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OCR for page 31
Device Physics:
Behavior at Elevated Temperatures
HIGH-TEMPERATURE EFFECTS:
FUNDAMENTAL, MATERIALS
RELATED PROPERTIES
Although technological considerations such as gate
and ohmic contact metallization, as well as dopant profiles
are an inextricable component of device behavior at
elevated temperatures, it is useful to specifically focus on
the device physics expected at elevated temperatures,
aside from the technologically relevant, but perhaps
addressable issues, such as increased diffusion of dopants.
The fundamental device physics, dependent on the
materials properties, include parameters such as carrier
concentrations, scattering, and leakage current, and are
described below. ~
Carrier Mobilities2
The mobilities of the charged carriers will determine
the speed of operation of the devices, and those mobilities
are expected to change with increased temperature of
operation. There are a variety of carrier scattering
mechanisms in the semiconductor material that will limit
carrier mobilities: principal among these are acoustic
phonon scattering (also referred to as lattice scattering and
ionized impurity scattering), which refers to the Coulom-
bic interactions between the charged carriers (electrons or
holes) with the ionized dopants that give rise to the free
carriers. To a first approximation, for a given material,
the mobility limitation from ionized impurity scattering
' For a more detailed consideration of some of We properties described
In this chapter, it is suggested that a basic semiconductor text be
consulted, such as S.M. Sze's Physics of SemiconductorDevices (1981).
2 Furler discussion on this topic can be found in Ridley (1993).
31
scales with temperature as T3/2 (that is, the mobility
decreases at lower temperatures), while that from acoustic
phonon scattering scales as T-3/2 (mobility decreases at
higher temperatures). However, the mobility also depends
on factors such as the density of ionized impurities, which
also varies with temperature. In addition, for polar
semiconductors, such as GaAs, optical phonon scattering
becomes important. Moreover, the critical, current-carry-
ing area of some devices comprises a thin sheet of charge
near a hetero-interface. This characterizes the inversion-
layer charge at the interface of silicon and silicon dioxide
in metal-oxide semiconductor field effect transistors
(MOSFETs). In this case, other factors will affect the
charge scattering and hence the mobility. Some of these
factors include the presence of charged defects in the
oxide, or at the interface, as well as morphological
roughness of the interface. The total mobility at a given
temperature is then determined by summing up all the
significant contributions to carrier scattering.
Figure 3.1 shows the calculated dependence of
mobility on temperature for undoped 6H-SiC and 3C-SiC
(Shur et al., 1993~. The mobility of the 6H-SiC decreases
from 420 cm2/V s at 25 °C to around 120 cm2/V s at
200 °C. The room-temperature mobility for SiC doped in
the 10~7 cm~3 range is expected to be lower ~ ~ 250
cm2/V s), since the impurity scattering has been in-
creased. Figure 3.2 shows the calculated electron mobility
as a function temperature for e-type GaN, doped
10~7 cm~3.
Intrinsic Carrier Concentrations:
Dependence on Bandgap Energy and Temperature
Thermal energy, kT, can be sufficient to promote
electrons from the valence band to the conduction band,
giving rise to a thermally generated current, which is not
OCR for page 32
1 200
1 000
2 800
cat
,` 600
._
n
o
400
200
n
\ 3C-SiC
\ \
\ \
\ \
~ burl \ ~
L I
200 300 400 500 600 700
Temperature (K)
FIGURE 3-1 Calculated electron mobility as a function of temperature
for undoped 6H-SiC and 3C-SiC. SOURCE: Shur et al. (1993).
controlled by explicit device operation. In fact, at suffi-
ciently high temperatures, the thermally generated intrin-
sic carrier concentration can exceed the density of explic-
itly introduced dopants: thus the ability is lost to control
the concentration of the charge carriers in the device. The
intrinsic carrier density for a given material is given by
ni = (2n h-2) Ohm/ e~p(-~;), (3.1)
where k and h are Boltzmann's and Planck's constants; T
is the absolute temperature; Mae and math are the electron
and hole effective masses, expressed in multiples of the
1 200
1 000
-
cn
2 800
cut
-
~ 600
o
o
c'
UJ
400
200
Materials for High-Temperature Semiconductor Devices
free electron mass; and Eg is the bandgap energy. In an
intrinsic semiconductor, ni = n = p, where n is the
electron density and p is the hole density. The simple
effect of increased temperature on Equation (3.1) is to
increase ni, hence leakage current. This is a critical
concern for technologies that depend on an insulating or
semi-insulating substrate, which is the case for GaAs
metal semiconductor field effect transistors (MESFETs).
A semi-insulating GaAs substrate will show approximately
six orders-of-magnitude decrease in bulk resistivity from
25-300 °C, resulting in a substrate leakage current that
cannot be controlled by the device gate (Look, 1989; Lee
8°° et al., 19951.
For a given temperature, the intrinsic carrier density
is less for a larger bandgap material. These data are
plotted in Figure 3.3 for silicon, GaAs, and SiC (the
temperature dependence of Eg is neglected here). As the
temperature is increased, ni increases rapidly. Moreover,
most semiconductors have bandgaps whose magnitudes
decrease as the temperature is increased. This is shown in
Figure 3.4 for silicon, doped with either e-type or p-type
at different concentrations. For example, the bandgap
energy in intrinsic silicon decreases by 0.083 eV from
room temperature to 300 °C. This represents an increase
in intrinsic carrier density at 300 °C of 2.5 times over the
room-temperature intrinsic carrier density.
1 E + 20 1
~ ~GaN
co
'A 1E+ 18
c 1E+ 16
~ 1E+ 14
.c' 1E + 12
.m
.~
' 1E + 10
_
_
O - 1 1 1 1 1 1
200 300 400 500 600 700 800 1E +
Temperature (K)
FIGURE 3-2 Calculated electron mobility as a function of temperature
for GaN doped e-type, 10~7 emu. SOURCE: Shur et al. (1993).
T 8 1
0 200
1 1
400 600
800 1 000
Temperature (°C)
FIGURE 3-3 Intrinsic carrier density for silicon, GaAs, and SiC.
32
OCR for page 33
Device Physics: Behavior at Elevated Temperatures
S 0.6
a)
-
0.4
a)
c
8 0.2
~ 0.0
o
Al -0.2
._
Cal
A, 0.4
-
(5 -0.6
llJ
z
_ E
N = ~cm~
Intrinsic Level ~
0 200 400 600
Temperature (K)
FIGURE 3-4 Decrease In silicon bandgap win increasing temperature.
SOURCE: Sze (1981).
PREDICTING HIGH-TEMPERATI)RE
DEVICE PERFORMANCE:
MATERIALS-RELATED [IGlJRES OF MERIT
The high-temperature effects described above have
been reported to have first-order effects on device operat-
ing characteristics (Khan et al., 1993b), which are de-
scribed in greater detail below. Figures of merit, based on
the materials parameters affecting devices, provide a
useful basis of comparison for the possible technologies.
In the figures of merit discussed below, the highest values
possible are desirable. Johnson's Figure of Merit (JFM;
Johnson, 1965) relates to the frequency and power product
of a semiconductor transistor, and is given by
JFM = (Emvs) (3.2)
where Em is the avalanche breakdown electric field and VS
is the carrier saturated velocity. JFM can also be viewed
as the square of the quotient of the breakdown voltage of
a semiconductor layer and the intrinsic transit time for
carriers moving through the layer. JFM accounts for the
fact that in an intrinsic device (i.e., one without parasitic
resistance or reactance) there is a tradeoff between the
time a carrier spends gaining energy in an electric field as
it drifts through a device and the response time of the
device. JFM is related to electronic properties and does
not account for thermal effects.
The Keyes' Figure of Merit (KFM; Keyes, 1972)
takes into account the thermal properties of a material and
IS given by
KFM = ~ ( CVs ) 1/2 (3 3)
where ~ is the material thermal conductivity, c is the
velocity of light in vacuum, and ~ is the material dielectric
constant. Keyes assumes that smaller devices are inherent-
ly faster in response at fixed electronic-input impedance
level. However, devices cannot be made smaller without
increasing the thermal resistances, thereby limiting the
power output and introducing thermal conductivity as a
factor. The breakdown field is not significant in this
figure of merit since KFM addresses a thermal rather than
an electronic limit.
Baliga (1982) noted the role of the saturation velocity,
vs. in both the JFM and KFM, which is important for
high-speed electronics but not necessarily the major
parameter for devices to be used in power applications.
Instead, he emphasized the role of high carrier mobility
and large electric field at breakdown (Em). For lower-
frequency power devices, where conduction loss in the
on-state is the dominant power loss, the figure of merit is
BFM = e,uEm3 . (3.4)
At higher frequencies, switching losses due to
charging and discharging of the device capacitance
assumes greater importance. In that case,
BHFM = epEm2 . (3.5)
Recently, Chow and Tyagi (1994) have carried out a
comparison of figures of merit of various semiconductors
for high-power and high-frequency unipolar devices. A
portion of their results are shown in Table 3-1. The
calculations are made at room temperature, rather than at
some elevated temperature of operation, as has been the
norm for these calculations. It is generally a safe assump-
tion that the materials advantages leading to higher figures
of merit at room temperature will persist at higher
temperatures. Breakdown voltages, mobilities, and
saturation velocities will be degraded in all cases, but
should fall off with temperature less precipitously for the
wide bandgap materials than for silicon or GaAs. For
JFM, the high breakdown field dominates, making all of
the wide bandgap materials attractive compared to silicon,
germanium, and GaAs. Since the figures of merit weight
various properties, they do not appear to provide a clear
33
OCR for page 34
Materials for High-Temperature Semiconductor Devices
TABLE 3-1 Companson of Normalized Figures of Ment of Vanous Semiconductors for High-Power and High-Frequency Unipolar Devices
Material JFM KFM BFM BHFM
Silicon 1.00 1.00 1.00 1.00
Germanium 0.13 0.28
Diamond (a) 5,330 31 14,860 1,080
Diamond (p) 6,220 32 11,700 850
A1N (p = 14) 5,120 2.6 390 14
AlN (p = 1,090) 5,120 2.6 31,670 1,100
AlAs 630 7.3 2.0
GaAs 11 0.45 28 16
GaN 790 1.8 910 100
6H-SiC 260 5.1 90 13
3C-SiC 110 5.8 40 12
4H-SiC 410 5.1 290 34
SOURCE: Chow and Tyagi (1994).
choice among the different wide bandgap materials. The
high thermal conductivities of the wide bandgap materials
increase the values of KFM. Also, their lower dielectric
constants reduce the capacitance per unit area, thereby
further increasing KFM.
The figures of merit suggest that devices whose
limitations are principally due to their electronic limita-
tions, such as saturated velocity or breakdown electric
field, can achieve higher power density in the wide
bandgap materials. Field effect transistors are among this
class of devices. For devices whose operation is limited
by thermal considerations, such as the material thermal
resistance, higher power density should also be achievable
in the wide bandgap materials. Among these devices are
bipolar transistors. These predictions must be moderated
by the fact that figures of merit generally provide a rough
estimate of performance, since only the "intrinsic" device
is considered. The figures of merit do not fully account
for parasitic resistance and other detailed effects that limit
device performance and that require more careful exami-
nation for a particular device technology.
34
Device Physics at High Temperatures
More-detailed discussions of the effects of high
temperature on device performance are given in Appendi-
ces A and B for silicon- and GaAs-based technologies
operating at high temperatures. Appendix C presents
detailed considerations of the influence and issues that
surround insertion of the wide bandgap semiconductors
into microwave devices. Specific issues of consequence
for high-temperature device operation are (1) the effect on
device conductivity in the on-state, and (2) the effect of
leakage currents in the off-state. The examples below
provide an introduction to the issues that are of concern
for device technologies, building upon the high-tempera-
ture changes in carrier density and mobility.
Junction Leakage: pen Junctions and Diodes
Shenai et al. (1989) have approximated junction
reverse leakage currents to have the following depen-
dence, at elevated temperatures:
OCR for page 35
OCR for page 37
OCR for page 38
Representative terms from entire chapter:
leakage current
Device Physics: Behavior at Elevated Temperatures
1
10-1
10-2
a:
~ 10-3
in
a)
~5
a)
a)
a)
10-4
10-5
10-6
10-7
\~
\~`
_ '\ ~
>~` GaAS
\ \\`
si \
_ ~
t
it\
,
,
, ~
, ~
-
\ GaP \ N~
6H - SiC \
creased temperature can cause increased base and collec-
tor resistances. This in turn can bring about an increased
collector-emitter saturation voltage, V`e
Materials for High-Temperature Semiconductor Devices
1.0 _
A 0.5 _
-
a)
~_
0.0 _
o
en
a) _
-0.5 _
-1 .0
~__
._e-Channel
-__
~ ~ /
, ~1 1 , 1, 1 , 1 ~1 1 1 1
0 50 100 150 200 250 300
Temperature (oC)
FIGURE 3-6 Variation In threshold voltage versus temperature for n-
and p-channel MOSFET devices. SOURCE: D.M. Brown et al. (1994).
Leakage currents represent the ultimate degradation
factor with respect to high-temperature operation of
silicon MOSFETs and metal-oxide semiconductor (MOS)
integrated circuits. At high enough temperatures, the drain
to body leakage currents can increase by orders of
magnitude and become comparable to the drain channel
currents; the transistor can no longer be turned off by the
gate. Such leakage lowers noise margins, making the
design of static digital devices difficult, and making
dynamic circuits and memories impossible to produce. In
addition, mobility degradation in MOSFETs at elevated
temperatures causes the transistor transconductance to
decrease.
Choice of High-Temperature
Device Technologies
The impetus for high-temperature, high-power
operation is sufficiently compelling, and there have
already been demonstrations of such operation in silicon
and GaAs devices. In addition, temperature-dependent
extrapolations of device performance and materials-
dependent figures of merit allow us to make some assess-
ment of device performance at elevated temperatures for
the newer wide bandgap materials such as SiC and GaN.
These factors are the basis for the predictions of Figure 3-
7, suggesting the materials technologies that will be
appropriate for the various temperatures of operation.
36
In general, the limitations of leakage currents at
elevated temperatures places a greater constraint on small-
signal devices, compared to digital logic. Reduced
mobility and leakage sets a more stringent limitation yet
on the performance of microwave devices. Dynamic
random access memories (DRAMs) may be further
limited by leakage of charge stored in capacitors. We note
that the temperature range of -55 °C to 125 °C are the
current military specifications that many device technolo-
gies must satisfy.
For silicon technology, transistor operation has been
demonstrated up to 450 °C (Migitaka and Kurachi, 1994~;
since leakage currents are already large at this tempera-
ture, it is unlikely that silicon devices will be operated at
significantly higher temperatures. Since analog-device
operation is more strongly affected by the changes in
leakage current, gain, and threshold voltages that occur
with temperature, it is estimated by the committee that
high-temperature operation of these devices will hold good
to only 350 °C. Microwave devices, operating at high
power densities, would be subject to more severe temper-
ature limitations than small-signal analog devices; hence
the committee extrapolates a maximum operating tempera-
ture of 200 °C. Power devices have been demonstrated,
operating at temperatures as high as 250 °C. Since
conventional DRAMs are strongly affected by the leakage
currents associated with higher temperature, the commit-
tee estimates the maximum temperature of operation to be
150 °C. Devices that were specifically designed for high-
temperature operation, with larger charge storage capaci-
tors or based on other memory-cell architectures, perhaps
with larger areas, would be capable of a somewhat
higher-temperature operation. Finally, reasonable reliabili-
ty has been demonstrated at 250 °C for bipolar logic
(Migitaka and Kurachi, 1994) and at 200 °C for comple-
mentary metal-oxide semiconductor (CMOS) logic (Foyt,
1994~.
The higher bandgap of GaAs is expected to lead to a
somewhat higher-temperature operation than silicon.
Transistor operation has indeed been demonstrated at
500 °C (Shenoy et al., 19941. The alloy system AlGaAs,
with still larger bandgaps, should allow still higher
temperature operation.
Small-signal analog devices have been shown to
operate at 300 °C (Bottner et al., 19911; the extrapolation
to operation at 400 °C is based on the observation that
analog applications are more strongly affected than digital
Device Physics: Behavior at Elevated Temperatures
-55 0 100 200
Silicon Microwave
Silicon Digital Logic
Silicon Small Signal
Silicon Power
Silicon DRAM
GaAs Power
GaAs Digital Logic
GaAs Small Signal
GaAs Power N/A
GaAs DRAM N/A
GaAs Static RAM
SiC Power
SiC Digital Logic
SiC Small Signal
SiC Power N-C MODSFET
SiC DRAM
Nitrides (e-type)
Nitrides Microwave
Temperature (°C)
300 400 500
600 700 800
Amp
Current operating temperature - -
FIGURE 3-7 Operating temperatures for different devices per material.
by the changes in leakage currents, gain, and threshold
voltages caused by high temperature. Microwave devices
have been operated at 300 °C (Wurfl et al., 1994), and
the extrapolation to 350 °C operation reflects again the
more constraining effects of temperature on these devices,
compared to small-signal analog devices.
DRAMs, in the conventional sense, are not made
using GaAs. As for power devices, conventional metal-
insulator semiconductor field effect transistors
(MISFETs), popular power devices in silicon, are not
possible in GaAs, due to the high interface recombination
velocity. Other power devices, including the thyristor-
based devices, have not yet been attempted, even with the
excellent development of other device types and integrated
circuits in this materials system.
Although the wide bandgap materials still represent
nascent technologies, there have already been demonstra-
tions of devices that have exhibited DC transistor opera-
tion at considerably higher temperatures than available
through silicon or GaAs devices. SiC transistors have
shown operation at temperatures as high as 650 °C
37
chug
.
Projected operating temperature
(Palmour et al., 1991), and they are projected to operate
at still higher temperatures. Based on material fundamen-
tals, operation to over 1000 °C should be possible. There
is as yet little information on SiC digital logic. Xie et al.
(1994) demonstrated some SiC enhancement-mode n-
channel metal-oxide semiconductor (NMOS) circuits to
300 °C, which included simple gates, latches, and flip-
flops. Based on the demonstrated transistor characteristics,
and on material fundamentals, operation to at least 650 °C
should be possible. D.M. Brown et al. (1994) have
demonstrated operational amplifier operation to 350 °C.
The extension to 550 °C is based on estimates similar to
those for silicon and GaAs. Similarly, the same estimates
apply to the expected performance of power devices at
elevated temperatures. The high-frequency results for SiC
are currently not available. Thus the committee was
unable to make an accurate appraisal of its microwave
potential. The lack of a SiC CMOS for analog could
restrict potential device applications, however.
Device work in the nitrides is in the very early
stages; however, the results are already very exciting. The
Materials for High-Temperature Semiconductor Devices
current devices are made in epitaxial layers with very high
defect densities. Nevertheless, reasonable transistor action
has already been achieved to 350 °C (Binari et al., 19941.
Based on fundamentals, operation to over 1000 °C should
be possible. Although it is too early to give an accurate
estimate of high-temperature operation of most applica-
tions (e.g., digital logic small-signal analog, power, and
DRAM), if the estimates based on operation of transistor
action are reasonably accurate, operation to very high
temperatures should be possible. The projected operating
temperatures in Figure 3.7 are based on estimates similar
to those for silicon, GaAs, and SiC. As for transistor
38
operation, the results for microwave devices at this early
stage of materials development are very encouraging.
Maximum frequencies of oscillation as high as 35 GHz
have already been reported (Khan et al., forthcoming).
Figure 3.7 should serve only as an approximate basis
of comparison, as is true for the figures of merit, repre-
senting informed extrapolations. To uncover the true
limitations or capabilities will require further experimenta-
tion. The intention is to more clearly focus on desired
operating temperatures and the concomitant most promis-
ing device technologies in those temperature regimes.
