C3683-01 View Datasheet(PDF) - Unspecified
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Characteristic and use
1. Basic principle
A PSD basically consists of a uniform resistive layer
formed on one or both surfaces of a high-resistivity semi-
conductor substrate, and a pair of electrodes formed on
both ends of the resistive layer for extracting position
signals. The active area, which is also a resistive layer,
has a PN junction that generates photocurrent by means
of the photovoltaic effect.
Figure 1-1 PSD sectional view
OUTPUT IX1
XB
XA
OUTPUT IX2
ELECTRODE X1
PHOTOCURRENT
INCIDENT
LIGHT
ELECTRODE X2
P LAYER
By finding the difference or ratio of Ix1 to Ix2, the light input
position can be obtained by the formulas (1-3), (1-4), (1-7)
and (1-8) irrespective of the incident light intensity level
and its changes. The light input position obtained here cor-
responds to the center-of-gravity of the light beam.
2. One-dimensional PSD
Figure 2-1 Structure chart, equivalent circuit (one-dimensional PSD)
ANODE (X1)
Rp
P
ANODE (X2)
CATHODE
(COMMON)
D Cj Rsh
COMMON
ELECTRODE
I LAYER
N LAYER
P : CURRENT GENERATOR
D : IDEAL DIODE
Cj : JUNCTION CAPACITANCE
Rsh: SHUNT RESISTANCE
Rp : POSITIONING RESISTANCE
KPSDC0006EA
RESISTANCE LENGTH LX
KPSDC0005EA
Figure 1-1 shows a sectional view of a PSD using a simple
illustration to explain the operating principle. The PSD has
a P-type resistive layer formed on an N-type high-resistive
silicon substrate. This P-layer serves as an active area for
photoelectric conversion and a pair of output electrodes
are formed on the both ends of the P-layer. On the
backside of the silicon substrate is an N-layer to which a
common electrode is connected. Basically, this is the
same structure as that of PIN photodiodes except for the
P-type resistive layer on the surface.
When a spot light strikes the PSD, an electric charge
proportional to the light intensity is generated at the
incident position. This electric charge is driven through the
resistive layer and collected by the output electrodes X1
and X2 as photocurrents, while being divided in inverse
proportion to the distance between the incident position
and each electrode.
The relation between the incident light position and the
photocurrents from the output electrodes X1, X2 is given by
the following formulas.
l When the center point of PSD is set at the origin:
LX - XA
LX + XA
IX1 = 2
× Io ......... (1-1) IX2 = 2
× Io ...... (1-2)
LX
LX
Figure 2-2 Active area chart (one-dimensional PSD)
LX
X1
X2
x
ACTIVE AREA
KPSDC0010EA
l Position conversion formula (See Figure 2-2.)
IX2 - IX1 = 2x ........ (2-1)
IX1 + IX2 LX
In the above formula, IX1 and IX2 are the output currents
obtained from the electrodes shown in Figure 2-2.
3. Two-dimensional PSD
Two-dimensional PSDs are grouped by structure into duo-
lateral and tetra-lateral types. Among the tetra-lateral type
PSDs, a pin-cushion type with an improved active area
and electrodes is also provided. (See “3-3”.) The position
conversion formulas slightly differ according to the PSD
structure. Two-dimensional PSDs have two pairs of output
electrodes, X1, X2 and Y1, Y2.
IX2 - IX1 = 2XA ............ (1-3) IX1 = LX - 2XA .............. (1-4)
IX1 + IX2 LX
IX2 LX + 2XA
3-1 Duo-lateral type PSD
On the duo-lateral type, the N-layer shown in the sectional
l When the end of PSD is set at the origin:
IX1 = LX - XB . Io ............. (1-5) IX2 = XB . Io ................. (1-6)
LX
LX
view of Figure 1-1 is processed to form a resistive layer,
and two pair of electrodes are formed on both surfaces as
X and Y electrodes arranged at right angles. (See Figure
3-1.) The X position signals are extracted from the X elec-
IX2 - IX1 = 2XB - LX ...... (1-7) IX1 = LX - XB ................ (1-8)
IX1 + IX2
LX
IX2
XB
trodes on the upper surface, while the Y position signals
are extracted from the Y electrodes on the bottom surface.
As shown in Figure 3-1, a photocurrent with a polarity op-
Io : Total photocurrent (IX1 + IX2)
posite that of the other surface is on each surface, to pro-
IX1: Output current from electrode X1
IX2: Output current from electrode X2
LX: Resistance length (length of the active area)
duce signal currents twice as large as the tetra-lateral type
and achieve a higher position resolution. In addition, when
compared to the tetra-lateral type, the duo-lateral type of-
fers excellent position detection characteristics because
XA: Distance from the electrical center of PSD to the light input position
the electrodes are not in close proximity. The light input
XB: Distance from the electrode X1 to the light input position
position can be calculated from conversion formulas (3-1)
5
and (3-2).
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