Download 1.3 Historical View of Microwave Transistors

Chapter 1.
Background on
Microwave Transistors
Mar. 7th, 2006
1
1.1 Introduction
• Since the invention of the bipolar transistor in 1947,
semiconductor electronics has been advancing and
evolving at an enormous pace.
• Microprocessors now contain hundreds of millions of
transistors and Gbit DRAMs are commercially available.
Si VLSI
• In past 30 years, the minimum feature size of production
stage Si ICs decreased by a factor of about 0.7 and the
capacity of DRAMs increased by a factor of 4 every 3
years.  Moore’s law
• Besides Si VLSI, there are other emerging fields in
semiconductor electronics. Microwave electronics with
microwave transistors is the most prominent one.
2
3
• ~1980s: military microwave application
(performance was most important)
• 1990s: consumer applications (sufficient
performance at lowest cost)
• The only semiconductor material used in Si VLSI
is silicon. Furthermore, only 2 basic types of
transistors are widely in the Si VLSI: MOSFETs
and BJTs.
• For microwave electronics, a large variety of
different semiconductor materials have been
employed: Si, SiGe, GaAs, InP, further III-V
compounds, and wide bandgap materials
4
• Microwave transistors
–
–
–
–
–
MESFETs (Metal-Semiconductor FETs)
HEMTs (High Electron Mobility Transistors)
MOSFETs
BJTs
HBTs (Heterojunction Bipolar Transistors)
• Circuits using microwave transistors: LNA, PA,
mixers, frequency converters and multipliers,
attenuators, and phase shifters
• Divided into in principle 2 groups: low-noise
transistors and power transistors
5
1.2 Microwave Transistor Figures of Merit
1.2.1 The concept of 2-Port Networks
Y parameters i1   y11 y12  v1 
i    y
 2   21
y22  v2 
6
y11 
y11 
y11 
y11 
i1
v1
i1
v1
i1
v1
i1
v1
v2  0
Input admittance
Reverse transfer admittance
v2  0
Forward transfer admittance
v2  0
v2  0
Output admittance
7
• S parameters are not defined as quotients of
currents and voltages but as ratios of the powers of
traveling waves.
b1   s11 s12  a1 
b    s
 a 
s
 2   21 22   2 
• Despite that Y parameters cannot be measured in
the microwave range, they are widely used for
discussing the properties of microwave transistors.
• Y parameters are more closely related to device
physics and are more interpretable for device
engineers than S parameters.
8
9
1.2.2 The Problem of Stability
• A microwave transistor is capable of power
amplification or sustained oscillation.
• Whether the transistor in a circuit will oscillate or
not depends on the transistor itself, and on the
source and load impedance.
• The stability behavior of a transistor can be
described by the stability factor k:
k
2 Re( y11 ) Re( y22 )  Re( y12 y21 )
| y12 y21 |
• k>1: unconditionally stable
k<1: conditionally stable
10
1.2.3 Power Gain Definitions
• The power gain is the ratio of the power P2
delivered from the transistor output to the load to
the power P1 delivered from the signal source to
the transistor input.
• The matching conditions between the signal
source and transistor and between the transistor
and load influence the power transfer.
• If a transistor is to achieve the maximum power
gain, then power matching is required. (k>1) 
conjugate matching  the maximum available
gain
y
MAG 
21
y12
(k  k 2  1)
11
• If k <1, auxiliary external admittances have to be
connected. The overall stability factor is
K
2  Re( y11 )  Re( y1 ) Re( y22 )  Re( y2 )  Re( y12 y21 )
| y12 y21 |
12
• If the input and output of the whole network is
conjugately impedance-matched, the maximum
stable gain is
y21
MSG 
y12
• The unilateral power gain U:
| y21  y12 |2
U
4  Re( y11 ) Re( y22 )  Re( y12 y21 ) 
13
1.2.4 The Characteristic Frequencies fT and fmax
• The cutoff frequency fT is the frequency at which
the magnitude of h21 (short-circuit current gain) =
1.
• The maximum frequency of oscillation fmax is the
frequency at which the U = 1 (still provides a
power gain).
U ( f )  20log f  20log f max
14
15
1.2.5 Minimum Noise Figure and Associated Gain
• A figure of merit describing the amount of
intrinsic noise produced in microwave transistors
NF [dB]  10 log
PSi / PNi
PSo / PNo
• The magnitude of NF is dependent on the
matching conditions at the input of the transistor,
bias condition, and frequency.
• The power gain obtained from the transistor
biased and matched for minimum noise is called
associated gain Ga.
 TN 
NF[dB]  10log 1  
 T0 
16
1.2.6 Output Power and Power-Added Efficiency
Pout [dBm] = 10 log Pout [mW]
PAE 
Pout (hf )  Pin (hf )
Pin (dc)
17
1.3 Historical View of Microwave Transistors
1.3.1 The Early Years
• Ge BJTs developed in 1958-1959 were the first
transistors operating above 1 GHz.
• By 1963, Si BJTs became competitive and in 1970
almost all microwave transistors were Si BJTs.
18
• It became clear in the early 1960s that Si is not the
optimal semiconductor for microwave transistors.
• GaAs (having a 6-fold electron mobility and a
higher maximum electron drift velocity compared
to Si) is a far better material for high-speed
transistors.
• In 1966 Mead presented the first GaAs MESFET.
• The first GaAs MESFET with practical
microwave performance was reported in 1967 and
showed a fmax of 3 GHz. In 1970, the record fmax of
GaAs MESFET increased to 30 GHz.
• Si BJTs and GaAs MESFETs were the 2 only
microwave transistor types in use in 1970s and
early 1980s.
19
20
21
• The critical dimensions to obtain good microwave
performance are wB and L, both of which should
be as small as possible.
22
1.3.2 Development of Microwave Transistors with
Heterostrutures
• The use of heterostructures after 1980 offered the
opportunity of tremendous progress toward
improved high-frequency performance of
microwave transistors.
• A heterostructure is a combination of at least 2
layers of different semiconductors with distinct
bandgaps.
• 2 growth techniques
– MBE: Molecular beam epitaxy
– MOCVD: Metal-organic chemical vapor deposition
23
• Mobility of sequences of n-type AlGaAs and undoped
GaAs exceeds those of doped bulk GaAs or AlGaAs at
room and lower temperatures.
• Selectively doped heterostructure FET (SDHT),
modulation doped FET (MODFET), two-dimensional
electron gas FET (TEGFET)
• The different bandgaps of AlGaAs and GaAs cause a
bandgap difference ΔEG resulting in band offsets ΔEC and
ΔEV in the conduction and valence bands at the
heterointerface.
• In HEMT, a large ΔEC is desired. The transferred electrons
are confined to a region only a few nanometers thick in the
GaAs layer near the heterointerface, called the twodimensional electron gas (2DEG ).
24
• Because the 2DEG electrons are spatially
separated from the donors, ionized impurity
scattering is suppressed and the electron mobility
in the channel is increased.
25
• The second type using heterostructures is the HBT.
• In the early 1980s, practical HBTs using the AlGaAs/GaAs
system could be successfully fabricated.
• The key part of a HBT is the emitter-base heterojunction
with the bandgap of the emitter being larger than that of
the base.
• Hole injection from the base into the emitter is strongly
suppressed, and higher current gains compared to those in
homojunctions BJTs can be obtained.
• The emitter injection efficiency (Ge)
 EG 
N DE
Ge 
exp 

N AB
k
T
 B 
• A large valence band offset ΔEV is desirable for HBT.
26
• Using a high base doping density produces
– A low base resistance resulting in higher fmax and lower
NFmin,
– A very thin base, leading to a short base transit time and
thus a high fT.
27
• When considering the lattice constants of the
semiconductors, 3 different heterostructure types
can be found: lattice matched, pseudomorphic,
metamorphic heterostructures
28
• It is possible to grow good quality heterostructures
from materials with different lattice constants,
provided the thickness of the grown layer does not
exceed a certain critical value tc.
• If the grown layer is thinner than tc, its crystalline
structure accommodates to that of the substrate
material.  a lattice deformation in the grown
layer (pseudomorphic layer)
• Since 1986, pseudomorphic
AlGaAs/InGaAs/GaAs heterostructures with In
contents in the range of 15-25% were successfully
grown on GaAs substrates and used in
pseudomorphic HEMTs.
29
30
• Metamorphic type: uses a substrate material
(GaAs) and overgrows a graded buffer layer
(InAlAs) with a thickness much greater than tc.
– Because the buffer is extremely thick, dislocations
arising at the interface substrate/buffer barely influence
the electrical properties of the device layer on top of the
buffer.
– The main advantage is that inexpensive GaAs
substrates can be used to obtain high ΔEC values, and
thus InP-HEMT-like performance can be attained with
GaAs.
31
1.3.3 Recent Developments
• 2 new research directions in 1990s
• The application of the Si MOSFET as a
microwave device.
– CMOS microwave circuits with operating frequencies
up to 5 GHz.
– Si power MOSFET up to 2.5 GHz used in base stations
of wireless communication systems.
• The investigation of wide bandgap
semiconductors, such as SiC and III-nitride, for
power transistors in the GHz range.
– Allows operating temperatures far exceeding those for
Si and III-V transistors  new application in
automobile and aircraft.
32
33
34
1.4 State of the Art of Microwave Transistors in
2001
1.4.1 III-V FETs
• Both the HEMT and GaAs MESFET are widely
used microwave devices due to their simple
structure and superior high-frequency performance.
• When the gate length is reduced down to the deep
submicron range, the resistance of the small gate
strip becomes large,
 a negative influence on the gain and noise
behavior at high frequencies.
 Mushroom gates are frequently used to achieve
a short gate length and a small gate resistance
35
36
• Since the late 1980s, the focus of HEMT research has been
shifted to systems that offer large conduction band offsets,
such as pseudomorphic heterostructures on GaAs as well
as lattice-matched and pseudomorphic structures on InP, all
using InGaAs channel layers.
37
38
• During the 1990s, m-HEMTs grown on GaAs
substrates were investigated.
39
40
41
42
43
1.4.2 BJTs and HBTs
• The first successfully realized HBTs for
microwave applications were based on GaAs.
44
• The major disadvantage of InP HBTs in general in the
brittle and expensive InP substrate.
• Also, the technology of InP HBTs is relatively immature
compared to that of GaAs HBTs.
• A main advantage of III-V HBTs compared to III-V FETs
is that high fT and fmax can be obtained without the
limitation of photolithography.
45
46
47
• III-V HBTs have inferior high-frequency noise
behavior compared to III-V FETs.
• The main application of III-V HBTs is in
microwave power amplifiers. HBTs offer much
higher power densities than III-V FETs, which
makes impedance matching easier and leads to
smaller chip sizes.
48
• SiGe HBT processes a huge advantage over the
III-V HBTs in that SiGe HBT can be fabricated
with the existing Si CMOS technology with only a
few more steps added.
• The layer sequence from the bottom: Si substrate,
n+-Si subcollector, n-Si collector, p-strained-SiGe
base, and n-Si emitter
49
• SiGe HBT – commercial status in the late 1990s.
– Unlike III-V HBTs, SiGe HBT noise figures are quite
low.
– The main problem of Si and SiGe bipolar power
transistors is the relatively low breakdown voltage.
– From the economical perspective, the main advantage
of SiGe HBTs compared to III-V HBTs are (1) large
diameter and inexpensive Si substrates, and (2) existing
Si technology available for the production of SiGe
HBTs.
50
51
52
53
1.4.3 Wide Bandgap Transistors
• Since 1990, wide bandgap semiconductors such as SiC and
group III nitrides (AlN, GaN and AlGaN) have received
increasing attention for high-power microwave
applications due to their high breakdown field and their
high electron peak and saturation velocities.
• In the case of SiC, there is more than 100 different
polytypes with different crystal structures (4H SiC is the
favorite material).
• The group III nitrides exist in 2 crystal types: wurtzite
(hexagonal) and zincblende (cubic)
• The first commercial SiC MESFET was announced in
1999. Another class of wide bandgap FETs is AlGaN/GaN
HEMTs.
54
55
56
57
1.4.4 Si MOSFETs
• The advances in CMOS processing, continuous scaling of
gate length, progress in SOI (silicon on insulator)
technology, and development of Si LDMOSFETs (laterally
diffused MOSFET)  suitability of MOSFETs and CMOS
for microwave applications
• The SOI concept seems to be more promising because of
the ease of integration with other high-performance
microwave components also fabricated on insulators.
• The noise performance of Si MOSFET will always be
inferior to that of advanced III-V FETS because of their
poorer electron transport properties.
58
• Another problem of microwave Si MOSFETs is
the fact that good fT, fmax, and NFmin can only be
obtained with extremely scaled MOSFETS.
• The breakdown voltages of such MOSFETs may
by too low for many practical applications.
• However, in the mass consumer markets, where
cost and the ability for integration are of major
concern, microwave Si MOSFETs have clear
advantages over other microwave devices.
59
1.5 Outlook
60
61
62
63
64
65
66
HW1
1. Solve and explain the Schrödinger wave equation
2. Explain the energy band, Fermi level and
bandgap.
3. Investigate simple crystal structures and explain
them.
67