Hamamatsu - PSD Principle and Features
Hamamatsu Corporation
360 Foothill Road
P.O. Box 6910
Bridgewater, NJ 08807-0910
TEL: (908) 231-0960
FAX: (908) 231-1539
Operating Principle and Features of PSD
Structures
As Shown in Fig. A, a sectional structure view, the PSD consists of
three layers: P-layer at one surface, N-layer at the other surface,
and I-layer between them, over a planar silicon substrate. Incident
light falling on the PSD is converted photoelectrically and detected
by the two electrodes on P-layer (P- and N-layers for duo-lateral
types) as photocurrent.
Principle
When a light spot falls on the PSD, an electric charge proportional to
the light energy is generated at the incident position. This electric
charge is driven through the resistive P-layer and collected by the
electrodes. Since the resistivity of the P-layer is uniform, the
photocurrent collected by an electrode is inversely proportional to
the distance between the incident position and the electrode. It is
possible to obtain the following formulas for the photocurrents I1 and
I2 collected by the electrodes, where L and I0 respectively stand for
the electrode interdistance and the total photocurrent.
- When the center point of the PSD is set at the original point:
- When the end of the PSD is set at the original point:
By finding the difference or ration of I1 and I2 thereby, the
incident position of light can be found by the formulas (3), (4), (7)
and (8) irrespective of the energy of incident light.
One-Dimensional PSD
The PSD is classified into two types: for one-dimensional position
detection and two-dimensional position detection. Fig. B shows the
structure of the one-dimensional PSD.
Fig. C is an equivalent circuit of the one dimensional PSD. Since
the PSD has the distributive circuits Cj and Rp, its time constant
acts as a decisive element for waveform response. By using a position
signal integration circuit, however, the one-dimensional PSD can be
used for position detection of a laser beam of 100ps pulse width.
Two-Dimensional PSD
The two dimensional PSD is classified by structure into two types:
duo-lateral type and tetra-lateral type.
- Duo-Lateral Type
As shown in Fig. D, the duo-lateral type has electrodes on both
surfaces (front surfaces and rear surfaces) of a photodiode. As shown
in the equivalent circuit in Fig. E, each position signal
(photocurrent) is only divided into two parts by two resistive layers,
and therefore, this type has a high position detecting ability
(smaller position detection error and higher resolution).
- Tetra-Lateral Type
As show in Fig. F, this type has 4 electrodes on the front surface of
a photodiode. Photocurrent is divided into 4 parts by the same
resistive layer and output as a position signal. (See Fig. G) As
compared with the duo-lateral type, this type has a distortion that is
greater in the circumference. However, it features a bias that can be
applied easier, a dark current that is smaller, and a response time
that is faster.
- Pin Cushion Type (Improved Tetra-Lateral Type)
This is the improved tetra-lateral type, with the improved sensitive
surface and electrodes. In addition to small dark current, fast
response and easy bias application, which are advantages of the
tetra-lateral type, distortion in the circumference has been greatly
reduced.
Conversion Formulas
The position of the light spot on the PSD can be found by the
following formulas, whereas X1, X2, and Y1, Y2 represent the output
signals (photocurrent) of each electrode, and x, y are
the coordinate positions of the light spot.
- One-Dimensional PSD
- Two-Dimensional PSD (Square Type)
- Two-Dimensional PSD (Pin Cushion Type)
Operating Circuit Example for 1D PSD
PSD Specifications
Joe Wagner