Improved High Frequency Performance by Composite Emitter
AlGaAs/GaInP Heterojunction Bipolar Transistors Fabricated using
Chemical Beam Epitaxy
J.-W. Park, D.Pavlidis, S. Mohammadi, C. Dua§, J.C. Garcia§
Department of Electrical Engineering and Computer Science, The University of Michigan,
Ann Arbor, MI 48109-2122 USA
Thomson-CSF, Laboratoire Central de Recherches, Domaine de Corbeville,91404 Orsay Cedex,
France§
Abstract. A new emitter design based on composite AlGaAs/GaInP approach is described which allows significant reduction of CBE and improved high frequency performance. Self-aligned HBTs were fabricated on CBE
layers grown with TBA/TBP precursors as applied to composite AlGaAs/GaInP and traditional emitter designs. CBE
of composite emitter HBTs is significantly lower than for traditional designs and does not show significant variation
with collector current. This leads to enhanced fT characteristics for composite emitter HBT designs and confirms the
theoretical expectation
1. Introduction
GaInP/GaAs Heterojunction Bipolar Transistors(HBTs) offer significant advantages over AlGaAs/GaAs
devices such as large valence band discontinuity and excellent etching selectivity as demonstrated by the
authors[1] and other laboratories. Excellent microwave properties have been obtained using GaInP
HBT[2] and Chemical Beam Epitaxy(CBE) using TBA/TBP precursors has been reported for material
growth of such devices[3]. A common limitation in high speed performance of HBTs has been their relatively large base-emitter capacitance(CBE) which is limited by mobile carrier transport in the emitter
region[4]. Mobile carrier transport takes place in traditional HBT designs by diffusion and results in
charge accumulation in the emitter and thus increased CBE. To reduce the impact of this effect, a composite AlGaAs/GaInP emitter design was employed as shown in Table 1. A compositionally graded
AlGaAs layer forms an electron launcher at the interface with the GaInP layer which injects the electrons at a high kinetic energy towards the remaining part of the emitter, thus resulting in lower free carrier concentration and smaller CBE, especially at high current drives (JC). Although the dynamic
resistance of the HBT also increases with JC, the CBE increase in traditional designs plays a predominant
role, dominating therefore the emitter time constant(τE). This paper addresses a new emitter design
based on composite AlGaAs/GaInP approach which allows significant reduction of CBE and improved
high frequency performance.
2. Layer Structure and Device Fabrication
A new emitter design HBT structure is shown in Table 1. The emitter layer consists of a compositionally
graded AlGaAs(Al 0 -> 0.22) layer and follows by undoped GaInP which can be used for reducing the
spike created in the conduction band at the heterointerface. To show the advantages of the new emitter
design, an abrupt junction GaInP/GaAs traditional HBT was also fabricated for comparison. A common
layer of two HBT structure is a GaInP etch stop layer which can be used to form the laterally etching
undercut. The desirable undercut profile was achieved by just wet etching solution for GaAs(NH4OH :
H2O2 : H2O) collector layer. Fig. 1(a) shows the cross-section of the device profile including laterally
etching undercut. A 2 x 30µm2 single emitter finger selfaligned HBT was fabricated on CBE grown layers using
composite AlGaAs/GaInP and traditional emitter designs
where the entire emitter is made from GaInP. An entirely
wet-based chemical etching process was used for fabrication. TiPtAu and PtTiPtAu non-alloyed ohmic contacts
were employed for emitter/collector and base contacts
respectively. A nominal device geometry with commonemitter structure used in this work is shown in Fig. 1(b).
Emitter metal
Base metal
Collector
LEU
Fig. 1.(a) Corr section of the device profile with laterally etching
technique. (b) Common-emitter device geometry used in
this work
3. Results
Typical DC characteristics of the composite emitter and the traditional emitter device are compared in
Fig. 2. DC gain of 30 and 28, base ideality factors of 1.72, 2.26 and collector ideality factors of 1.26,
1.27 and a collector-emitter breakdown voltage of above13.5V are obtained for the composite emitter
and traditional emitter device respectively. Offset voltage(Voffset) of both devices shows same 0.15V.
For microwave performance, The microwave properties of the HBTs with common-emitter configuration were also measured on wafer using network anlayzer(HP8510B) from 0.5 ~ 25.5GHz and their
power and current gain versus frequency characteristics are compared in Fig. 3. These results shows that
the devices present cutoff frequencies(fT) extrapolated from measured |H21| using a -6dB/Oct slope are
60GHz for the composite emitter design HBT, versus 43GHz for the traditional emitter design HBT and
maximum oscillation frequencies(fmax) from Mason’s U are extrpolated to 75GHz for the composite
design, versus 60GHz for the traditional design at VCE=2V and IC=18.12mA for the composite design
and IC=16.51mA for the traditional design.
0.006
35
Ib=200µA
30
0.005
Ib=150µA
0.003
25
Gain[dB]
IC[A]
0.004
U
20
Ib=100µA
H21
15
0.002
Ib=50µA
0.001
10
0
5
-0.001
0
0
0.5
1
1.5
2
VCE[V]
2.5
3
3.5
- 6dB/Oct
4
1
10
Frequency[GHz]
fT
fMAX
100
After microwave measurements, small signal parameters are extracted from a complete HBT T model
equivalent circuit using direct analysis of S-parameter data developed in our group.[6] Total time
delay(τd) and forward transit time(τF=τB + τC) are calculated from S-parameter data. Total time
delay(τd) is defined as follows [5], [6];
τd ≈ τE + τB + τC + τC’ = ∠ ([Z12-Z21]/[Z22-Z21]) (1)
τF = τB + τC = - tan-1(Re[αZBC]/ Im[αZBC])
where τE ≈ CbeRbe, τC’= CbcRc
(2)
Fig. 4 shows the calculated τd, τF as function of frequency from the extracted small signal parameters.
In case of the composite emitter design, 2.33psec of τd and 2.22psec of τF are achieved which means
emitter delay time(τE) is just 0.071psec after subtraction of τC’= CbcRc = 0.059psec. On the other hand,
total delay time(τd) is 3psec and forward transit time(τF) is 1.66psec for the traditional emitter design.
Consequently emitter delay time(τE) for the traditional design i.e. 0.485psec can be calculated from
Eq(1). From this result, we can figure out emitter delay time for the composite design is much shorter
than that of the traditional design so that the composite design enhance the cutoff frequency performance by 10~15GHz than the traditional design. The CBE and fT dependence on JC manifests distinct
features for composite and traditional emitter designs as shown in Fig. 5 for 2 x 30µm2 single emitter
devices. CBE of composite emitter HBTs is significantly lower than for traditional designs and does not
present the strong JC dependence. This feature is representative of the new design as expected by theory
and leads to increased fT performance.
200
70
60
60
150
fT[GHz]
30
VCE= 2V
20
Cbe
10
5
(A)
10
50
150
40
Cbe[fF]
Cbe[fF]
40
Cbe
50
fT[GHz]
fT
50
0
200
70
fT
100
30
VCE= 2V 50
20
10
15
IC[mA]
20
25
0
0
5
10
15
IC[mA]
20
25
0
Best microwave performance for composite emitter HBTs was fT=60 GHz and fmax=75GHz for a
2×30µm2 emitter geometry. Overall, we have applied CBE growth technology using TBA/TBP precursors to the demonstration of self-aligned composite emitter AlGaAs/GaInP designs and showed the
superior properties of such designs for reduced emitter-base capacitance and enhanced fT performance
through all collector current region.
References
[1] Y. J. Chan, D. Pavlidis, M. Razeghi, F. Omnes, Int. Symp. on GaAs and Related Compounds, 1989,
p891
[2] M. Feng, G.E. Stillman, IEEE Electron Device Letters, 1996, vol.17, No. 5 p226
[3] G.I. Ng, D. Pavlidis, J.C. Garcia, Conf. on Indium Phosphide and Related Materials, 1994
[4] J. Hu, D. Pavlidis, IEEE Electron Device Letters, 1993, vol. 14, No12, p563
[5] J. M. M. Rios, Leda M. Lunardi, S. Chandrasekhar, Y. Miyamoto, IEEE Transaction on Microwave
theory and techniques, 1997, vol. 45, No. 1, p39
[6] D. R. Pehlke, D. Pavlidis, IEEE Transaction on Microwave theory and techniques, 1992, vol. 40,
No. 12, p2367