Download HEMT and HBTstructuresR1

Table 1. EE5323/7312 GaAs based High Electron Mobility Transistor (HEMT) Structure grown
by IntelliEPI.
Layer
Com m ent
Material
x
11
Contact
GaAs
-
400
10
Cap
GaAs
-
9
8
7
Etch-Stop
Gate
Upper delta doping
In(x)Ga(1-x)P
Al(x)Ga(1-x)As
0.48
6
5
4
3
2
Upper Spacer
Channel
Low er spacer
Low er delta doping
AlGaAs buffer
Al(x)Ga(1-x)As
In(x)Ga(1-x)As
Al(x)Ga(1-x)As
0.25
0.15
0.25
-
50
150
50
Al(x)Ga(1-x)As
200
Buffer
GaAs/AlGaAs
0.25
-
1
periods Thickness
(Å)
0.25
-
Dopant
Type
Si
Level
(/cm 3)
1-3e18
400
Si
3-5e17
n
40
300
Si
3-5e17
n
Si
Si
-
3-5e17
UID
n
Si
-
UID
UID
UID
UID
n
-
6,000
S.I. GaAs Substrate
Table 2. EE5323/7312 InP based Single Heterojunction Bipolar Transistor (SHBT) Structure
grown by IntelliEPI. Figure 27 below shows a Double Heterojunction Bipolar Transistor
(DBHT).
Laye r
com m ent
6
Emitter Cap 1
In(x)Ga(1-x)As
.530
Thick ne s s
(Ѓ)
1,500
Si
N+
5
Emitter Cap 2
In(x)Ga(1-x)As
.530
1,000
Si
N-
4
Emitter
100
Si
N
3
Base
In(x)Ga(1-x)As
.530
700
C
P+
2
Collector
In(x)Ga(1-x)As
.530
3800
Si
N-
1
Sub Collector
In(x)Ga(1-x)As
.530
3000
Si
N+
Substrate
Material
x
InP
InP
Dopant
Type
n+
n
-
-
Integration of Heterojunction Bipolar Transistors and
Grating-outcoupled Surface-Emitting Lasers
(excerpted from a joint IntelliEPI/Photodigm/SMU proposal:
Wayne Jones, Chris Boehme, Paul Pinsukanjana, ….)
1.1 Background on Electronic and Photonic Integration of HBTs and GSE lasers
Although optoelectronic integration has been the subject of considerable R&D effort over the
past 20 years, the problem of monolithically integrating a laser and its associated control
circuitry is a difficult problem without a low-cost production solution to date. Through
integration of the laser with the driver circuit the number of discrete components required in the
transmitter is reduced from 2 chips to 1 monolithic driver/laser circuit, which reduces assembly
and test time. The power required to operate the transmitter can also be reduced because the
electrical link between the driver and the laser no longer needs to be matched to a 50-ohm
transmission line. The load resistor consumes more than half of the power in a transmitter for
this matching purpose so a bigger driver is required. The power can be significantly reduced if
the driver and the laser are monolithically integrated (Fig. 25).
Fig. 25. A monolithic Electronic Photonic Integrated Circuit (EPIC) for a laser module integrating both GSE
laser/Modulator and HBT electronic driver.
In the initial step, IntelliEPI and SMU will collaborate to develop a combined epitaxy structure
that will integrate a long wavelength diode laser with an InP HBT epi structure [17]. This
combined epi structure will serve as a basis for the development of a high performance
monolithic integrated laser module. The lack of progress in merging photonics and electronics
onto the same substrate is in large part due to historically divergent development paths taken by
the photonics and electronics research groups. The primary technology for 1310 and 1550 nm
long wavelength telecom lasers have been edge emitters which typically require labor intensive
laser bar cleaving for mirror facets, an approach not compatible with the planar processing of
electronic driver circuits. Furthermore, InP-based epitaxy materials for 1310 and 1550 nm Edge
Emitting (EE) lasers have mostly been developed with Metal Organic Chemical Vapor
Deposition (MOCVD) processing, while the high-speed laser drivers, especially for OC-768 and
beyond, have been dominated by Molecular Beam Epitaxy (MBE) grown epi materials. The
following key issues outline the difficulties for integrating the lasers and electronic drivers:
 Incompatible substrate: n+ InP for EE Laser, and semi-insulating InP for high-speed
electronic driver.
 Incompatible growth technology from the early development days: MBE and MOCVD.
 EE Laser requires non-planar processing making them more difficult to integrate with
electronic processing.
The advent of new advances such as the solid source P valved cracker and high p-doping for
InGaAs with Be or CBr4 allows production MBE technology to now be able to encroach upon
the fabrication of long wavelength InP-based lasers that had traditionally been dominated by
MOCVD. With the availability of the phosphorous-valved cracker, MBE can also address the
growth of InGaAsP quaternary layers, which had previously been difficult to control by MBE.
For the InP materials system, MBE has the following key advantages:
 Higher and less doping diffusion of p-InGaAs for HBTs with Be up to 2e19 cm-3 without
diffusion, and much higher with CBr4.
 No hydrogen passivation issues.
 Very good control of thickness and composition [18, 19] of InAlGaAs quaternary layer
growth.
The advent of new advances such as the solid source P valved cracker and high p-doping for
InGaAs with Be or CBr4 allows production MBE technology to now be able to encroach upon
the fabrication of long wavelength InP-based lasers that had traditionally been dominated by
MOCVD. With the availability of the phosphorous-valved cracker, MBE can also address the
growth of InGaAsP quaternary layers, which had previously been difficult to control by MBE.
For the InP materials system, MBE has the following key advantages:
 Higher and less doping diffusion of p-InGaAs for HBTs with Be up to 2e19 cm-3 without
diffusion, and much higher with CBr4.
 No hydrogen passivation issues.
 Very good control of thickness and composition [18, 19] of InAlGaAs quaternary layer
growth.
Integrating a GSE laser with an HBT driver is an advanced innovative EPIC that has broad
applications in telecommunications, information processing, and data communications with
specific applications to broadband optical fiber networks requiring high-speed data throughput.
1.1.1 InP-HBT Technology Background
For high-speed, low-power analog, digital, and optoelectronics applications InP-based HBTs
offer numerous advantages over GaAs and SiGe HBTs due to the following superior properties
[20]:
 Higher carrier mobilities, which translate to superior minority-carrier transport and bulk
resistance.
 A higher  valley separation, which
gives pronounced velocity overshoot
and shorter space-charge-layer transit
time.
 A lower surface recombination velocity,
which reduces 1/f noise and surface
recombination current density.
 Excellent specific contact resistance for
non-alloyed ohmic contacts to n- and ptype InGaAs.
 A smaller bandgap, which reduces the
turn-on voltage and minimizes power
dissipation.
 Compatibility with 1.3 and 1.5 µm light
wave communication systems.
Professor Feng’s group at UIUC
reported a record device ft of over 600 GHz
with InP Single Heterojunction Bipolar
Fig. 26. World’s highest integration of InP based IC. A
mixed signal circuit at 43 GHz fabricated from IntelliEPI
Transistors (SHBT) [21] made from MBE
InP HBT materials. (Courtesy of Vitesse Semiconductor
grown epi materials supplied by IntelliEPI.
Corporation)
With the relative maturity of the InP HBT
technology, digital, analog, and mixed signal electronic circuits at both 10 GHz and 40 GHz are
now commercially available from several foundries. An example of available InP products is a
mixed signal circuit at 43 GHz from Vitesse Semiconductor (Fig. 26). This circuit was
processed by Vitesse’s 4” fabrication line based on InP SHBT using its VIP-1 process and with
MBE grown epi materials from IntelliEPI. The die size is ≈ 3mm square with approximately
5000 transistors.
1.1.2 Monolithic Integration of Laser and HBT-based Electronic Driver
Previous attempts at monolithic integration of 980 nm [22] and 1310 nm [23] lasers and laser
drivers have selectively regrown the HBT epi materials on top of processed EE Lasers by
MOCVD. In both the 980 nm and 1310 nm cases, n+ substrates were used for the laser growths.
Therefore, a semi-insulating buffer had to be re-grown on top of the n+ substrate for electronic
device isolation prior to HBT growth. Further, the DHBT used for the InP-based 1310 nm lasers
Vdd
HBT
n-Emitter Contact
InGaAs
n-Emitter
InP
p-Base
InGaAs
n-Collector
InP
HBT & Laser
Shared n-Contact
InGaAs
n-Cladding
InP
n-GRIN
InAlGaAs
mQW Active Region
InAlGaAs
p-GRIN
InAlGaAs
p-Cladding
InP
p-Contact
InGaAs
Semi-Insulating InP Substrate
R1
Q16
D1
Q4
X
Q15
Q3
Y
Laser
Fig. 27. Epitaxial structure for integrating GSE
lasers and other photonic components with HBT
based circuitry
R2
Q1
Vip
Q2
Vin
Q5
Vb
Q14
Q6
Ibias
Imod
Q7
Q8
Q9
Q10
Q11Q12
Gnd
Fig. 28. Circuit diagram of a monolithic integrated
semiconductor laser and HBT-based electronic laser driver.
Q13
had a very low p-InGaAs base doping just below 1e18 cm-3 compared to today’s base doping
range in the 2 – 5e19 cm-3 range.
This proposal will take a different approach, leveraging off of the existing and well
developed electronic InP-based HBT technology where epi materials are routinely grown on 4”
semi-insulating substrates. The proposed EPIC will be developed based on a single epi growth,
which stacks HBT layers on top of GSE laser layers as shown in Fig. 27. Combining both
together on the same circuit will dramatically simplify EPIC processing and down-stream
module packaging. Higher speed and greater performance will be possible. With this combined
single epi approach, possibilities also exist for creating new device functionalities such as an inline ridge photodetector for GSE laser power monitoring, and diodes for laser temperature
monitoring and stabilization. Further, these added device functionalities could be integrated
directly with the laser and the electronic driver circuit.
Figure 28 shows a driver circuit for a monolithically integrated semiconductor laser and
HBT based laser driver. The last stage of the driver circuit employs a differential pair (Q5 and
Q6) that steers its tail current to the laser or to a dummy load (made of Q15 and Q16).
Differential drivers provide many important advantages over single-ended circuits such as
maintaining a relatively constant supply current thus achieving low switching noise and reducing
crosstalk if the signals remain symmetric. In this bipolar implementation, the output transistors
(Q5 and Q6) must be large enough to handle the peak current without experiencing high-level
injection. Owing to large device dimensions, the output stage normally suffers from a substantial
input capacitance. Therefore, emitter followers (Q3 and Q4) precede the output stage to provide
buffering. The followers lower the output impedance of the pre-driver circuits (Q1 and Q2),
improving the speed at nodes X and Y. In Fig. 28, transistors Q7-Q14 are biasing circuitry.