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Bandwidth Enhancement in Metal-Semiconductor-Metal Photodetectors by Internal Electric Field Modification

IP.com Disclosure Number: IPCOM000120864D
Original Publication Date: 1991-Jun-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 5 page(s) / 174K

Publishing Venue

IBM

Related People

Arjavalingam, G: AUTHOR [+3]

Abstract

The metal-semiconductor-metal photodetector (MSM-PD) offers the fundamental advantages of low device capacitance and process compatibility with MESFETs. This leads to optical receiver advantages in speed [1], noise immunity [2] and low cost (large area PDs are easier to align to optics). This device is being explored in both GaAs and InP material systems, for applications at different transmission wavelengths.

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Bandwidth Enhancement in Metal-Semiconductor-Metal Photodetectors
by Internal Electric Field Modification

      The metal-semiconductor-metal photodetector (MSM-PD)
offers the fundamental advantages of low device capacitance and
process compatibility with MESFETs.  This leads to optical receiver
advantages in speed [1], noise immunity [2] and low cost (large area
PDs are easier to align to optics).  This device is being explored in
both GaAs and InP material systems, for applications at different
transmission wavelengths.

      For gigabit-speed opto-electronic integrated circuit (OEIC)
receivers used in optical interconnects, PD responsivity and speed
must be achieved even under the constraint of low power supply
voltage compatible with high-speed logic.  As the semiconductor
absorption lengths at the wavelengths of interest are relatively
large (/1 mm) and the field distribution is non-uniform [3], a
significant proportion of photo-generated carriers have relatively
long path lengths and experience fields considerably smaller than the
surface field.  Thus, a modified field structure is needed in the
MSM-PD to realize high-speed operation at low bias voltages.

      The introduction of an internal field perpendicular to the
MSM-PD surface is shown which increases hole conduction towards the
surface and significantly improves the bandwidth.  This field is
produced by doping n-type a high bandgap epilayer just below the
photo-absorption layer, with the charge concentration low enough to
be depleted (Figs. 1a and 1b).  Data from two GaAs/AlGaAs wafers is
presented which have an identical layer structure except that only
one sample (the test sample) is intentionally doped as described
above.

      At low frequencies, the control and test MSM-PDs show identical
behavior.  The measured responsivity of 0.3A/W corresponds to 100%
internal efficiency.  At 10 V, dark currents were less than 1 nA for
2, 3 and 4 mm finger spacing (FS) PDs.  There was no observable
differences between the PD capacitances which were measured to be
200, 130 and 90 fF/(100 mm2) for 2, 3 and 4 mm FS PDs, respectively,
in both the control and test samples.  These are typical DC
characteristics of well-grown devices.

      Figs. 2 and 3 show the bandwidth versus bias voltage for 4 and
3 mm FS MSM-PDs with an average incident intensity of 1 mW.  The
measurement system bandwidth was 3 GHz.  Both figures show that the
control photodiodes have a higher bandwidth at low voltages.
However, the test PD bandwidths surpass the control PD bandwidths at
larger voltages.  The crossover occurs because extra bias is required
to overcome the internal field that is inhibiting electron conduction
towards the surface.  The crossover voltage for the 4 mm FS PD...