A Single-Stage Three-Terminal Heterojunction Bipolar Transistor ...

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 16, NO. 4, APRIL 1998

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A Single-Stage Three-Terminal Heterojunction Bipolar Transistor Optoelectronic Mixer Yoram Betser, Dan Ritter, C. P. Liu, A. J. Seeds, and A. Madjar

Abstract— A high-conversion gain three-terminal heterojunction bipolar transistor (HBT) optoelectronic mixer has been demonstrated. The maximum obtained intrinsic conversion gain was 10.4 dB. The mixing performance was measured as a function of the dc bias of the device and local oscillator power level. A SPICE-based large signal model was employed to simulate the device. The main nonlinear effects which contributed to the mixing process were the voltage dependence of the dynamic emitter resistance, and the variation of the current gain in the saturation regime.

I. INTRODUCTION

I

N OPTICAL subcarrier multiplexed (SCM) systems [1], [2] the modulated laser beam is detected, down-converted, and amplified, to recover the original baseband information. An heterojunction bipolar transistor (HBT) with an optical access to the base is a natural candidate to carry out all three tasks simultaneously. The inherent nonlinearity of three-terminal InP/GaInAs HBT’s, and their excellent high frequency performance as front end components [3], are very attractive for SCM systems. In this publication, we report on the performance of a single-stage InP/GaInAs HBT optoelectronic mixer (OEM). The mixing performance for an RF modulated optical signal with an electrical LO signal was measured at different bias conditions and local oscillator power levels. Previous reports on HBT OEM’s did not include modeling. Here, we use a SPICE-based large signal model to calculate the expected performance of the device. Good agreement with the experimental results was obtained when the HBT was biased in the active mode, whereas in the saturation mode only qualitative agreement was obtained. The SPICE model enabled us to identify the origin of the various nonlinear effects in the HBT. The measured mixing performance is compared to previously published results. II. THE DEVICE The epitaxial layers were grown on semi-insulating InP substrates by a compact metalorganic molecular beam epitaxy system [4]. Conventional wet etching and a self-aligned Pt/Ti/Pt/Au one step metallization process were employed

Fig. 1. Schematic diagram of layer structure, mesa structure, and a top view of the HBT. Emitter and base dimensions are 3.3 11 m2 and 22.5 m2 , respectively. Optical window (on the base mesa) size is 8.5 6 m2 5

2

2 2

to fabricate the devices. Polyimide passivation and Ti/Au bonding pads completed the fabrication process. A schematic diagram of the device is shown in Fig. 1. The emitter and base dimensions were 3.3 11 m2 and 2 2 8.5 22.5 m , respectively. The 5 6 m optical window was located on the base mesa in order to minimize device dimensions. Another advantage of locating the optical window on the base mesa rather than on the emitter mesa is that the current crowding effect [5] is avoided because the light is absorbed in the extrinsic base collector junction. An HBT with an optical window on the base mesa is basically similar to an HBT with the base terminal connected to a separate p-i-n photodiode. A detailed comparison of photoreceivers based on the p-i-n-HBT approach and the photosensitive HBT approach can be found in [3] and [6]. Small signal on-wafer RF measurements up to 40 GHz were carried out to evaluate the microwave electrical performance of the HBT. The obtained and were 65 and 35 GHz, respectively, at mA V. The relatively low was primarily due to the large area of the base mesa dictated by the presence of the optical window. III. THEORY

Manuscript received August 11, 1997. Y. Betser and D. Ritter are with the Department of Electrical Engineering, Technion—Israel Institute of Technology, Haifa 32000 Israel. C. P. Liu and A. J. Seeds are with the Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE U.K. A. Madjar is with RAFAEL, Haifa 32000 Israel. Publisher Item Identifier S 0733-8724(98)02205-1.

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MODELING

The main difference between the modeling of an optoelectronic HBT mixer and an electronic HBT mixer is that the RF input signal is represented as a current source in the optical case and as a voltage source in the electronic case. Therefore, in an electronic mixer the nonlinearity of the transconductance

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of the HBT determines the mixing efficiency, whereas in the OEM the nonlinearity of the current gain determines the mixing efficiency. Nonlinearity of the current gain of an HBT can be achieved in various configurations. First, the HBT can be switched by the LO between on and off states. The disadvantage of this mode of operation is that although the current gain changes dramatically, the LO power is large and the average frequency response of the HBT is poor. An alternative mode of operation is to bias the HBT in the saturation regime where the current gain depends strongly on the bias condition. The disadvantage of mixing in the saturation mode is that current gain is low at high frequencies because of the poor frequency response in this regime. The best mixing results were achieved in our experiments by biasing the HBT in the active mode. At first glance this is surprising because the dc current gain does not vary much in this regime. However, the high frequency ac because the dynamic current gain depends strongly on emitter resistance depends exponentially on the base emitter . The nature of the nonlinearity in the active mode voltage, is therefore exponential, as is the case in electronic HBT mixers. The strong nonlinearity combined with high gain at high frequencies in the active mode enabled us to obtain the high mixing efficiency described below. A large signal standard bipolar SPICE model was employed to simulate the mixing performance of the HBT-OEM. The schematic circuit diagram used in the simulations is shown in Fig. 2. The HBT-OEM was operating in a common emitter configuration. The LO signal was applied to the base from a 50- signal generator and the collector was connected to a 50- spectrum analyzer. This setup was designed to simulate closely a 50- broadband communication system. The IF signal was obtained from time domain large signal calculations followed by a fast Fourier transform (FFT) procedure performed with SPICE. The equivalent circuit shown in Fig. 2 included bond wire inductance, ideal bias-Tees, and 50- load resistors representing the spectrum analyzer and LO impedance. The LO signal was represented by a voltage . The photogenerated current was modeled as source, an RF current source, connected between the base and collector terminals. The HBT SPICE model parameters are listed in Table I. The parameters were extracted from dc measurements and small signal on-wafer RF measurements up to 40 GHz using the equivalent circuit extraction technique described in [7]. IV. OPTOELECTRONIC MIXING EXPERIMENT The experimental setup is shown in Fig. 3. The HBT base and collector were connected to SMA sockets for external connections using bond wires and two 1cm long 50- characteristic impedance microstrip lines. The emitter was connected to the common ground with a bond wire. The illumination of the device was normal to the epi-layer via the 5 6 m2 optical window in the base metallization. A DFB laser of 1.55 m wavelength was directly modulated with a 10 dBm incident RF signal at 3 GHz. The average optical power, on the focusing lens was 0.37 mW. Since the lens had a very small loss, was assumed to be the average incident optical

Fig. 2. SPICE-based large signal model of the HBT optoelecronic mixer. TABLE I LARGE SIGNAL SPICE MODEL PARAMETERS

REE RCC Rb Cbc(v=0) Cbe(v=0) Nf Nr Is Ics Beta

8 3 15 120 fF 32 fF 1.1 1.2 1 fA 6 nA 80 0.85 ps 1 ns

power on the HBT. An optical power meter in position A monitored the laser output power. The modulation index of the laser beam was 0.24 or 6.2 dB as measured by an HP70810B lightwave signal analyzer. The peak modulated component of the incident optical power was therefore mW W If would have been detected by an ideal photodetector with 100% quantum efficiency, terminated by a 50 load resistor, the electrical power generated in the load would have been dBm, where is the electron charge, Planck’s constant, optical frequency and In order to evaluate separately the system performance and the intrinsic device performance we defined an extrinsic and an intrinsic conversion (system) conversion gain, gain, The extrinsic conversion gain, is the ratio of the IF output power to the ideally detected RF power, defined above. The intrinsic conversion gain, is the ratio of the IF output power to the RF power, detected by the base-collector junction without amplification. The two definitions are related by where is the external quantum efficiency. In the mixing experiments the base was electrically pumped by a 3.1 GHz local oscillator signal. The 100 MHz IF output

BETSER et al.: HETEROJUNCTION BIPOLAR TRANSISTOR OPTOELECTRONIC MIXER

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Fig. 3. Experimental arrangement for measurement of mixer conversion gain.

of the HBT mixer was measured at the collector terminal by a spectrum analyzer. Measurements of the IF power were and carried out as a function of the base-emitter voltage collector-emitter voltage for different LO power levels. V. RESULTS

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DISCUSSION

In Fig. 4(a) the mixing performance of the HBT biased in the active mode is presented. The measured and calculated IF power is plotted as a function of for different LO power levels with V. The mixing efficiency exhibited a maximum at V. The measured and calculated optimum is readily explained as follows. The HBT served as a mixer and a current amplifier simultaneously. The main nonlinear effect in this bias range was the voltage dependence of the dynamic emitter resistance, which determined the current gain of the HBT at high frequencies. At low values of was large and the amplification level of the IF signal increased the high frequency gain of the was low. As HBT increased as well, leading to a larger IF output power. However, at high values of had a small effect on the high-frequency ac current gain, because of its small value. This resulted in a smaller nonlinearity and therefore, a reduction of the mixing efficiency of the HBT at high values of As is evident from Fig. 4(a) the simulations agreed very well with the experimental results for low to moderate base and for moderate emitter voltages. However, at high and large LO power, the simulations did not agree well with the measured data. We believe the discrepancy was due to the inadequacy of the SPICE model parameters in the saturation regime, which is further discussed below. Although the HBT was biased in the active mode, at high or LO power levels the instantaneous voltage drop across the 50- collector load, momentarily forward biased the base-collector junction and shifted the HBT toward the saturation regime. In the saturation mode the current gain of HBT’s is strongly bias dependent, and it is therefore of interest to find out

(a)

(b) Fig. 4. Calculated and measured IF (100 MHz) power as a function of base-emitter voltage. Peak RF modulated optical power = 88 W: (a) VCE = 1:5 V and (b) VCE = 0:8 V.

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Fig. 5. IF (100 MHz) power as a function of collector to emitter voltage. VBE = 0=8V and LO (3.1 GHz) power is 6 dBm. Peak RF modulated optical power = 88 W:

0

whether this nonlinearity enhances the mixing efficiency. In Fig. 4(b), the measured and calculated IF power is plotted as with equal to 0.8 V. Since was a function of constant in this experiment, at low values of the base collector junction was reverse biased, and the HBT operated the base collector in the active mode. At high values of junction was forward biased, and the HBT operated in the saturation mode. A clear minimum in the measured and calculated IF power was obtained in the transition region between the linear and saturation modes. We attribute this effect to an opposite phase of the two different nonlinear effects in the saturation mode and in the linear mode. An additional indication to the opposite phase of the two nonlinear effects was obtained by inspecting the harmonics of the LO signal in the SPICE simulation. The power of the third harmonic was considerably larger than that of the second harmonic, as expected when two nonlinear effects with opposite phases sum up. We finally note that in the saturation mode the RF current gain of the HBT was much lower than in the active mode. However, the mixing efficiency in the saturation mode was comparable to that in the linear mode indicating a strong nonlinearity in the saturation regime. The simulated IF power in the saturation mode did not agree well with the experimental data. We believe this was because of the complicated behavior of the base-collector capacitance and collector resistance in the saturation mode, which were difficult to extract and implement in the SPICE model. As mentioned above, the collector to emitter voltage determined whether the HBT was biased in the active or saturation mode. In Fig. 5, the measured IF power is plotted versus for V and LO power of 6 dBm. The best mixing results were achieved when the HBT was biased values). For V the in the active mode (higher IF power saturated because the high frequency response of the HBT and the nonlinear effects did not vary much with . The maximum extrinsic conversion gain obtained in our experiments was calculated from the data presented in Fig. 5. The maximum IF power was 45.9 dBm and hence in dBm) dB.

The external quantum efficiency of the HBT was found to be by measuring the ratio of the dc primary photogenerated current with V to the equivalent current that would be generated by a 100% quantum efficient photodetector when illuminated by the same incident optical power. This relatively low external quantum efficiency agrees well with a calculation based on the total thickness of the base and collector layers which was about 3 times shorter than the absorption depth of the light, and 30% reflection of the incident light. Since was 16%, was 5.2 dB. An improvement of the mixing efficiency was achieved by eliminating some of the leakage of the ac photocurrent to the LO port. A three-stub tuner was inserted between the LO source and the base and was adjusted to obtain maximum IF power. The insertion of the three-stub tuner increased the IF output by 5.2 dB, the measured RF power by 4.1 dB and the LO output power by only 0.4 dB, indicating that the improvement of the conversion gain was due to a more efficient collection of the primary photocurrent into the base, and not due to better matching of the LO network. The extrinsic and intrinsic conversion gains with the three-stub tuner became 5.5 dB and 10.4 dB, respectively. This result compares favorably with Gext of 21.1 dB, 26 dB and 29 dB for three-terminal HBT, HEMT, and JFET, optoelectronic mixers calculated from a previous report [8]. The authors have also recently demonstrated a two-terminal edge-coupled heterojunction phototransistor (HPT) optoelectronic mixer [9] and achieved a 7dB extrinsic conversion gain. The reason for the better extrinsic conversion gain with this device is that since the light was edge-coupled to the base, the length for optical absorption was equal to the lateral dimension of the transistor which was several microns long, yielding a high external quantum efficiency. VI. CONCLUSION We have reported on the experimental performance of a high conversion gain three-terminal HBT OEM. The results were compared to calculations carried out using a large signal model. The largest mixing efficiency was achieved when the HBT was biased in the active mode. The main nonlinear effects in the HBT OEM were the voltage dependence of the dynamic emitter resistance and the variation of the current gain in the saturation regime. Further improvement of the mixing efficiency can be achieved by increasing the quantum efficiency (increasing the collector thickness), RF isolation of the base terminal, and reducing the collector load impedance. REFERENCES [1] T. E. Darcie, “Subcarrier multiplexing for multiple-access lightwave networks,” J. Lightwave Technol., vol. LT-5, pp. 1103–1110, Aug. 1987. [2] R. Olshansky, V. A. Lanzisera, and P. M. Hill, “Subcarrier multiplexed lightwave systems for broad-band distribution,” J. Lightwave Technol., vol. 7, pp. 1329–1342, Sept. 1989. [3] S. Chandrasekhar, L. M. Lunardi, A. H. Gnauck, R. A. Hamm, and G. J. Qua, “High speed monolithic pin/HBT HPT/HBT photoreceivers implemented with simple phototransistor structure,” IEEE Photon. Technol. Lett., vol. 5, pp. 1316–1318, Nov. 1993. [4] R. A. Hamm, D. Ritter, and H. Temkin, “A compact MOMBE growth system” J. Vacuum Sci. Technol., vol. A12, pp. 2790–2794, 1994.

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[5] J. K. Twynam and R. C. Woods, “Current crowding effects in GaAs/AlGaAs heterojunction phototransistors,” Inst. Elec. Eng. Proc., vol. 135, pt. J, no. 1, pp. 52–55, 1988. [6] E. Sano, M. Yoneyama, S. Yamahata, and Y. Matsuoka, “InP/InGaAs double heterojunction bipolar transistors for high speed optical receivers,” IEEE Trans. Electron Devices, vol. 43, pp. 1826–1832, Nov. 1996. [7] S. J. Spiegel, D. Ritter, R. A. Hamm, A. Feygenson, and P. R. Smith, “Extraction of the InP/GaInAs heterojunction bipolar transistor small signal equivalent circuit,” IEEE Tran. Electron Devices, vol. 42, pp. 1059–1064, June 1995. [8] Z. Urey, D. Wake, D. J. Newson, and I. D. Henning, “Comparison of InGaAs transistors as optoelectronic mixers,” Electron. Lett., vol. 29, p. 1796, 1993. [9] C. P. Liu, A. J. Seeds, and D. Wake, “Two-terminal edge-coupled InP/InGaAs heterojunction phototansistor optoelectronic mixer,” IEEE Microwave Guide Wave Lett., vol. 7, pp. 72–74, Mar. 1997.

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Dan Ritter received the B.Sc., M.Sc., and Ph.D. degrees from the Technion— Israel Institute of Technology, Haifa, in 1981, 1984, and 1989, respectively. From 1989 to 1992, he was a Consultant at AT&T Bell Laboratories. In 1992, he joined the Department of Electrical Engineering, the Technion, as a Senior Lecturer. His current research interests are metalorganic molecular beam epitaxy of InP-based semiconductors and optoelectronic and electronic devices based on InP.

C. P. Liu, photograph and biography not available at the time of publication.

A. J. Seeds, photograph and biography not available at the time of publication. Yoram Betser received the B.Sc. and M.Sc. degrees from the Technion— Israel Institute of Technology, Haifa, in 1992 and 1995, respectively. He is currently working towards the Ph.D. degree at the Department of Electrical Engineering, the Technion, carrying out research on HBT device physics, high-frequency modeling, and optical control of HBT’s.

A. Madjar, photograph and biography not available at the time of publication.