Structural Parameters Improvement of an Integrated HBT in a Cascode Configuration Opto- Electronic Mixer
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Home Search Collections Journals About Contact us My IOPscience Structural parameters improvement of an integrated HBT in a cascode configuration opto- electronic mixer This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 J. Semicond. 34 094001 (http://iopscience.iop.org/1674-4926/34/9/094001) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 137.99.31.134 This content was downloaded on 16/05/2015 at 10:04 Please note that terms and conditions apply. Vol. 34, No. 9 Journal of Semiconductors September 2013 Structural parameters improvement of an integrated HBT in a cascode configuration opto-electronic mixer Hassan Kaatuzian, Hadi Dehghan Nayeri, Masoud Ataei, and Ashkan Zandi Photonics Research Laboratory (PRL), Electrical Engineering Department, AmirKabir University of Technology, Hafez Avenue No. 424, Tehran-15914, Iran Abstract: We analyze an integrated electrically pumped opto-electronic mixer, which consists of two InP/GaInAs hetero junction bipolar transistors (HBT), in a cascode configuration. A new HBT with modified physical structure is proposed and simulated to improve the frequency characteristics of a cascode mixer. For the verification and calibrating software simulator, we compare the simulation results of a typical HBT, before modifying it and com- paring it with empirical reported experiments. Then we examine the simulator on our modified proposed HBT to prove its wider frequency characteristics with better flatness and acceptable down conversion gain. Although the idea is examined in several GHz modulation, it may easily be extended to state of the art HBT cascode mixers in much higher frequency range. Key words: cascode; down conversion gain; mixer; opto-electronic; photo HBT; simulation DOI: 10.1088/1674-4926/34/9/094001 EEACC: 2570 1. Introduction Opto-electronic mixers (OEMs) based on InP/GaInAs HBT are attractive components for optical sub carrier multi- plexed systemsŒ1. InP/GaInAs HBTs exhibit a large inherent nonlinearityŒ2 that’s a key property for mixing operation effi- ciency. Both single and cascode configuration of HBTs have been examined for better performances in frequency response. In this paper, first of all, in Section 2, we develop a software simulator, to simulate behaviors of a cascode opto-electronic mixer which has already been fabricated and empirically tested by Betser et al.Œ3. In Sections 3 to 5, we use the experimental results to verify our software with suitable fitting parameters. Then in Section 6, we take a further look at the physical struc- Fig. 1. T model for bipolar transistor. ture of a typical HBT to modify its physical dimensions in a way to obtain better performances. Physical dimensions are converted into Y -parameters (systematic model), and then this model is used in our developed software to prove better perfor- mances of the new proposed HBT. We also have a conclusion section. 2. Setting up a simulation work space The model we used for simulation was based on the P-SPICE charge control model. The P-SPICE large-signal modelŒ4 is based on the T model shown in Fig. 1. The following relationships are used in this modelŒ4: Fig. 2. P-SPICE model for transistor. Is Ä Â VBE à Is Ä ÂVBC à ib exp 1 exp 1 ; (1) D ˇF nVT C ˇR nVT Ä Â VBE Ã Ä ÂVBC à iB and iC are obtained from the T model expressed before iT Is exp 1 Is exp 1 ; (2) D nVT nVT in Fig. 1: iT is a controlled current source that is controlled with base– emitter and base–collector voltages. The complete model that Is  VBC à is used in the large-signal P-SPICE model is shown in Fig. 2. iC iT exp 1 ; (3) D ˇR nVT † Corresponding author. Email: [email protected] Received 13 February 2013, revised manuscript received 17 March 2013 © 2013 Chinese Institute of Electronics 094001-1 J. Semicond. 2013, 34(9) Hassan Kaatuzian et al. Table 1. Values of the equivalent circuit components. ˇF Is CBC CBE0 n 130 36.2 fA 43 fF 100 fF 1.217 1.8 ps Fig. 3. P-SPICE model for transistor used for our simulation.  Is àVBE à iE iT exp 1 : (4) D C ˇF nVT Base–emitter and base–collector capacitances are variable Fig. 4. Schematic diagram of the epitaxial layer structure and mesa capacitances which include both diffusion and depletion capac- structure. The optical window is located on the base mesa. itances. The diffusion part of base–emitter capacitance is expressed as: Cde Fgm; (5) D in which W 2 F ; (6) D 2Dn and the depletion part can be expressed asŒ4: Fig. 5. Experimental setup for measuring the opto-electronic cascode Cje0 mixer. Cje 2Cje0: (7) D V m Š Â1 BE à V0e ˇ is the collector current gain. Base collector capacitance W is the base width and Dn is the diffusivity of electrons is reported to be 43 fFŒ3. Values of the equivalent circuit com- in base. ponentsŒ3 are shown in Table 1. Our model is the mentioned model in Fig. 2 that is to some extent simplified. The above model works in both active and saturation regimes, but our research is confined to the active 3. Experimental arrangement mode, so the model for the transistor can be simplified. In A schematic diagram of the epitaxial layer structure and Ref. [3] the base emitter capacitor is expressed as: mesa structure is shown in Fig. 4. The epitaxial layers were grown on a semi-insulating substrate by a compact molecular VBE Is exp beam epitaxy (MBE). The emitter and base dimensions were VT 2 2 CBE CBE0 2Cje0: (8) 4 11 m and 9 23 m respectively. Obtained FT and D C VT Š Fmax of the single device were 70 and 50 GHz respectively. is the collector-to-emitter transit time, and CBE0 is the Small signal S-parameters of the single device and the cas- base–emitter capacitance for VBE 0. Œ3 D code pair were measured up to 40 GHz . The schematic dia- The model we used for the transistor is shown in Fig. 3. gram of the experimental arrangement is shown in Fig. 5. The Œ5 RBi is the intrinsic base resistance that is expressed as : 5 6 m2 optical window was located on the base mesa of HBT Q1 in Fig. 5. 1 We RBi b ; (9) The light detection, mixing and amplifying were per- D 12 LeXb formed by HBT Q1, while HBT Q2 served as a low input re- Œ5 in which b is the resistivity of the base material : sistance unity. In Fig. 5 the base of Q1 is the input port and collector of HBT Q2 is the output. A 50 local oscillator and 1 a DC voltage source were connected via bias T to the base port. b : (10) D qbNb The output power was measured using a spectrum analyzer. The base of HBT Q2 was connected to a dc voltage source us- We, Le and Xb are emitter width, emitter length and base thickness. ing a DC probe with a 120 pF capacitor to provide a radio fre- quency (RF) ground. A distributed feedback laser emitting at The diode relationship is expressed as: 1.55 m was externally modulated by a Mach Zender modula- VBE tor. The modulated light was amplified using an erbium doped ib Is exp : (11) fiber amplifier (EDFA). The light was focused onto the opti- D nVT 094001-2 J. Semicond. 2013, 34(9) Hassan Kaatuzian et al. Fig. 6. Schematic of the simulation of single HBT mixer. cal window using a microscope. The modulation index was re- ported to be 27% ( 6:7 dB). In the experiment an average op- tical power typically of about –15.3 dBm is incident on HBT’s optical window. Both intrinsic and extrinsic conversion gains Fig. 7. Down conversion gain comparison of simulation results and are useful figures of merit. experimental results. The intrinsic conversion gain Gint is defined as the ratio of the output power to Pprime, the primary photo-detected RF power. Pprime is the photo induced RF electrical power detected by the base–collector junction that was measured by shorting the base–emitter junction. The extrinsic conversion gain is de- fined as the ratio of the output power of up or down converted signal to the equivalent electrical RF power Pin that would have been detected by an ideal photo diode with equal load resis- tance. The relation between the incident peak modulated com- ponent of the optical power, Pmod, and the equivalent electrical 2 RLOAD input power is pin .qPmodhv/ Á, where q is the elec- D 2 tric charge, hv is the photon energy and RLOAD 50. Pprime D and Pin are related by the external quantum efficiency of the 2 base–collector photo diode (), Gext Gint . The external quantum efficiencyŒ6 was reported toD be 29% or 2 Fig. 8. Schematic of the simulation for cascode pair and comparison –10.8 dBŒ3. D D with experiment. 4. Simulation of the experiment of single HBT sults, because our simulator doesn’t take into account the sat- The simulation is based on the previous transistor uration effects. The figure shows that for various VBEs down schematic. Here we used an inductance and capacitance with conversion gain varies widely with base emitter bias, because large magnitude for simulating bias T of the base. The out- of the nonlinearity effect of input impedance that varies widely put bias T is omitted for simplification of calculations. The with base–emitter bias (the mixing efficiency is dependant on schematic of the simulation is shown in Fig.