The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Fabrication and Modelling of SOI and GaAs MSM Photodetectors and a GaAs-Based Integrated Photoreceiver

Katri Honkanen

Dissertation for the degree of Doctor of Technology to be presented with due permission for public examination and debate in Auditorium S5 at Helsinki University of Technology (Espoo, Finland) on the 30th of November, 2001, at 12 o'clock noon.

Dissertation in PDF format (ISBN 951-22-5697-5)   [3670 KB]
Dissertation is also available in print (ISBN 951-22-5732-7)

Abstract

In this work metal semiconductor metal photodetectors (MSM PDs), pseudomorphic high electron mobility transistors (pHEMTs), and a monolithically integrated photoreceiver are studied. The motivation for this work is to develop a solution for high-speed data transfer in telecommunication systems. The goals are to develop the fabrication technology using electron beam (e-beam) lithography, and to achieve higher bandwidths for the detector, the transistor, as well as the photoreceiver, with submicron devices.

First, MSM detectors on different materials, i.e. silicon-on-insulator (SOI) and semi-insulating gallium arsenide (S.I. GaAs) are fabricated using both optical and e-beam lithography. The detectors are characterized with current-voltage (I-V), capacitance-voltage (C-V), scattering-parameter (S-parameter) and transient time measurements. At a bias voltage of 3 V the dark current is measured to be 135 pA for a submicron S.I. GaAs detector, which corresponds to a dark current density of 31 μA/cm2. For micrometer feature size SOI detectors with SiN-passivation the dark current is reduced significantly, being in the pA range up to 30 V. The capacitance of the interdigitated electrode structure is shown to be very low: e.g. a GaAs-based detector with submicron finger dimensions and comprising an area of 10 x 35 μm2 obtains a capacitance of 32 fF. The instrumentation limited rise time and full width at half maximum (FW H M) for a submicron MSM on S.I. GaAs (finger spacing 0.3 μm, finger width 0.2 μm) are 18 and 27 ps, respectively. The transient characteristics measured for SOI detectors indicate that with decreasing thickness of the photoactive layer the bandwidth increases. The instrumentation limited rise time and FW H M for a SOI PD (different measurement setup than for GaAs detectors) with 0.5 μm top Si layer (finger spacing 3 μm, finger width 3 μm) are 64 and 100 ps, respectively. In contrast a SOI PD with 1 μm top Si layer exhibits a rise time and FW H M of 77 and 142 ps, respectively. The responsivity for the GaAs detectors is reasonable and according to theoretical values around 0.25 A/W at a wavelength of 780 nm. The SOI MSM PDs, however, perform responsivities in the mA/W- range, attributed to the relatively thin photoactive top Si layer: e.g. a device with top Si thickness of 4 μm demonstrates a responsivity of 93 mA/W (quantum efficiency 14.5 %), measured at a bias voltage of 6 V and a wavelength of 800 nm. It can be concluded, that a thicker layer leads to enhanced photoresponse.

Second, GaAs based pHEMTs with various gate lengths (ranging from 1 μm down to 0.2 μm) are fabricated. The e-beam lithography technique is developed for both triangular and mushroom shaped gates. DC and RF measurements are carried out, as well as simulations based on models available in the microwave design system (MDS). The extrinsic cut-off frequency ft and maximum frequency of oscillation fmax for a standard transistor (lg = 1.1 μm, W = 200 μm) exhibit 29 GHz and 52 GHz, respectively, whereas for a submicron triangular shaped transistor (lg = 0.35 μm, W = 160 μm) they are 61 GHz and 80 GHz, respectively. For a mushroom shaped pHEMT (top 0.6 μm, footprint 0.2 μm, W = 200 μm) ft and fmax are 73 GHz and 116 GHz, respectively. The transconductance is observed to increase with decreasing gate length. A transistor with a gate length of 0.34 μm and a width of 40 μm obtains a peak extrinsic transconductance of 539 mS/mm at Vds = 2 V and Vgs = -0.25 V (Idsat = 217 mA/mm).

Finally, a GaAs-based photoreceiver is investigated, consisting of an MSM PD and a transimpedance amplifier. Two different kind of circuits are realized, either applying only optical (standard) lithography, or a combination of optical and e-beam lithography. For the standard receivers the transistors have a gate length of 1 μm and the MSMs finger spacings and widths of 1 μm, whereas the e-beam circuits have submicron transistors for the amplification stage, and a submicron MSM. The electrical, as well as opto-electronic response, is measured for the different receivers. The submicron receiver shows the largest electrical 3 dB bandwidth, as expected, 9.5 GHz (ZT = 1.2 kΩ) compared to 8.4 GHz (ZT = 1.3 kΩ) that of a standard receiver. According to simulations the 3 dB bandwidth of a receiver solely consisting of submicron transistor yields 16.9 GHz, indicating that reduced gate length is a major factor in achieving higher bandwidths.

Keywords: MSM photodetector, SOI detector, GaAs-based pHEMT, photoreceiver, transimpedance amplifier, e-beam lithography

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© 2001 Helsinki University of Technology

Last update 2011-05-26