The Plasma Bipolar Junction Phototransistor:Coupling Electron-hole and Electron-ion Plasmas

Benben Li1, Thomas Houlahan, Jr.1, Clark J. Wagner1, Paul A. Tchertchian3, Dane J. Sievers2, J. Gary Eden11Laboratory for Optical Physics and Engineering, 2Department of Electrical and Computer Engineering,

University of Illinois at Urbana-Champaign, Champaign, IL 618203Present address: Exponent, Inc, 149 Commonwealth Drive, Menlo Park, CA 94025

Email: bli9@illinois.edu

Abstract—Coupling semiconductor (e−-h+) and gas phaseplasmas with a strong electric field yields a transistor exhibitingphotosensitivity and voltage gain but also a light-emitting col-lector whose radiative output can be switched and modulated.

I. INTRODUCTION

Low temperature, weakly ionized plasmas in the gas phaseand electron-hole (e−-h+) plasma in semiconductors areclosely related. Having a common mathematical and physicalancestry, both plasmas are described by virtually identicalrelationships for charged particle drift, diffusion, and recom-bination.

We report the interaction of e−-ion and e−-h+ plasmasacross a narrow potential barrier, mediated by a strong electricfield provided by the sheath of the gas phase plasma. Theintegration of a gas phase plasma with a semiconductor emitterand base yields a phototransistor having a light-generating col-lector and the ability to modulate and switch (fully extinguish,then reignite) the collector plasma with voltages as small as<1 V applied to the emitter-base junction.

Fig.1 shows a qualitative energy band diagram for an n+pnPlasma Bipolar Junction Transistor (PBJT). In the forward-active mode of operation, the forward-biased emitter-basejunction injects electrons into the base. After diffusing acrossthe base, electrons accumulate at the base-collector (i.e., Si-plasma) interface where they reside in a conduction bandpotential well created in part by the internal electric field (Eint)that is normally present at the surface of Si in vacuum [1].As discussed in [2], bandbending induced by Eint at the Sisurface is reinforced by the electric field (Es) generated bythe sheath of the microplasma and directed toward the base.Therefore, the presence of the sheath electric field at the base-collector interface has an effect similar to as the imposition ofa reverse bias on the base-collector junction of a conventionalBJT except for the creation of the interface potential barrierwhose height is estimated to be no more than the siliconaffinity (4.05 eV).

II. DEVICE DESIGN AND PERFORMANCE

Two distinct PBJT designs have been fabricated and testedto date. The initial design shown in Fig. 2 is based on a SOIwafer whose device layer forms the base-emitter junction bythermal diffusion and the handle layer is etched to produce the

Fig. 1. Qualitative energy level diagram (not to scale) for the PBJT whenthe emitter-base junction is unbiased

Fig. 2. Cross section of a npn PBJT made from a SOI wafer

cavity in which the collector plasma resides [2]. In order toensure optical access to the critical base-collector junction, themore recent design has the pn junction in a form of circularmesa produced by KOH wet etching on an epitaxial wafer.The electrical and optical characteristics shown in Fig. 3 wererecorded with the devices made from SOI wafer.

Fig. 3 (a) shows the collector current (iC) as a function ofbase current (iB) for selected anode volatge (VCC) for a PBJThaving a collector cavity diameter of 3 mm and operatingwith a collector gas of 25 Torr Ne. These data were acquiredwith 5 kΩ and 67.2 kΩ resistors in series with the base andcollector, respectively. Throughout these measurements, theemitter was grounded and the base was driven by a 200 Hz

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Fig. 3. Output characteristics of a PBJT having a 3 mm diameter collectorand operating with a collector gas pressure of 25 Torr Ne; (a) iC -iB curvesfor 14 values of VCC ranging from 270 to 400 V; (b) relative optical emissionintensity recorded over the same ranges in iB and VCC as those of part (a).

sinusoidal voltage waveform having an RMS amplitude of2.8 V.

From the data of Fig. 3, the largest value of hfe (the smallsignal current gain) is approximately three. It is immediatelyapparent in Fig. 3 that the iC-iB profiles exhibit hysteresis,particularly at the lowest values of iB accessible for a givenvalue of VCC . This effect is tentatively attributed to the RCtime constant associated with the base-emitter capacitance,the current-limiting resistor (5 kΩ) in series with the baseand, ultimately, charge stored in the base. Another factorpotentially contributing to the hysteresis is the base currentproduced by photogenerated e−-h+ pairs in the base by above-bandgap photons originating in the collector plasma. When therelative emission intensity of the collector plasma is recordedwith a photodiode while varying VCC and iB , the set ofcharacteristics presented in the lower half of Fig. 3 is obtained.Similar to the iC-iB data of Fig. 3, these photoresponsecurves confirm the expected correlation between iC and opticalemission from the collector.

By lowering VCC such that it is near the threshold forplasma turn-off, it is possible to switch (fully extinguish andreignite) the collector plasma by modulating the base with

Fig. 4. Impact on iC -iB characteristics of irradiating the PBJT base with632.8 nm photons from a He-Ne laser.

a 2 Vpp signal superimposed onto a 0.4 V DC offset volt-age. Preliminary simulation results suggest that the effectivesecondary electron emission coefficient for Si as a cathodediffers for a forward-biased and a reverse-biased emitter-basejunction, thereby altering the plasma breakdown criterion.

In an effort to investigate the impact of e−-h+ pair pro-duction in the base on PBJT operation, experiments wereconducted in which the base of the PBJT was irradiated by anexternal He-Ne laser of approximately 0.4 mW. The effect ofsupplemental e−-h+ production on the iC-iB characteristicsof the transistor when VCC=440 V is presented in Fig. 4.The data acquired when laser photons impinge on the base(indicated by the red dots) are displaced to the left, owing toincreases in both |iB | and ic. From the measurements of Fig. 4,∆iB=0.2 mA for a fixed value of iC . This photogeneratedcurrent is equivalent to the absorption of 0.4 mW of 632.8 nmphotons and the complete conversion of the absorbed powerinto base current.

III. CONCLUSION

By interfacing a semiconductor’s electron-hole plasma witha gas phase plasma, the authors have demonstrated a new typeof light-emitting phototransistor, whose light output can bemodulated and switched.

REFERENCES

[1] W. Monch, P. Koke, and S. Krueger, “On the electronic structure of clean,2×1 reconstructed silicon (001) surfaces,” J. Vac. Sci. Technol., vol. 19,p. 313, 1981.

[2] C. J. Wagner, P. A.Tchertchian, and J. G. Eden, “Coupling electron-holeand electron-ion plamsas: Realization of an npn plasma bipolar junctionphototransistor,” Appl. Phys. Lett., vol. 97, p. 134102, 2010.

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