IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 12, DECEMBER 2004 2013

AlGaAsSb–InGaAsSb HPTs With High Optical Gainand Wide Dynamic Range

M. Nurul Abedin, Member, IEEE, Tamer F. Refaat, Member, IEEE, Oleg V. Sulima, andUpendra N. Singh, Member, IEEE

Abstract—Novel heterojunction phototransistors based onAlGaAsSb–InGaAsSb material systems are fabricated and theircharacteristics are demonstrated. Responsivity of a phototran-sistor is measured with applied bias voltage at four differentwavelengths. The maximum responsivity around 1400 A/W andminimum noise equivalent power of 1 83 10

14 W/Hz1 2

from this phototransistor with bias of 4.0 V at a wavelength of2.05 m were measured at 20 C and 20 C, respectively. Noiseequivalent power of the phototransistor is considerably lowercompared with commercially available InGaAs p-i-n photodiodes.Collector current measurements with applied incident power areperformed for two phototransistors. Currents of 400 nA at lowintensity of 0.425 W/cm2 and of 30 mA at high intensity of 100mW/cm2 are determined. Collector current increases nearly byfive orders of magnitude between these two input intensities. Highand constant optical gain of 500 in the 0.46-nW to 40- W inputpower range is achieved, which demonstrates high dynamic rangefor such devices at these power levels.

Index Terms—Collector current, dynamic range, gain, hetero-junction, optical power, photodiode, phototransistor, responsivity.

I. INTRODUCTION

HETEROJUNCTION phototransistors (HPTs) have beenstudied for the last several decades as a promising

alternative to p-i-n photodiodes or avalanche photodiodes foroptical communication systems [1]–[10]. Recently, there hasbeen considerable interest in HPTs in the 1.8- to 2.2- mwavelength range for potential application to laser remotesensing [11]. Several important performance parameters, suchas high responsivity, low noise, and hence high gain, as well aslarge dynamic range are considered very important propertiesof the detectors in order to satisfy the requirements of theoptical communication and laser remote sensing systems. Ingeneral, p-i-n photodiodes and avalanche photodiodes (APD)are the detectors of choice for these systems, but HPTshave also attracted considerable attention to satisfy manyof the detector requirements for applications to the opticalcommunication and laser remote sensing systems. These HPTs

Manuscript received May 17, 2004; revised September 14, 2004. This workwas supported by the Laser Risk Reduction Program under NASAs Earth Sci-ence Technology Office and NASAs Enabling Concepts and Technologies Pro-gram. The review of this paper was arranged by Editor L. Lunardi.

M. N. Abedin is with the Laser and Electro-Optics Branch, NASA LangleyResearch Center, Hampton, VA 23681 USA (e-mail: m.n.abedin@nasa.gov).

T. F. Refaat is with the Science and Technology Corporation, NASA LangleyResearch Center, Hampton, VA 23681 USA.

O. V. Sulima is with the Electrical and Computer Engineering, University ofDelaware, Newark, DE 19716 USA.

U. N. Singh is with System Engineering Competency, NASA Langley Re-search Center, Hampton, VA 23681 USA.

Digital Object Identifier 10.1109/TED.2004.838328

can offer large optical gain without excess noise and highbias voltages.

Numerous types of heterojunction transistors, such asGaAs–Ge [12], ZnSe–Ge [13], CdS–Si [14], InAlAs–InGaAs[15], and AlGaAs–GaAs [16] material systems have demon-strated small current gains around 30–80. An improvedtransistor based on AlGaAs–GaAs was successfully fabricatedand a current gain of 350 was reported [17]. Another group[18] studied the AlGaAs–GaAs transistor and evaluated thecurrent gain at around 300. Again, Konagai et al. [19] hadshown a maximum current gain of 1600 for AlGaAs–GaAsphototransistor. Campbell and his group [20] fabricatedInGaAs–InP HPT in the 0.9- to 1.6- m wavelength rangeand reported a gain as high as 1000. Wright et al. [3]and Tobe et al. [4] fabricated quaternary InGaAsP on InPHPTs in the 0.9- to 1.3- m spectral range and Tobe etal. [4] demonstrated a gain of 1000.

Recently, the quaternary AlGaAsSb–InGaAsSb HPTs inthe 1.0- to 2.2- m wavelength range have been developedand fabricated at AstroPower in collaboration with NASALangley Research Center [11], [21]. These devices havebeen characterized at NASA Langley Research Center, andencouraging results including high responsivity, low noise,and high optical gain have been obtained.

The HPT device discussed in this paper is a two-terminaldevice with a floating base. Electromagnetic radiation with

2- m wavelength incidents on the wide-bandgap n-typeAlGaAsSb emitter, passes through the emitter unattenuatedand is absorbed in the p-type InGaAsSb part of the p-typeAlGaAsSb–InGaAsSb composite base, base–collector depletionregion, and n-type InGaAsSb collector region. The holesphotogenerated in the p-type narrow bandgap part of thecomposite base or swept into base from the collector increasethe forward bias of the base–emitter junction, which causesa large electron injection from emitter to collector. Providedthat the diffusion length of the injected electrons in thebase is larger than the base thickness, the current gain isachieved as in usual transistors.

In this paper, characterization of AlGaAsSb–InGaAsSbphototransistors is reported. Responsivity measurements areperformed for a phototransistor (HPT1: sample#A1-a2) withapplied bias voltages at four different wavelengths (1.0,1.55, 2.05, and 2.2 m). A comparison of noise equivalentpower (NEP) with a phototransistor (HPT1) and InGaAsp-i-n photodiodes (G5852 and G5853, Hamamatsu, Inc.) at

C has been made. Collector current measurements arecarried out on HPTs (HPT1 and HPT2: sample#A1-b3) over

0018-9383/04$20.00 © 2004 IEEE

2014 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 12, DECEMBER 2004

Fig. 1. AlGaAsSb–InGaAsSb HPT layered structures.

a wide range (49 dB) of radiation intensity. High optical gainsof about 500 are achieved from these HPTs as comparedto multiplication gain 60 of similar material APD [22],which are the highest values ever reported on Sb-basedmaterial structure to date. This gain over a 49-dB powerrange demonstrates the highest dynamic range of HPTs at2.05- m wavelength.

II. MATERIAL GROWTH AND DEVICE FABRICATION

The phototransistor mesa structure (Fig. 1) fabricated andstudied in this work is composed of AlGaAsSb and InGaAsSblayers with bandgaps of and eV,respectively. The layers are lattice matched to a GaSb substrateand were grown by liquid-phase epitaxy (LPE) using a horizontalslideboat technique [23]. LPE inherently provides superiormaterial quality both with respect to defect density and purity.Thismaybeattributedtothepreferentialsegregationofimpuritiesto the liquid phase, the high mobility of atoms in the liquidphase, and near-equilibrium growth conditions. The recentexample of the excellent quality of the LPE-grown materialsclose to those discussed in this paper, is the demonstration ofthe first room temperature photoluminescence of LPE-grownInGaAsSb lattice matched to InAs [24]. The phototransistorstructure includes an n-type AlGaAsSb emitter ( cm ,0.9 m thick), p-type composite base consisting of AlGaAsSb( cm , 1.3 m thick) and InGaAsSb ( cm ,1 m thick) layers, and an n-type InGaAsSb ( cm ,5 m thick) collector. Mesa phototransistors with a 400- mdiameter of the total area and a 200- m diameter of the activearea were defined using photolithography and wet chemicaletching.

Backside planar and front side annular ohmic contacts (to-gether with a bonding pad) were deposited by electron-beamevaporation of Au–Sn and Ti–Ni–Au, respectively. A polyimidecoating was spun on the front of the device. The polyimideserved several functions including planarization of the topsurface, mesa isolation, and edge passivation. After dicing,1-mm pieces with a single device in the middle of each squarewere mounted to TO-18 headers using silver conducting epoxyand wire bonded. No antireflection coatings were applied.

Fig. 2. Responsivity measurements with applied bias voltage at 1-, 1.55-,2.05-, and 2.2-�m incident radiation for phototransistor (HPT1) operating at20 C.

III. EXPERIMENTAL RESULTS AND DISCUSSIONS

Responsivity of the phototransistor is obtained by using a PbScalibrated detector. The output signal of the phototransistor isacquired and then compared with that of a spectrally calibratedPbS detector. The spectral responsivity, (in A/W), of thephototransistor is determined by using the following equation:

(1)

where is the PbS photodiode response (in volts), isthe phototransistor response (in volts), is the ratio of thePbS detector area to the phototransistor area, (in V/A) is thepreamplifier gain setting, and (in V/W) is the spectralresponsivity of the PbS detector.

Fig. 2 shows the responsivity of the AlGaAsSb–InGaAsSbphototransistor (HPT1) as a function of applied bias voltage at20 C. The maximum responsivities around 35, 709, 1400, and190 A/W are determined at 4.0 V for 1.0-, 1.55-, 2.05-, and2.2- m wavelengths, respectively. The maximum responsivityis measured for 2.05 m, which demonstrates that this light is

ABEDIN et al.: AlGaAsSb–InGaAsSb HPTs WITH HIGH OPTICAL GAIN 2015

Fig. 3. NEP measurements with an applied bias voltage at 20 C and�20 Cfor a phototransistor (HPT1) and at �20 C for InGaAs p-i-n photodiodes(G5852 and G5853).

absorbed close to the base–collector p-n junction, where condi-tions for the carrier separation are the most favorable. A shorterwavelength light (1.55 and 1.0 m) is absorbed closer to theemitter at a larger distance from the base–collector p-n junction.Evidently, some of the carriers are lost due to a higher proba-bility of recombination in this case. The lower value of the re-sponsivity at 2.2 m compared to that obtained at 2.05 m, canbe explained by a low absorption that occurs at energies closeto the bandgap of InGaAsSb used in this study. As expected,the responsivity of the HPT1 has a strong bias dependence. Re-sponsivity increases by more than three orders of magnitudeat 2.05 m when increasing bias voltage, V (collector–emittervoltage, ), from 0 to 4.0 V, which is due to more efficientemitter injection, faster transport of injected electrons throughthe base at higher voltages, and also possibly due to a decreaseof the recapture mechanism [25].

To measure the NEP of the test detector, the amount of theoptical power incident on a test detector that produces an outputsignal equal to the noise signal in a 1-Hz bandwidth. NEP (inwatts per square root of hertz ) is defined by

(2)

where (in ampere per square root of hertz) is the noise cur-rent density and (in A/W) is the responsivity of the pho-totransistor. Fig. 3 shows NEP characteristics of a phototran-sistor (HPT1) and of commercially available p-i-n photodiodes(G5852 and G5853) with bias voltage at different wavelengths.Thus, bias dependencies of NEP can be considered for an op-timum operation of the phototransistor. The bias dependenceof NEP for the p-i-n photodiodes shows a small increase ofthe noise, which is only due to the high bias voltage. Thesetwo photodiodes have two different cutoff wavelengths, such as2.6 m for G5853 and 2.3 m for G5852; and NEP (top solidlines) is determined around six times lower for G5852 Cas compared to G5853 C by applying 2.05- m radi-ation at 4.0 V. For the measured NEP for the HPT1 deviceat 2.05- m radiation wavelength, the bias dependence of NEP

(bottom solid line: NEP W/Hz at 20 C) isaround seven times smaller at 4.0 V with respect to photodiodeG5852 (NEP W/Hz at C). Comparisonwith the best of the tested p-i-n photodiodes measured at Cshows that at 4.0 V the phototransistor achieves near an orderof magnitude lower NEP despite the 40 higher operating tem-perature (20 C). Besides 20 C, this phototransistor was alsooperated at C and NEP was measured as low as

W/Hz with incident radiation of 2.05 m at 4.0 V.Fig. 4 shows the collector current, as a function of

intensity (incident flux) at various bias voltages for HPTs. A2.05- m CW (continuous wave) laser beam was focused ontothe HPT front surface and a current controller was used toprovide a signal to the laser and to obtain the signal from theHPT output. As one might expect, the HPT current increaseswith both the bias voltage and the incident intensity. Currents of400 nA at low 0.425- W/cm input intensity and of 30 mA athigh 100-mW/cm input intensity are determined. At relativelysmall bias voltages (1, 2, and 3 V), both linear and quadraticdependences of the current on the incident intensity are ob-served. However, the quadratic dependence disappears whenbias voltage is 4.0 V. For these measurements, we intentionallyselected two HPTs with different quality. A significant discrep-ancy in collector currents and saturation levels are detectedat 1, 2, and 3 V of these two HPTs. But, this discrepancy iseliminated at 4.0 V and the collector current is linear over fiveorders of magnitude. In addition, the variation of responsivitiesaround 0.7, 17.2, 58.0, and 276.4 A/W for HPT1 [see Fig. 4(a)]and 0.8, 1.7, 24.3, and 191.9 A/W for HPT2 [see Fig. 4(b)] at0.0, 1.0, 2.0, and 3.0 V are determined, but responsivity around1000 A/W was calculated for both HPTs at 4.0 V. The saturationcollector currents were detected at 0.002, 0.545, 2.3, 8.6, and27.1 mA for HPT1 and at 0.006, 0.9, 3.8, 6.5, and 7.5 mA forHPT2 with applied biases 0, 1, 2, 3, and 4 V, respectively.

Optical gain of the HPT was determined as a function of in-cident optical power. This gain was calculated by measuring thecollector current of the HPT as a function of incident op-tical power at a specific bias voltage. This gain canbe determined using the following equation:

(3)

where is the collector current, is the wavelength of the inci-dent photon, is Planck’s constant, is the speed of light, and

is the electron charge.Fig. 5 shows optical gain versus optical power for

HPT1 and HPT2 calculated using (3). An optical gain of about500 is obtained in the 0.46-nW to 40- W input power range(over 49-dB range) for both HPT1 and HPT2. It is observedthat the gains of HPT1 and HPT2 are independent of incidentoptical power at intermediate range. At low and high incidentpower, the gain is found to have small power dependence withbiases of 1, 2, and 3 V.

Finally, the measured responsivities for the HPT1 at 4.0 V are35, 709, 1400, and 190 A/W for incident radiation of 1.0, 1.55,2.05, and 2.2 m, and the highest value 1400 A/W is about threeorders of magnitude higher than that of the same phototransistor

2016 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 12, DECEMBER 2004

Fig. 4. Collector current of AlGaAsSb–InGaAsSb phototransistors, (a) HPT1 and (b) HPT2, as a function of incident flux with applied biases (0, 1, 2, 3, and 4V) at 20 C.

Fig. 5. Optical gain of AlGaAsSb–InGaAsSb phototransistors, (a) HPT1: A1-a2 and (b) HPT2: A1-b3, as a function of incident power with applied biases (0, 1,2, 3, and 4 V) at 20 C.

at 0.0 V. The NEP for the HPT1 is much improved with re-spect to the p-i-n photodiodes at C. Collector current de-tection and optical gain calculation are performed by applyingfour different voltages. The maximum collector current is de-termined around five orders of magnitude in the 0.425- W/cmto 100-mW/cm incident flux range. High and constant opticalgain of 500 in the 0.46-nW to 40- W input optical power range(over 49 dB range) is obtained, which demonstrates high opticalgain and wide dynamic range for such devices on Sb-based ma-terial systems.

IV. CONCLUSION

We have demonstrated the characterization of novel Al-GaAsSb–InGaAsSb HPTs and obtained high responsivity, low

noise, high gain, and large dynamic range for these devices.Bias voltage in a device significantly improves the performanceof a phototransistor. Responsivity measurements at 1.0-, 1.55-,2.05-, and 2.2- m wavelengths show great promise of thisHPT for many applications. NEP measurements of the HPT1were compared with the p-i-n photodiodes and the measuredNEP of the HPT1 is more than one order of magnitude lowerthan that of the p-i-n photodiodes with bias of 4.0 V at

C. Collector current of five orders of magnitude andoptical gains of 500 were obtained from these HPTs at biasvoltage of 4.0 V. The characterization results reported hereon Sb-based phototransistors may eventually lead to opticalcommunications and remote sensing systems employing in-tegrated phototransistors.

ABEDIN et al.: AlGaAsSb–InGaAsSb HPTs WITH HIGH OPTICAL GAIN 2017

ACKNOWLEDGMENT

The authors wish to thank G. Komar, C. Moore, and F. Perifor their constant support and F. Amzajerdian for assistance withthe 2- m laser.

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M. Nurul Abedin (M’00) received the Ph.D. degreein solid state physics from Rensselaer Polytechnic In-stitute, Troy, NY, in 1989.

He has been with NASA since 1999, and withindustry/university at large since 1989. He is LeadDetector Technologist, Laser and Electro-OpticsBranch, NASA Langley Research Center, Hampton,VA, where he plans, directs, and coordinates researchand technology development programs dealing withdetectors of the earth and space science missions.With over 18 years experience in semiconductor and

materials characterization, his research activities include studies of GaAs–Al-GaAs quantum wells, GaAs–AlAs superlattices, YaBaCuO superconductors,GaAs–Si heterostructures, GaAs, InP, Si, CCDs, HgCdTe FPAs, InGaAs p-i-ndiodes, InGaAsSb p-n/SAM APD, phototransistors, multicolor devices, andmodeling of infrared detectors.

Dr. Abedin is a member of AIAA, Optical Society of America (OSA), andSPIE, and is also Langley’s representative on the AIAA Sensor Systems Tech-nical Committee. He is an Associate Editor and a Member of the Editorial Boardof the IEEE SENSORS JOURNAL

Tamer F. Refaat (M’00) received the B.S. (withhonors) and M.S. degrees in electrical engineeringfrom Alexandria University, Alexandria, Egypt, in1991 and 1995, respectively. He recevied the Ph.D.degree from Old Dominion University, Norfolk, VA,in 2000, working on the development of water vaporlidar detection systems.

He was a Teaching Assistant in the ElectricalEngineering Department, Alexandria University. In1996, he joined the NASA Langley Research Center,Hampton, VA as a Research Assistant through Old

Dominion University. He is currently with NASA Langley Research Centerworking on the development and characterization of optical detectors for lidarapplications.

Dr. Refaat is a member of SPIE.

Oleg V. Sulima received the M.S. degree in electricalengineering from the Electrical Engineering Institute,St. Petersburg, Russia, in 1979 and the Ph.D. degreein physics and mathematics from Ioffe Institute, St.Petersburg, in 1986.

He joined Ioffe Institute in 1979, where heremained until 1991. From 1989 to 1991, he wasalso an Associate Professor at the Institute of FineMechanics and Optics, St. Petersburg. In 1991, hereceived the Aleksander-von-Humboldt Fellowshipand continued his research in the field of III-V

semiconductors at the Fraunhofer Institute for Solar Energy Systems (ISE),Freiburg, Germany. His research work from 1992 to 1997 was carried outat both the Ioffe Institute and the Fraunhofer ISE and was focused on thedevelopment of high-efficiency solar cells for terrestrial and space application.From 1997 to 2000, he developed low-bandgap photovoltaic cells at theFraunhofer ISE and at the Freiburg Material Research Center. In 2000, hejoined AstroPower, Newark, DE, where he was involved in the design anddevelopment of advanced optoelectronic devices. Since 2004, he has been aScientist at GE Energy, Newark, and Adjunct Associate Professor, Universityof Delaware, Newark.

2018 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 12, DECEMBER 2004

Upendra N. Singh (M’00) received the Ph.D.degree in physics from the University of Pierre andMarie Curie, Paris, France, in 1985, the Diplômed’Etude Approfondis degree from the Universityof Franchè-Compté, Besançon, France, in 1982.He received the M.Phil., physics, M.Sc., appliedphysics, and the B.Sc. (Honors) degrees in physicsduring 1979, 1980, and 1974, respectively, from IIT,Kanpur, India, B.I.T. Mesra, India and L.N. MithilaUniversity, Bihar, India respectively.

He is the Chief Technologist, Systems EngineeringCompetency, NASA Langley Research Center, Hampton, VA. Currently, he isthe Principal Investigator of NASA’s Laser Risk Reduction Program, a multi-center NASA program dedicated toward developing an end-to-end lidar capa-bility leading to space-based remote sensing. He has authored or coauthoredmore than 150 research papers. He is also on the Board of Editors, Journal ofOptics and Lasers in Engineering, Elsevier Science Ltd., U.K.

Dr. Singh has chaired 20 international conferences/symposia in the area ofactive and passive remote sensing of the Earth’s atmosphere. He is a Fellowof SPIE and member of Optical Society of America (OSA). He is also servingon the SPIE Symposia Committee. He has received numerous awards includingNASA Outstanding Leadership Medal in 2001 for “Significant contributions anddistinguished, internationally recognized, scientific and technical leadership ofNASA programs in the area of active and passive remote sensing of the atmos-phere.”