Advances in Electrical Engineering Systems (AEES)`
Vol. 1, No. 4, 2013, ISSN 2167-633X
Copyright © World Science Publisher, United States
www.worldsciencepublisher.org
196
Enhanced Emitter Transit Time for Heterojunction Bipolar
Transistors (HBT)
1
Mazharul Huq Chowdhury, 2Mohammad Saad Alam
1
Nxteer Automotive, Saginaw, MI, USA
*2
Auburn Hills, MI, USA
Email: saadphd@gmail.com
Abstract – Simulation results are presented to study the performance of improved emitter transit time using AlGaAs-GaInP
composite emitter heterojunction bipolar transistor (HBT). This composite emitter HBT shows significant reduction of
emitter- base capacitance CBE and improved high frequency performance. In our simulation we use Medici (2-D simulator)
as a simulator. The composite emitter HBT has been compared with the conventional HBT. Results show superior
performance of the composite emitter design over conventional one in terms of reduced CBE. The CBE achieved with the
composite emitter design was 55% lower compared to conventional designs. However, there are no variations with the
collector current which provides enhanced frequency response for a composite emitter design.
Keywords – emitter transit time, composite emitter heterojunction bipolar transistor, performance, enhanced frequency
1. INTRODUCTION
For
next
generation
optical
communication,
GaInP/GaAs heterojunction bipolar transistors (HBTs) are
suitable due to their high reliability, high speed, high power
capacity and high reliability. GaInP/GaAs HBTs offer
significant advantages over AlGaAs/GaAs devices such as
large valance band discontinuity (high electron injection
efficiency), high etching selectivity between GaInP/GaAs
(increases yield), low surface recombination velocity
(lower noise) [1]-[2]. GaInP/GaAs HBTs do not suffer from
impurities related to oxygen and thus improve the mean
time failure (MIF) [3].
A very good microwave
performance of GaInP HBT has been achieved (cut off
frequency, fT = 140 GHz and fmax = 230 GHz) using various
designs such as collector under cut, tunnelling emitter and
strained InGaAs base [3]-[6]. However, a common
disadvantage in high speed performance of HBTs has been
their large base-emitter capacitance (CBE). This high baseemitter capacitance is caused by limited mobile carrier
transport and thus charge accumulation in emitter region
[7]. A low base-emitter capacitance has been reported at
low collector current density (JC) by using lightly doped
emitter [8]. However, it does not solve the problem at high
current density in conventional HBTs. Mobile carrier
transport takes place in conventional HBT by diffusion
through the base-emitter region. This causes a charge
accumulation in the emitter and increased the base-emitter
capacitance (CBE). To reduce this effect a composite emitter
design has been reported [9] - [10]. In this work, MEDICI
simulation results of a composite emitter design
(AlGaAs/GaAs) have been presented which allows
significant reduction of CBE and thus improved the high
frequency performance. In this design, a graded AlGaAs
layer forms an electron launcher at the interface with a
GaInP layer. The electron launcher injects the electron
towards the emitter with high velocity which lowers the
free carrier concentration in base-emitter junction and
reduces the CBE. This low carrier concentration is achieved
without degradation of transconductance because of the
high electron velocity in the launcher region. Therefore, the
overall performance is improved compared to the
conventional HBT design.
2. Composite emitter HBT design
In a composite emitter design a graded layer of AlGaAs
is introduced which acts as an electron launcher at the
interface with GaInP layer. This launcher layer injects
electrons with a very high velocity into a GaInP layer and
ensures that electrons have very high velocity before they
enter into the base region. This creates a low free carrier
concentration in the emitter and hence the transit time (τE)
decreased.
In this composite emitter design the GaInP emitter layer
blocks the holes to get back into the emitter and separated
Mazharul Huq Chowdhury, et al., AEES, Vol. 1, No. 4, pp. 196-200, 2013
197
the electron launched region from the base region and by
this base-emitter capacitance (CBE) is decreased. Figure.1
shows the structure of a conventional HBT design which is
used in this work to compare the performance with the
composite structure. Conventional HBT starts from emitter
cap of n-type GaInP with concentration of 1×1019 cm-3 with
the thickness of 700 A°. Then there is a 2000 A° thick layer
of emitter which is n-type and concentration of 3×1017 cm-3.
After this there is a blocker layer of 2000 A° of n-type
GaInP with concentration of 3×1017 cm-3. This layer
provides high frequency performance. After this there is a
base layer of p-type GaAs of 600 A° thickness and having
Figure 2. Structure of composite emitter GaInP/GaAs HBT design
By using this composition an electron launcher is made
with a height of 0.125 eV. Next to this layer there is a layer
of un-doped GaInP with thickness of 100 A°. The undoped GaInP layer reduces the spikes created in the
conduction band of AlGaAs-GaInP interface. Next to this
there is a 400 A° thick n-type GaInP layer with a
concentration of 5×1016 cm-3 which acts as an emitter
region. After this there is a base region of 500 A° of GaAs
with a concentration of 6×1019 cm-3. The last layer is n-type
GaAs of 7000 A° thick with a concentration of 1.5×1016 cm3
. It acts as a collector region. In this design there is also an
emitter cap of n-type GaInP of 700 A° thick with
concentration of 1×1019 cm-3.
Figure 1. Structure of conventional GaInP/GaAs HBT design
concentration of 4×1019 cm-3. And at the end there is a layer
of 7000 A° thickness of n-type GaAs with concentration of
1.5×1016 cm-3 and it acts as a collector region.
On the other hand, in a composite emitter HBT design
(Figure. 2) there is a layer of AlGaAs with a concentration
of 5×1017 cm-3. The aluminum (Al) composition is chosen
as 0.22 to prevent inter valley scattering.
3. Medici simulation study of conventional and
conventional and composite emitter
GaInP/GaAs design
In our simulation we have used Medici as a simulator.
Medici is a 2D device simulator that can model the
electrical, thermal and optical characteristics of
semiconductor devices such as MOSFETs, BJTs, HBTs,
and power devices, IGBTs, HEMTs, CCDs, photo
detectors. It solves Poisson’s, Thermal, Current-Continuity
and Energy-Balance equations for electron and hole. We
consider the following assumptions for our simulation
through MEDICI1.
2.
3.
In0.49Ga0.51P is chosen as a spacer layer. About
92% of the band gap differences transforms to a
positive valence band offset.
Current under thermionic field emission is
designed as thermionic current multiplied by
tunnelling coefficient (HJTEM used as ICCG
parameter in Medici).
To make ohmic contacts polysilicon is used. It
increases gain by reduction in IPE (recombination
velocity).
Mazharul Huq Chowdhury, et al., AEES, Vol. 1, No. 4, pp. 196-200, 2013
4.
5.
In un-doped regions the concentration is about
1×102 cm-3 (which is default value in Medici)
In Medici eg.model is chosen as 4 as our layer
designing is considered to be as compound
semiconductor material.
Figure 3 shows the energy band diagram of (a)
Conventional Emitter and (b) composite emitter of
GaInP/GaAs HBT. In composite emitter design, there is a
notch created in bas-emitter region which helps electron to
tunnel through the emitter region quickly. This quick
movement of electron through the emitter region reduces
the free carrier concentration in the base-emitter region and
improves frequency response by decreasing base-emitter
capacitance, CBE.
198
design HBT has been presented in Figure. 4. In a composite
design at x= 0.35 µm distance there is a very high electric
field of 200Kv/cm as compared to 5Kv/cm electric field
present in conventional design. This high electric field
helps electron to move quickly through the emitter region
before they reach base region. This lowers the free electron
concentration in base-emitter junction and thus reduces the
CBE.
On contrary, there is low electric field in the emitter of
the conventional emitter HBT design. Therefore carriers
move slowly through the base-emitter junction and
accumulate in the base-emitter junction. This increases the
electron density in the base-emitter junction and also
increases the base-emitter capacitance, CBE.
(a)
(a)
(b)
(b)
Figure 3. Energy band diagram of (a) Conventional emitter GaInP/GaAs
HBT (b) Composite emitter GaInP/GaAs HBT
Electric fields are very strong in a composite emitter
design compare to the conventional emitter design. The
electric field versus distance profile for GaInP/GaAs (a)
conventional emitter design HBT and (a) composite emitter
Figure 4. Electric field (a) Conventional GaInP/GaAs HBT (b) Composite
emitter GaInP/GaAs HBT
The drift velocity of compositionally graded AlGaAs
design has been presented in Figure. 5(b). This figure
emphasis on enhancement of velocity of composite design
which improves frequency characteristics. In the case of
conventional emitter design the electron velocity is lower
as compare to composite emitter design (Figure. 5(a)). The
Mazharul Huq Chowdhury, et al., AEES, Vol. 1, No. 4, pp. 196-200, 2013
high electric field value in composite design speeds up the
electron mean velocity to 1.65×107 cm/s at x= 0.35 µm
distance which is very high as compared to 0.7×107 cm/s of
conventional design. From the simulations results it is
apparent that the that CBE for composite emitter design is
very small as compared to conventional emitter HBT
because of stronger electric fields in composite emitter
design (Figure 4. and 5).
The CBE of composite emitter HBT’s is lower as
compared to conventional emitter HBT design. This leads
to enhancement of frequency response of composite emitter
design. It has been found that for collector current, Ic=1
µAmp/µm composite design gives out
199
collector can be used for the same breakdown voltage and
thus reduces the size of the device. On the other hand,
narrow bandgap material ensures high carrier mobility
through the device and this will further improve the transit
time of the device.
(a)
(a)
(b)
Figure 6. CBE versus Collector current (a) conventional emitter
GaInP/GaAs HBT (b) composite emitter GaInP/GaAs HBT
4. Conclusion
(b)
Figure 5. Electron mean velocity (a) Conventional emitter GaInP/GaAs
HBT (b) Composite emitter GaInP/GaAs HBT
1.2 µf capacitance which is about 55% smaller than that
value of 2.7 µf in conventional design (Figure. 6).
The proposed model can be improved by using
composite collector which is the combination of both wide
and narrow bandgap materials. The wide bandgap material
provides high breakdown voltage. Therefore, a thinner
Simulation results are presented for both conventional
and composite emitter AlGaAs/GaInP HBT design. Both
the composite and conventional emitter HBT have been
studied and analyzed on the basis of MEDICI simulator.
The characteristics of composite emitter designs are found
very superior as compared to conventional design in terms
of low base-emitter capacitance. The base-emitter
capacitance, CBE for a composite emitter design is found to
be 55% lower compared to conventional emitter design.
This lower base-emitter capacitance enhances frequency
performance as carriers move faster through the device.
Mazharul Huq Chowdhury, et al., AEES, Vol. 1, No. 4, pp. 196-200, 2013
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