Page 23 - Photodetection and Measurement - Maximizing Performance in Optical Systems

P. 23

Photodetection Basics

16 Chapter One

the junction bandgap energy (Sze 1969). Different processing parameters
during manufacture can have a large effect on C p. In Chaps. 2 and 3 we will see
how important is C p to the speed and noise performance of an ampliﬁed
photodetector.

1.7.6 Leakage resistance
All photodiodes act as though they have a resistance shunting the diode (R sh in
Fig. 1.7). Its value depends on the processes used to fabricate the diode sub-
strate and junction and decreases with increasing area. A high quality 1-mm 2
silicon device (e.g., PIN040 from UDT Sensors Inc.) speciﬁes a leakage resist-
ance of typically 1GW and not less than 200MW at room temperature. Tem-
perature increase reduces R sh . Other manufacturers publish instead a leakage
current or dark current, which is just the current that will ﬂow through the
shunt resistance for a speciﬁed terminal voltage. If the diode voltage in the
actual circuit is zero, then no current will ﬂow and the effects of R sh will be
minimized. We will see later that R sh can have a signiﬁcant effect on the detec-
tion signal-to-noise ratio. For low values of R sh even a few millivolts offset from
an operational ampliﬁer can lead to signiﬁcant leakage current.
Leakage can be greatly modiﬁed by process parameters, although signiﬁcant
reduction is usually at the expense of other diode characteristics. For example,
2
the 7.6-mm detection area BPW34 photodiode manufactured by Siemens is
speciﬁed with a leakage of 2nA at 10-V reverse bias and capacitance of 72pF at
zero bias. The very similar BPW33 exhibits only 20-pA dark current, but the
parasitic capacitance increases to 750pF under the same conditions.

1.7.7 Series resistance
Series resistance comes from the bulk resistance of the photodiode substrate,
from the ohmic contact diffusions, and from the leads. It is probably the least
troubling diode characteristic for normal laboratory detection needs but
becomes important in high bandwidth systems and in power generators using
solar cells.

1.7.8 Table of representative photodiodes
Table 1.2 shows a few representative photodiodes. The BPW34 is a common
silicon device in a clear plastic package and has been available for 30 years. The
BPW33 looks similar, with the same active area. However, as noted, the dark
leakage current has been reduced signiﬁcantly, at the expense of a tenfold
increase in junction capacitance. Items 3 and 4 are modern design silicon devices
and show the change in dark current and capacitance incurred by changing the
active area. These also offer very low temperature coefﬁcients of responsivity
at less than 100ppm/°C. Items 5 and 6 are InGaAs detectors. The ﬁrst has an
2
area of 1mm and a capacitance of 90pF. Item 6 is similar, except that a spheri-
2
cal lens has been used to accept about 1mm of beam area but focus it onto a

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