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Reduction of Space Charge Recombination Current with a Self Passivated GaAlAs/GaInP/GaAs HBT Structure
R. Bourguiga, J. Palmier, C. Dubon-Chevallier
To cite this version:
R. Bourguiga, J. Palmier, C. Dubon-Chevallier. Reduction of Space Charge Recombination Current with a Self Passivated GaAlAs/GaInP/GaAs HBT Structure. Journal de Physique III, EDP Sciences, 1997, 7 (11), pp.2153-2157. �10.1051/jp3:1997247�. �jpa-00249706�
Reduction of Space Charge Recombination Current with a Self Passivated GaAlAs/GaInP/GaAs HBT Structure (*)
R. Bourguiga (~'**), J-F- Palmier (2) and C. Dubon-Chevallier (2)
(~) Facultd des Sciences de Bizerte, 7021 Zarzouna, Tunisie
(~) FRANCE T#L#COM, CNET/PAB, Laboratoire de Bagneux. 196 Avenue Henri Ravera, BP 107, 92225 Bagneux Cedex, France
(Received 4 February1997, accepted11 August 1997)
PACS.85.30.-Z Semiconductor devices
PACS.73.40.-c Electronic transport in interface structures
Abstract. A Passivated HBT structure which includes a thin GaInP layer between the GaAIAS emitter and the GaAs base layer has been proposed in order to reduce surface and space charge recombination current, while keeping a low p-type ohmic contact resistivity The
optimization of the GaInP layer thickness has been carried out leading to a value of 30 nm.
1. Introduction
Heterojunction Bipolar Transistors (HBTS) are very attractive devices for a large range of
applications, from ultrahigh speed logic to microwave power devices. However, this device presents several limitations such as the current gain size effect ill. We have recently proposed
a new Self Passivated HBT structure [2j to overcome this problem. The present study concerns
the optimization of the SP-HBT structure, and more specifically the GaInP layer thickness.
The Self Passivated HBT structures (SP-HBTS) were grown by CBE, using carbon and sili-
con as p-type and n-type dopants, respectively. The principle of this new structure [2j is to
use two different high band gap semiconductor materials to form the emitter. Indeed, a thin GaInP layer is introduced between the C-doped GaAs base layer and the GaAIAS emitter layer.
Using GaInP in the emitter is very attractive, because it exhibits many interesting features,
such as high selectivity of chemical etching, low surface recombination velocity, and low inter- face recombination velocity (3j. Minority carrier lifetime in GaInP which was calculated by
R K. Ahrenkiel is markedly larger than in GaAlAs [4j. This has been confirmed by higher DC
current gain at low current obtained on GaInP/GaAs HBTS processed in our laboratory [5j.
The associated non self aligned double mesa technology (Fig. 1) takes advantage of GaInP selective etch. The n-type GaInP layer is kept on top of the extrinsic base layer, to achieve the device passivation. The Aumn p type ohmic contact is then directly deposited on the n type GaInP layer and annealed at 300 °C. The contact thickness and alloy schedule are identical to those of reference [6j.
(*) This paper was presented at the "Journdes Maghr6bines sur les Sciences des Mat6riaux" held at Hammarnet the 8, 9 and 10 November 1996
(** Author for correspondence
© Les #ditions de Physique 1997
2154 JOURNAL DE PHYSIQUE III N°11
AuGeNilAg/Au
Mn/AuirilAu
,
Mn/AUirVAU
E GeAlAs n
~ n 210~~
B GeAs p+
AuGeNilAg/Au
C GaAs n
C' GeAs n+
GeAs substrate
Fig 1 SP-HBT schematic cross section
G lnP 2x10' f-50nm
G~AS P
e
GaAs n P'
~- GaAs n~
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~
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~o ,
~z /
S ,'
"
~ ,
~
G ,
,
m ,
E
o "
u '
,d
-o 4
~-70 lo 30 40
Thickness of Ga)nP (nm)
Fig 2. Aumn p type ohmic contact resistivity as a function of GaInP layer thickness.
2. Optimization of the GaInP layer thickness
The influence of the GaInP layer thickness on the base contact resistivity has first been in-
vestigated. In Figure 2, the contact resistivity measured on TLM t)st devices, is plotted as a function of the thickness of the n-type GaInP layer It was found to be
as low as 5 x lo ~~ Q cm~
when the GaInP layer was 30 nm thick, close to the value obtained when directly deposited on the GaAs base layer(2 x 10~~Qcm~). From Figure 2, it appears cllarly that the GaInP layer
thickness should not exceed 30 nm.
The HBT current gain is classically limited by extrinsic base surfice recombination current
~ Q
~ c
a i .-.---*
v'
c i
o ,
~ ,
fl 08 /,
o
~
z .
~ 3 '
~ 0 /
~
<
,
~ ,
~ ,
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o m
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~ ,
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lo 20 30 40 50 60
GalnP thickness Inm)
Fig. 3 Recombination rate in GaInP to total SCR recombination rate ratio.
and space charge region recombination current (SCRC). In the SP-HBT structure. the extrinsic base surface recombination is drastically reduced [7j. Space charge recombination current, which can not be neglected at the low VBE biases required for low circuit consumption, can
also be reduced thanks to the GaInP layer. To investigate this point, we have carried out an extensive simulation work with our well established ID electrical software (GIBI-ETHER) [8j, based on drift diffusion equations, already used to optimize HBT structures [9j The simulations
were carried out with a measured GaInP/GaAs £iEc of150 mev [10j, while a value of £iEc
of 70 mev was taken for GaAlAs/GaInP hetero junction assuming a £iEc of 220 mev ii ij for
GaAlAs/GaAs hetero junction. We have calculated the recombination rate in the space charge region with SRH equations, for structures with different GaInP thicknesses We present in
Figure 3 the ratio (RI of the recombination rate in the GaInP part of the SCR to the total recombination rate in the GaAlAs/GaInP SCR. The curve presented in Figure 3 demonstrates that to decrease the total recombination rate, the GaInP layer thickness should be higher than 30 nm.
3. Results
SP-HBT structures with the optimum 30 nm GaInP thickness were processed, with the double
mesa technology previously presented. The characteristics obtained on large dimension devices
(180 x 100 ~Jm~ emitter area), which allows to get rid of surface recombination were carefully analysed. This devices exhibit nearly ideal I-V characteristics (Fig. 4) demonstrating the absence of SCR recombination. Ideality factor measured on Gummel-Plots for collector and base currents were 1.02 and 1.16 respectively, with a dispersion of 0.3% across a whole 2"
wafer, demonstrating that the base current is only controlled by bulk recombination in the base in
= 1) [12j. Typical nb values for classical large dimension GaAlAs/GaAs HBTS are between 1.5 and 2. The current gain is similar to that obtained with GaInP/GaAs HBTS.
2156 JOURNAL DE PHYSIQUE III N°11
TBH GaAlAs/GalnP/GaAs C
0~~
q~ n~=1.012 n~=1 16
m
o 6 0 8 2 4
V~ IV)
Fig. 4. Gummel-plots of optimized GaAlAs/GaInP/GaAs SP-HBT, the ideality factor for the base current is 1.16.
Small dimension devices with emitter length of10 ~Jm and emitter of i, 1.5, 2 or 3 ~Jm,
were also processed and did not exhibit any surface recombination demonstrating the elimination of the current gain size effect [7j.
4. Conclusion
We have demonstrated that by optimizing the GaInP the SP-HBT structure,
which allows us to eliminate surface recombination current, we can drastically reduce space charge recombination current (nb
" 1 16) while keeping a low ohmic contact resistivity
(5 x 10~~ Qcm~). This is of particular interest to reduce the of critical circuits for optical distribution and switching systems.
Acknowledgments
The authors wish to thank H. Sik, J. Dangla, J.L. Benchimol, an~ F. Alexandre for fruitful
discussions, A-M- Duchenois, C. Besombes and L. Bricard for devic~ processing.
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