>3W/mm) where reliability becomes the relevant 
factor due to high junction temperature (>150°C) 
[1-2-3], a lot of work has been done in the past to 
raise the MTF value. Successful developments and 
results have been obtained in C & X bands in 
pulsed and CW modes. Defence and Spatial 
applications are in these cases , the main 
consumers. However, the recent emerging of very 
high power amplifiers for base stations offer an 
attractive civil application in L band [4] to the HBT 
community. As it has been many times stressed in 
the past, the thermal management is the key 
limiting factor of this technology. State of the art 
multi-finger HBT delivers 1.2 W at 10 GHz with 
50% min. of PAE. The design is based on a parallel 
topology combining up to 8 fingers of 40µm length 
spaced roughly by the same number. For L to C 
bands, the fishbone topology offers the best 
compromise between performance and 
compactness. In this case, the finger length could be 
upgraded to 100 and 70µm respectively. 
To increase the power density of these transistors, 
we have developed a new power cell (Bi-Cell), 
basically designed by the combination of 2 fingers. 
More details on this will be given in part II. The 
first results given in this summary assess this 
solution to demonstrate a 2.5 W power X band 
transistor. An equivalent doubling of the output 
power is expected for L band application with 
output power ranging from 10 to 20 W by 
transistor. Notice that improved thermal 
management has been developed to sustain the 
increase in dissipated power. Depending on the 
mode of operation (CW or pulsed) and the final 
application, different solutions could be used. 
 
II – New basic power cells  
 
In figure (1a), we give a schematic cross section of 
a classical one finger HBT transistor in comparison 
to the new power cells developed in this study (b) 
& (c).  In Fig. (1b), we observe the combination of 
2 fingers sharing only one central base stripe. Note 
that the surface is unchanged in comparison to the 
surface of 2 independent fingers.  In Fig. (1c), a 
more common combination is used. The width of 
the central base stripe is adjusted to homogenise the 
input base signal (RF and DC) between the two 
fingers. Significant improvement are expected with 
this structure. First, a drastic reduction of the output 
parasitic collector capacitance should be obtained. 
The main contribution of this parameter comes 
from the common surface shared between the 
extrinsic n- collector layer and the thick metal used 
to support the thermal spreader. The distance d 
(Fig. 1a) multiplied by the emitter length L gives 
the surface of an equivalent MIM parallel plate 
capacitance. In Fig. (1b) & (1c), this effect 
disappears. Moreover, the base-collector 
capacitance should present a lower value due to 
reduced border effects. Fig (1c) would be more 
suitable for high frequency operation due to a lower 
base access resistance. From a technological 
processing point of view, the width of the window 
used to etched the BCB layer (BenzoCycloButhene) 
is increased in comparison to Fig. (1a). Better 
uniformity and yield are expected. This enlarged 
access on the emitter contacts would also make 
easier the thermal cooling of the active layer. 
Different multi- finger transistors have been 
designed to test the potentialities of these new basic 
cells. In figures 2 (a) and 2 (b), we make a 
comparison between a 8 finger transistor of 2x40 
µm2 (320 µm of total emitter length) and a 8 Bi-cell 
transistor (640 µm of total emitter length). Note that 
the scale is the same for the 2 photos.  
 
 
 2 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1 : From top to bottom (a) : schematic 
view of 2 cells. (b) : BiCell concept with one base 
stripe. (1c) : Bi-Cell concept with 3 base stripes 
 
 
 
 
 
 
 
 
 
 
 (a)    (b) 
Figure 2 : Comparison between a 8 finger HBT 
of 2x40µm2 (0.32mm) and a 8 Bi-Cell HBT 
(0.64mm). Chip size : 1.0x0.6mm2 
 
We make the same comparison for a fishbone 
topology for which we present in Fig (3), a 20 Bi-
cell of 2x2x100 µm2 (4.0 mm). Fig 2b corresponds 
to an expected 2.5 W X- Band transistor and Fig 3b 
to a 20 W L- Band transistor. The surfaces of the 
chips are mentioned in comments under the photos.  
 
 
 
 
 
 
 
 
 
 
Figure 3 : Very high power design for L band 
application : 20 Bi-Cell of 2x2x100µm2 (4 mm). 
Chip size : 1.3x0.6mm2. 
 
III Thermal Analysis for a Transient Mode 
of Operation 
 
The distance between the two emitters has to be 
adjusted in respect of the mode of operation. 
Depending on the pulse length and the duty cycle,  
we use thermal modelling to optimise this 
parameter in order to limit further thermal heating 
to 5 °C for short and long pulses applications 
Typically, we got a factor two between the 2 
distances. The figure 4 gives an example of a cross 
section of one bi-cell for which 2 volumes of 
dissipated power have been defined in the collector 
regions corresponding to 4W/mm of RF power 
density. 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4 : Thermal simulation and cross section 
of a 8 bi - cell transistor. In short pulse (x10µs), 
the stabilised max. temperature is limited to 
150°C. 
 
IV – DC and RF Results  
 
Several lots of wafers have been fabricated. The 
epitaxial structures of InGaP HBTs have already 
been detailed respectively in [1] and [2] for L and 
Ku band applications. Corresponding to Fig. (2a) & 
(2b), a very thick Gold radiator (>20µm) has been 
processed on top of the emitter fingers. 
In this summary, we present a DC characteristic and 
S parameter on- wafer measurements corresponding 
to the schematic cross section (1b) and (1c). Fig. 5 
gives the base- emitter voltage (Vbe) drift versus 
the collector current for different collector- emitter 
voltage (Vce). This presentation is very common to 
Collector contact  
Base contacts 
Emitter contact 
Collector contact  
Base contacts 
Emitter contact 
Collector contact  
Base contacts 
Emitter d 
 3 
observe the thermal behaviour of power HBTs. 
From this characteristic, it appears that the trend is 
very similar between a 8 finger transistor of 
2x40µm2 and a 4 bi-cell transistor with the same 
total emitter length.  
In X-Band, at 10GHz, a power gain (MSG) close to 
12.5 dB is expected for a 2.5 W power transistor (8 
bi-cells of 2x2x40µm2).  In Fig. 6, we present the 
RF gain versus frequency for transistors based on 
the Bi- cell concept shown in fig (1b) and (1c). The 
bias point is fixed at Vce=6.0V (Bvce0 = 18V –  
Ib=1µA) and Ic=130mA in order to limit the 
overheating.  
Using a symmetrical base stripe like in Fig. 1c 
reduce the base resistance and increase this one by 
2 dB in comparison to Fig. 1b. Note also a slight 
decreasing of the base- collector capacitance by unit 
length in comparison to a standard cell (fig 1a).  
 
For L band application, we present in Fig. 7 the RF 
performances of a 16 Bi-cell transistors. The bias 
point is fixed at Vce=6.0V and Ic=260mA  (Bvce0 
= 31V – Ib=1µA). This last example is very 
promising and supports the idea that power HBT 
could challenged Si Bipolar and LDMOS 
technologies . 
 
 
 
 
 
 
 
 
 
 
 
 
Fig 5 : Vbe versus collector current Ic for a set of 
Vce. Comparison between a 8 finger of 2x40µm2   
and a 4 Bi-Cell of 2x2x40µm2 (same 
development). 
 
 
 
 
 
 
 
 
 
 
 
 
Fig 6 : RF gain of different 8 Bi-Cell transistors 
(0.64mm emitter length). Vce=6V – Ic=130mA. 
Blue curve corresponds to the fig 1b, red curve 
to the fig. 1c.  
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig 7 : RF gain of different 20 Bi-Cell L Band 
transistors (4mm of emitter length). Vce=6V –  
Ic=260mA. 
 
RF large signal measurements are underway on X-
band transistors and should be presented at the 
GaAs 2002 Symposium. Based on non linear fully 
electro-thermal model, with a special effort to fit 
tightly the thermal transient behaviour, we present 
in Fig 8 a non linear simulation from which a 
2.45W at 10GHz with 55% PAE is expected on the 
transistor of the Fig. 2b. 
 
Fig.8 : Non linear simulations of a 8 Bi-cell 
transistor (0.64mm of emitter length). Vce=9.5V. 
 
Similar results have been obtained for longer 
emitter finger (up to 100µm length). As it is well 
known, the power gain rapidly decrease in respect 
of the emitter length. Nevertheless, with a specific 
modification of the base stripes, we claim that this 
limit could be pushed from 40 to 80µm keeping 
safe the power gain. With this last evolution of the 
Bi-cell design, 4W X-Band power transistor could 
be achieved. 
 
V – Conclusion 
 
We have proposed in this summary a new basic cell 
used in multi finger power HBTs for application 
ranging from L to Ku bands. The basic power HBT 
transistor (1.2 to 1.5W as widespread value at 10 
 4 
GHz) in now doubled for the same surface keeping 
constant a high PAE of 55%.  
With this new topology, a significant improvement 
is expected in the design of very compact integrated 
or hybrid amplifiers. Non linear measurements 
would confirm the great advantages of this 
evolution for very high power applications. 
 
Acknowledgement 
 
The authors thanks P. Auxemery and H. Blanck 
from the UMS company for X Band device 
fabrication. This work is funded by Thales  
Airborne Systems, Thales Air Defence Business 
groups and by the French MoD under the contract 
00-34-063. 
 
Bibliography 
 
[1]  The correlation between material properties and 
HBT reliability. S. Delage et al - GaAs99 
Conference - p246. 
[2] HBT Technology and reliability for satellite 
applications, within the relatively new APOS 
project. O. Vendier et al - GaAs99 Conference - 
p65. 
[3] Industrial GaInP/GaAs Power HBT Process - H. 
Blanck et al - GaAs2000 Conference - p113. 
[4] High Voltage HBT technology. D. Hill - MTT 
Symposium 1998 - p 149. 
[5] Development and Characterisation of 
GaInP/GaAs HBTs for high voltage operation. - P. 
Kurpas - GaAs Mantech 2001. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 


InGaP Power HBTs : Basic power cells for High Power transistors

AMS Acta

 1 InGaP Power HBTs : Basic power cells for  High Power Transistors   D. Floriot*, E. Chartier, N. Caillas, S. Delage, JC Jacquet, S. Piotrowicz Thales Research and Technology, Domaine de Corbeville, 91404 Orsay France          * didier.floriot@thalesgroup.com  Abstract –Power HBT Technology offers today the best compromise for high power – high efficiency amplifiers up to Ku band. Many improvements have been published in the past to offer a better behaviour in terms of thermal heating and microwave performances. Since the reliability limiting factors have been solved, significant improvements could be proposed to get more power. In this paper, we report on the proposal of new elementary cells used in multi-finger transistors. Based on these, compact very high power amplifiers could be considered.   I – Introduction  GaAs based power HBT technology offers today the best compromise in terms of efficiency / power / linearity and cost depending on the application addressed. A large part of the manufactured devices or opened foundry services are proposed for communications based applications in which medium power and high linearity are required. The power handling capabilities and maximum frequency of use is thus limited to oriented mobile phone applications. For high power applications (>3W/mm) where reliability becomes the relevant factor due to high junction temperature (>150°C) [1-2-3], a lot of work has been done in the past to raise the MTF value. Successful developments and results have been obtained in C & X bands in pulsed and CW modes. Defence and Spatial applications are in these cases , the main consumers. However, the recent emerging of very high power amplifiers for base stations offer an attractive civil application in L band [4] to the HBT community. As it has been many times stressed in the past, the thermal management is the key limiting factor of this technology. State of the art multi-finger HBT delivers 1.2 W at 10 GHz with 50% min. of PAE. The design is based on a parallel topology combining up to 8 fingers of 40µm length spaced roughly by the same number. For L to C bands, the fishbone topology offers the best compromise between performance and compactness. In this case, the finger length could be upgraded to 100 and 70µm respectively. To increase the power density of these transistors, we have developed a new power cell (Bi-Cell), basically designed by the combination of 2 fingers. More details on this will be given in part II. The first results given in this summary assess this solution to demonstrate a 2.5 W power X band transistor. An equivalent doubling of the output power is expected for L band application with output power ranging from 10 to 20 W by transistor. Notice that improved thermal management has been developed to sustain the increase in dissipated power. Depending on the mode of operation (CW or pulsed) and the final application, different solutions could be used.  II – New basic power cells   In figure (1a), we give a schematic cross section of a classical one finger HBT transistor in comparison to the new power cells developed in this study (b) & (c).  In Fig. (1b), we observe the combination of 2 fingers sharing only one central base stripe. Note that the surface is unchanged in comparison to the surface of 2 independent fingers.  In Fig. (1c), a more common combination is used. The width of the central base stripe is adjusted to homogenise the input base signal (RF and DC) between the two fingers. Significant improvement are expected with this structure. First, a drastic reduction of the output parasitic collector capacitance should be obtained. The main contribution of this parameter comes from the common surface shared between the extrinsic n- collector layer and the thick metal used to support the thermal spreader. The distance d (Fig. 1a) multiplied by the emitter length L gives the surface of an equivalent MIM parallel plate capacitance. In Fig. (1b) & (1c), this effect disappears. Moreover, the base-collector capacitance should present a lower value due to reduced border effects. Fig (1c) would be more suitable for high frequency operation due to a lower base access resistance. From a technological processing point of view, the width of the window used to etched the BCB layer (BenzoCycloButhene) is increased in comparison to Fig. (1a). Better uniformity and yield are expected. This enlarged access on the emitter contacts would also make easier the thermal cooling of the active layer. Different multi- finger transistors have been designed to test the potentialities of these new basic cells. In figures 2 (a) and 2 (b), we make a comparison between a 8 finger transistor of 2x40 µm2 (320 µm of total emitter length) and a 8 Bi-cell transistor (640 µm of total emitter length). Note that the scale is the same for the 2 photos.     2                           Figure 1 : From top to bottom (a) : schematic view of 2 cells. (b) : BiCell concept with one base stripe. (1c) : Bi-Cell concept with 3 base stripes            (a)    (b) Figure 2 : Comparison between a 8 finger HBT of 2x40µm2 (0.32mm) and a 8 Bi-Cell HBT (0.64mm). Chip size : 1.0x0.6mm2  We make the same comparison for a fishbone topology for which we present in Fig (3), a 20 Bi-cell of 2x2x100 µm2 (4.0 mm). Fig 2b corresponds to an expected 2.5 W X- Band transistor and Fig 3b to a 20 W L- Band transistor. The surfaces of the chips are mentioned in comments under the photos.            Figure 3 : Very high power design for L band application : 20 Bi-Cell of 2x2x100µm2 (4 mm). Chip size : 1.3x0.6mm2.  III Thermal Analysis for a Transient Mode of Operation  The distance between the two emitters has to be adjusted in respect of the mode of operation. Depending on the pulse length and the duty cycle,  we use thermal modelling to optimise this parameter in order to limit further thermal heating to 5 °C for short and long pulses applications Typically, we got a factor two between the 2 distances. The figure 4 gives an example of a cross section of one bi-cell for which 2 volumes of dissipated power have been defined in the collector regions corresponding to 4W/mm of RF power density.             Figure 4 : Thermal simulation and cross section of a 8 bi - cell transistor. In short pulse (x10µs), the stabilised max. temperature is limited to 150°C.  IV – DC and RF Results   Several lots of wafers have been fabricated. The epitaxial structures of InGaP HBTs have already been detailed respectively in [1] and [2] for L and Ku band applications. Corresponding to Fig. (2a) & (2b), a very thick Gold radiator (>20µm) has been processed on top of the emitter fingers. In this summary, we present a DC characteristic and S parameter on- wafer measurements corresponding to the schematic cross section (1b) and (1c). Fig. 5 gives the base- emitter voltage (Vbe) drift versus the collector current for different collector- emitter voltage (Vce). This presentation is very common to Collector contact  Base contacts Emitter contact Collector contact  Base contacts Emitter contact Collector contact  Base contacts Emitter d  3 observe the thermal behaviour of power HBTs. From this characteristic, it appears that the trend is very similar between a 8 finger transistor of 2x40µm2 and a 4 bi-cell transistor with the same total emitter length.  In X-Band, at 10GHz, a power gain (MSG) close to 12.5 dB is expected for a 2.5 W power transistor (8 bi-cells of 2x2x40µm2).  In Fig. 6, we present the RF gain versus frequency for transistors based on the Bi- cell concept shown in fig (1b) and (1c). The bias point is fixed at Vce=6.0V (Bvce0 = 18V –  Ib=1µA) and Ic=130mA in order to limit the overheating.  Using a symmetrical base stripe like in Fig. 1c reduce the base resistance and increase this one by 2 dB in comparison to Fig. 1b. Note also a slight decreasing of the base- collector capacitance by unit length in comparison to a standard cell (fig 1a).   For L band application, we present in Fig. 7 the RF performances of a 16 Bi-cell transistors. The bias point is fixed at Vce=6.0V and Ic=260mA  (Bvce0 = 31V – Ib=1µA). This last example is very promising and supports the idea that power HBT could challenged Si Bipolar and LDMOS technologies .             Fig 5 : Vbe versus collector current Ic for a set of Vce. Comparison between a 8 finger of 2x40µm2   and a 4 Bi-Cell of 2x2x40µm2 (same development).             Fig 6 : RF gain of different 8 Bi-Cell transistors (0.64mm emitter length). Vce=6V – Ic=130mA. Blue curve corresponds to the fig 1b, red curve to the fig. 1c.               Fig 7 : RF gain of different 20 Bi-Cell L Band transistors (4mm of emitter length). Vce=6V –  Ic=260mA.  RF large signal measurements are underway on X-band transistors and should be presented at the GaAs 2002 Symposium. Based on non linear fully electro-thermal model, with a special effort to fit tightly the thermal transient behaviour, we present in Fig 8 a non linear simulation from which a 2.45W at 10GHz with 55% PAE is expected on the transistor of the Fig. 2b.  Fig.8 : Non linear simulations of a 8 Bi-cell transistor (0.64mm of emitter length). Vce=9.5V.  Similar results have been obtained for longer emitter finger (up to 100µm length). As it is well known, the power gain rapidly decrease in respect of the emitter length. Nevertheless, with a specific modification of the base stripes, we claim that this limit could be pushed from 40 to 80µm keeping safe the power gain. With this last evolution of the Bi-cell design, 4W X-Band power transistor could be achieved.  V – Conclusion  We have proposed in this summary a new basic cell used in multi finger power HBTs for application ranging from L to Ku bands. The basic power HBT transistor (1.2 to 1.5W as widespread value at 10  4 GHz) in now doubled for the same surface keeping constant a high PAE of 55%.  With this new topology, a significant improvement is expected in the design of very compact integrated or hybrid amplifiers. Non linear measurements would confirm the great advantages of this evolution for very high power applications.  Acknowledgement  The authors thanks P. Auxemery and H. Blanck from the UMS company for X Band device fabrication. This work is funded by Thales  Airborne Systems, Thales Air Defence Business groups and by the French MoD under the contract 00-34-063.  Bibliography  [1]  The correlation between material properties and HBT reliability. S. Delage et al - GaAs99 Conference - p246. [2] HBT Technology and reliability for satellite applications, within the relatively new APOS project. O. Vendier et al - GaAs99 Conference - p65. [3] Industrial GaInP/GaAs Power HBT Process - H. Blanck et al - GaAs2000 Conference - p113. [4] High Voltage HBT technology. D. Hill - MTT Symposium 1998 - p 149. [5] Development and Characterisation of GaInP/GaAs HBTs for high voltage operation. - P. Kurpas - GaAs Mantech 2001.                                                

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InGaP Power HBTs : Basic power cells for High Power transistors

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