APPLIED PHYSICS LETTERS 97, 134102 共2010兲
Coupling electron-hole and electron-ion plasmas: Realization of an npn
plasma bipolar junction phototransistor
C. J. Wagner,a兲 P. A. Tchertchian, and J. G. Eden
Laboratory for Optical Physics and Engineering, Department of Electrical and Computer Engineering,
University of Illinois, Urbana, Illinois 61801, USA
共Received 20 May 2010; accepted 23 August 2010; published online 29 September 2010兲
Coupling e− – h+ and gas phase plasmas with a strong electric field across a potential barrier yields
a transistor providing photosensitivity and voltage gain but also a light-emitting collector whose
radiative output can be switched and modulated. This optoelectronic device relies on the
correspondence between the properties of a low temperature, nonequilibrium plasma and those for
the e− – h+ plasma in an n-type semiconductor. Hysteresis observed in the collector current-base
current characteristics is attributed primarily to charge stored in the base, and the photogeneration
of e− – h+ pairs at the base-collector junction. Extinguishing the collector plasma requires an
emitter-base junction reverse bias of ⬍1 V. © 2010 American Institute of Physics.
关doi:10.1063/1.3488831兴
Low temperature, weakly ionized plasmas in the gas
phase and electron–hole 共e− – h+兲 plasmas in semiconductors
are closely related. Having a common mathematical and
physical ancestry, both plasmas are described by virtually
identical relationships for charged particle drift, diffusion,
and recombination 共for example兲. Early in the development
of solid state electronics, the close linkage between e− – h+
plasmas and nonequilibrium e−-ion plasmas was recognized.1
In the six decades since the invention of the transistor in
1947, however, the inherent correspondence between the
properties of the two plasma genres has largely been overlooked in plasma research and optoelectronic device design.
The first strides toward integrating gas phase and electronhole plasmas were taken in 2005 when, in the course of
characterizing the Si/microplasma photodetector,2 Ostrom
and Eden3 identified the pivotal role of coupling both plasmas with an electric field imposed across the semiconductor/
gas phase plasma interface.
In this letter, we report the interaction of e−-ion and
−
e – h+ plasmas across a narrow potential barrier, mediated by
a strong electric field provided by the sheath of the gas phase
plasma. Integration of a plasma with a semiconductor emitter
and base yields a phototransistor having a light-generating
collector and the ability to modulate and switch the collector
plasma with voltages as small as ⬍1 V applied to the
emitter-base junction. Not only does this device offer optoelectronic functionality not available previously in a single
device but it also provides a window onto the physics of
e− – h+ and e−-ion plasmas separated by a thin barrier and yet
linked by e− tunneling.
Figure 1 is a simplified drawing of the plasma bipolar
junction transistor 共PBJT兲. The primary difference between
this device and a conventional npn BJT is the substitution of
a gas phase plasma for the n-type collector. A potential difference between the anode at top and the p-type base generates and partially sustains a plasma confined within a cavity
fabricated in a dielectric 共blue in Fig. 1兲. For clarity, a portion of the dielectric has been cut away. The injection of
current into the plasma through the vacuum potential barrier
a兲
Electronic mail: cjwagner@illinois.edu.
0003-6951/2010/97共13兲/134102/3/$30.00
at the base-collector junction, and subsequent electron avalanche in the plasma, provides a mechanism for modulating
and extinguishing the gas phase plasma. As illustrated schematically in Fig. 2, the PBJT draws on the resemblance between the properties of a low temperature, gas phase plasma
and those of the e− – h+ plasma within an n-type semiconductor. In normal mode operation of the transistor, for example,
the emitter-base junction is forward-biased so as to inject
electrons from the emitter into the p-type base. Electrons
diffusing from the emitter-base junction are, upon reaching
the reverse-biased, base-collector junction, swept into the
collector. Because the electron mobility in the gas phase
plasma is much higher than that for the ions, the conductivity
of the collector is dictated by the electrons which drift to an
anode under the influence of the local electric field in the
ជ p.
plasma, E
Although the physics of the PBJT is similar in several
respects to that of a conventional npn BJT, the plasma collector has several distinguishing characteristics. Foremost
among these is the emission of atomic and/or molecular radiation owing to electron-neutral collisions in the gas phase
FIG. 1. 共Color兲 Drawing of a plasma BJT in which the collector plasma is
confined within a cavity fabricated in dielectric 共shown in blue兲, a portion of
which has been cut away for clarity. The cavity also serves to expose the
collector plasma to the base.
97, 134102-1
© 2010 American Institute of Physics
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134102-2
Wagner, Tchertchian, and Eden
Appl. Phys. Lett. 97, 134102 共2010兲
FIG. 3. 共Color兲 Cross-sectional diagram 共not to scale兲 of one configuration
of an npn PBJT that has been fabricated and tested to date.
FIG. 2. 共Color兲 Generalized diagram of a plasma BJT operating in the
normal mode in which the emitter-base junction is forward biased. All red
and blue arrows indicate the direction of local electron and hole flow, respectively. The green arrow represents plasma-produced photons incident on
the p-Si base.
plasma. Furthermore, photons with energies above the Si
band gap 共1.1 eV兲 are produced by the plasma, impinge on
the base, and generate e− – h+ pairs at or near the reversebiased base-collector junction. After tunneling into the collector, photogenerated electrons contribute to the total current flowing through the plasma whereas photogenerated
holes either replace holes lost in e− − h+ recombination in the
base or are injected into the emitter, thereby contributing to
both iB and iE. Because the base of the PBJT is exposed and
the collector plasma is generally optically thin, external radiation is also capable of producing e− − h+ pairs at the basecollector junction and the PBJT behaves as a phototransistor.
Although plasma radiation-generated e− − h+ pairs contribute
to the dark current of the device, judicious choice of the
collector gas共es兲 will minimize the impact of this process.
Much of the voltage required to sustain a gas phase
plasma appears across the sheath 共or cathode fall region兲4
which, in a PBJT, extends to the surface of the base, and the
depth of penetration of the collector’s electric field into the
base is dependent upon the base doping level. It is clear that
the built-in electric field normally present at the surface of Si
in vacuum5,6 is reinforced by this intense electric field that
exists in the cathode fall region of the collector plasma. Together they facilitate the tunneling of electrons through the
semiconductor/plasma potential barrier 共⬃4 eV兲,5 and possibly modify the secondary electron emission coefficient at
the base/collector junction. For an electron temperature and
number density of 2 eV and 1013 cm−3, respectively, and an
assumed width of the plasma sheath of 5␭D 共where ␭D is the
electron Debye length兲, the sheath electric field strength is
estimated to be ⬃105 V / cm for an applied voltage of 200 V.
This value is only slightly larger than that existing within the
depletion region of a reverse-biased, conventional n+ – p
junction for which the donor and acceptor number densities
are 1018 cm−3 and 1015 cm−3, respectively, and the bias is
⫺10 V. Therefore, the presence of the sheath electric field at
the base-collector interface has the same effect as the imposition of a reverse bias on the base-collector junction of a
conventional BJT.
A cross-sectional diagram of an npn PBJT structure
characterized in the present experiments is shown in Fig. 3.
All devices were fabricated on 100 mm diameter silicon-oninsulator wafers comprising two boron-doped Si regions,
350 ␮m and 15 ␮m in thickness, separated by a 2 ␮m
thick SiO2 layer. The background doping density of the base
is ⬃1015 cm−3 and its width is ⬃14 ␮m, which represents a
tradeoff between preventing punchthrough and minimizing
the loss of electrons in the base by recombination. A cylindrical cavity having a diameter in the 500 ␮m – 5 mm range
was etched into the Si handle layer by an inductivelycoupled plasma process and the base was exposed by wet
etching of the buried SiO2 film. Electrically isolating the
collector plasma from the handle Si layer required depositing
a thin dielectric film on the cavity wall and around the perimeter of the cavity aperture. Fabrication of the device was
completed by installing a collector anode which, for these
studies, was planar or a needle centered on the cavity axis.
For this initial demonstration of the device, the diameter of
the collector cavity was either 3 mm or 5 mm and the cavity
volume was several microliters. Neon served as the plasma
collector gas.
Figure 4 presents a set of 14 collector current 共iC兲-base
FIG. 4. 共Color online兲 Collector current 共iC兲-base current 共iB兲 characteristics
for a PBJT having a collector diameter of 3 mm and a Ne gas pressure of 25
Torr. The base-emitter junction is driven by a 200 Hz sinusoidal voltage
waveform with an rms value of 2.8 V. Throughout these measurements, the
collector anode was planar.
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134102-3
Appl. Phys. Lett. 97, 134102 共2010兲
Wagner, Tchertchian, and Eden
current 共iB兲 characteristics for a PBJT with a collector cavity
diameter 共d兲 of 3 mm. Data are given for VCC varied in 10 V
increments from 270 to 400 V, a Ne gas pressure 共p兲 of 25
Torr 共thus corresponding to a pd product of 7.5 Torr-cm兲,
5 k⍀ and 67.2 k⍀ resistors in series with the base and collector, respectively, and the base-emitter junction driven by a
200 Hz sinusoidal voltage waveform having an rms amplitude of 2.8 V. Several general properties of these curves are
immediately apparent. Among them is the observation that,
for all values of VCC, the characteristics exhibit hysteresis.
Testing of the emitter-base junction in the absence of a
plasma in the collector cavity indicates that, although other
factors may be involved, this effect is attributable to the RC
time constant dictated by the emitter-base junction and the
current limiting series base resistor employed in device testing. For the data of Fig. 4, the maximum value of hFE, the
small signal current gain, is approximately 3, although considerably higher gains are expected as the device structure is
optimized.
The base-collector junction of the PBJT 共reverse-biased
by the potential across the collector兲 serves as a photodetector. In Fig. 4, the influence of photogenerated carriers is apparent in the monotonic shift in the iC – iB curves to larger
values of iC and 兩iB兩 as VCC is increased, a trend which appears to be mediated by the Early effect.7 Raising the voltage
imposed on the collector enhances the magnitude of the collector current, optical emission, and the e− – h+ production
rate at the base. In this regard, the dashed line in Fig. 4 is the
locus of 共iB , iC兲 ordered pairs—one for each value of
VCC—for which transistor operation has ceased. The emitterbase junction is no longer forward biased and iC and iB are
equal. Both currents have two components: 共1兲 photocurrent
generated in the base by collector emission and 共2兲 the replenishment of base electrons lost to secondary electron
emission and the neutralization of atomic ions arriving at the
base/collector interface.
When the relative emission intensity of the collector
plasma is monitored 共with a photodiode兲 as a function of
VCC and iB, a set of characteristics similar to those of Fig. 4
is obtained. Experiments were also conducted in which the
base of the PBJT was irradiated by an external He–Ne laser.
With the collector plasma extinguished, approximately 1.4
mW of 632.8 nm radiation was estimated to reach the base.
The incoming optical beam was chopped at 100 Hz and, for
the sake of comparison, all other experimental parameters
were maintained at the same values as those of Fig. 4. The
impact of supplemental e− – h+ production on the iC – iB characteristics of the transistor 共Fig. 4兲 is to displace them to
higher 兩iB兩 and iC and, for a fixed value of iC, ⌬iB is measured
to be ⬃0.2 mA. This photogenerated current is equivalent to
the absorption of 0.4 mW of 632.8 nm photons in the base
and the complete conversion of this absorbed radiation into
base current. Accounting for optical power losses incurred by
reflection at the plasma-Si 共collector-base兲 interface and
resonant absorption as the incident beam traverses the collector plasma, calculations show that the measured value of
⌬iB is approximately the same as that expected on the basis
of e− – h+ generation alone.
To probe the temporal dynamics of the device, several
different waveforms were applied to the base of a PBJT for
which d is 5 mm and VCC = 300 V. A low frequency 共2 Hz兲
sinusoid having a peak-to-peak voltage of 2 V yields a
collector-emitter voltage 共VCE兲 waveform having a swing of
⬃54 V, thus yielding a voltage gain of 27, and the collector
current and VCE waveforms are mirror images of one another. Applying a 2 V, 400 Hz square wave to the base resistor results in a collector visible fluorescence waveform that,
except for a rise time of ⬃100 ␮s, replicates the input waveform. Owing to the large emitter-base junction capacitance at
present 共⬃2 nF兲, the maximum PBJT driving frequency has
thus far been limited to 4 kHz. Consequently, the device
performance is restricted by the semiconductor junction and
not the collector plasma in which electron mobilities are considerably higher than the corresponding values for Si. Despite the early stage of this technology, it is important to note
that collector plasmas can now be modulated and completely
extinguished with input voltages ⬍1 V. As one example, a
15 Hz sinusoid superimposed onto a 0.4 V dc offset voltage
is able to extinguish the collector plasma when the base voltage reaches ⫺0.5 V. Modulating the emission of the collector
plasma by ⬎80% while preserving the fidelity of VCE to the
input waveform has also been demonstrated. Although the
voltage 共VCC兲 required at present to produce and sustain the
collector plasma is ⱕ300 V, the ability to control the plasma
by modulation and switching with base voltages ⬍1 V is
unprecedented.
In summary, the first in a class of optoelectronic devices
has been realized by substituting a low temperature, nonequilibrium plasma for the n-type collector of an npn BJT. Interfacing the electron-hole plasma existing in the vicinity of an
n+ – p junction with its gas phase 共e−-ion兲 counterpart yields a
light-emitting phototransistor that offers the ability to modulate plasmas with base-emitter voltages well under 1 V. The
greater impact of the results reported here may well be the
window they provide onto the behavior of e− – h+ and gas
phase plasmas separated by a nanometer-scale potential barrier and coupled by a strong electric field.
Discussions with D. J. Sievers, the technical expertise of
T. C. Galvin and B. R. Flachsbart, and the support of this
work by the U.S. Air Force Office of Scientific Research, the
National Science Foundation, and the Department of Energy
under grant numbers FA9550-07-1-0003 and FA9550-10-10048, CBET 08-53739, and DE-SC0001540, respectively.
1
See, for example, W. Shockley, Electrons and Holes in Semiconductors
共Van Nostrand, New York, 1950兲.
S. J. Park, J. G. Eden, and J. J. Ewing, Appl. Phys. Lett. 81, 4529 共2002兲.
3
N. P. Ostrom and J. G. Eden, Appl. Phys. Lett. 87, 141101 共2005兲.
4
M. A. Lieberman and A. J. Lichtenberg, Principles of Plasma Discharges
and Materials Processing, 2nd ed. 共Wiley, New York, 2005兲.
5
I. Vaquila, J. W. Rabalais, J. Wolfgang, and P. Nordlander, Surf. Sci. 489,
L561 共2001兲.
6
W. Mönch, P. Koke, and S. Krueger, J. Vac. Sci. Technol. 19, 313 共1981兲.
7
J. M. Early, Proc. IRE 40, 1401 共1952兲.
2
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