5

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.

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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.

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

proportion to the distance between the incident position

and each electrode.

The relation between the incident light position and the

the following formulas.

l When the center point of PSD is set at the origin:

l When the end of PSD is set at the origin:

obtained from the electrodes shown in Figure 2-2.

l Position conversion formula (See Figure 2-2.)

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

3-1 Duo-lateral type PSD

On the duo-lateral type, the N-layer shown in the sectional

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-

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-

posite that of the other surface is on each surface, to pro-

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

the electrodes are not in close proximity. The light input

position can be calculated from conversion formulas (3-1)

and (3-2).

Figure 2-1 Structure chart, equivalent circuit (one-dimensional PSD)

Figure 2-2 Active area chart (one-dimensional PSD)

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Figure 1-1 PSD sectional view

=

............ (1-3)

=

.............. (1-4)

=

...... (1-7)

=

........ (2-1)

=

................ (1-8)

. Io ............. (1-5)

. Io ................. (1-6)

Characteristic and use

2. One-dimensional PSD

3. Two-dimensional PSD

2

......... (1-1)

× Io

2

...... (1-2)

× Io

PHOTOCURRENT

INCIDENT

LIGHT

P LAYER

COMMON

ELECTRODE

I LAYER

N LAYER

ACTIVE AREA

x

P

D

Cj

Rsh

Rp

: CURRENT GENERATOR

: IDEAL DIODE

: JUNCTION CAPACITANCE

: SHUNT RESISTANCE

: POSITIONING RESISTANCE

PD

Cj Rsh

Rp

CATHODE

(COMMON)