P. 02

Operating principle

P. 02

Characteristics

2-1

2-2

2-3

2-4

2-5

2-6

Dark current

Gain vs. reverse voltage characteristics

Noise characteristics

Spectral response

Response characteristics

Crosstalk

P. 06

How to use

3-1

3-2

Connection to peripheral circuits

APD modules

P. 08

Applications

4-1

4-2

4-3

4-4

Optical rangefinders

Obstacle detection

Remote sensing

LiDAR (light detection and ranging)

Technical note

Si APD

Contents

The APD (avalanche photodiode) is a high-speed, high-sensitivity photodiode that internally multiplies photocurrent

when reverse voltage is applied. The internal multiplication function referred to as avalanche multiplication features

high photosensitivity that enables measurement of low-level light signals. The APD’s ability to multiply signals reduces

the effect of noise and achieves higher S/N than the PIN photodiode. The APD also has excellent linearity.

Utilizing our unique technologies, we offer numerous types of Si APDs for various applications. We also offer custom-

designed devices to meet special needs. Hamamatsu Si APDs have the following features.

Type Features Applications

Short

wavelength

type

Low bias operation

Enhanced sensitivity in the UV to visible region

•

Low-light-level detection

•

Analytical instruments

Low terminal

capacitance

Near

infrared

type

Low bias operation Low bias voltage operation

•

FSO

•

Optical fiber communications

•

Analytical instruments

Low temperature

coefficient

Low temperature coefficient of the bias voltage, easy gain

adjustment

•

FSO

•

Optical fiber communications

800 nm band Type with enhanced sensitivity in the 800 nm band (λp=840 nm)

•

FSO

•

Optical fiber communications

•

Analytical instruments

900 nm band

Type with enhanced sensitivity in the 900 nm band (λp=860 nm)

•

FSO

•

Optical fiber communications

•

Analytical instruments

Type with enhanced sensitivity in the 900 nm band (λp=940 nm)

•

FSO

•

Analytical instruments

•

YAG laser detection

TE-cooled type High S/N

•

Low-light-level detection

Si APD (for general measurement)

Type Features Applications

700 nm band

Type with reduced dark current, expanded operating temperatures,

and enhanced sensitivity in the 700 nm band

•

LiDAR

•

Optical rangefinders

800 nm band

Type with reduced dark current, expanded operating temperatures,

and enhanced sensitivity in the 800 nm band

900 nm band

Type with reduced dark current, expanded operating temperatures,

and enhanced sensitivity in the 900 nm band

Si APD (for LiDAR)

High sensitivity High-speed response High reliability Select delivery in individual specifications is possible

0201

Operating principle

The photocurrent generation mechanism of the APD is the

same as that of a normal photodiode. When light enters a

photodiode, electron-hole pairs are generated if the light

energy is higher than the band gap energy. The ratio of the

number of generated electron-hole pairs to the number of

incident photons is defined as the quantum efficiency (QE),

commonly expressed in percent (%). The mechanism by

which carriers are generated inside an APD is the same as in

a photodiode, but the APD is different from a photodiode in

that it has a function to multiply the generated carriers.

When electron-hole pairs are generated in the depletion

layer of an APD with a reverse voltage applied to the PN

junction, the electric field created across the PN junction

causes the electrons to drift toward the N

side and the

holes to drift toward the P

side. The higher the electric

field strength, the higher the drift speed of these carriers.

However, when the electric field reaches a certain level,

the carriers are more likely to collide with the crystal

lattice so that the drift speed becomes saturated at

a certain speed. If the electric field is increased even

further, carriers that escaped the collision with the

crystal lattice will have a great deal of energy. When these

carriers collide with the crystal lattice, a phenomenon

takes place in which new electron-hole pairs are

generated. This phenomenon is called ionization. These

electron-hole pairs then create additional electron-hole

pairs, which generate a chain reaction of ionization. This

is a phenomenon known as avalanche multiplication.

The number of electron-hole pairs generated during the

time that a carrier moves a unit distance is referred to as the

ionization rate. Usually, the ionization rate of electrons is

defined as “α” and that of holes as “β.” These ionization rates

are important factors in determining the multiplication

mechanism. In the case of silicon, the ionization rate of

electrons is larger than that of holes (α > β), so the ratio at

which electrons contribute to multiplication increases. As

such, the structure of Hamamatsu APDs is designed so that

electrons from electron-hole pairs generated by the incident

light can easily enter the avalanche layer. The depth at which

carriers are generated depends on the wavelength of the

incident light. Hamamatsu provides APDs with different

structures according to the wavelength to be detected.

[Figure 1-1]

Schematic diagram of avalanche multiplication (near infrared type)

Schematic diagram of avalanche multiplication (near infrared type Si APD)

Electric field strength E

High voltage

Avalanche

layer

KAPDC0006EC

Characteristics

Dark current

2 - 1

The APD dark current consists of surface leakage

current (Ids) that flows through the PN junction or

oxide film interface and generated current (Idg) inside

the substrate [Figure 2-1].

[Figure 2-1] APD dark current

APD dark current

Carriers that are not multiplied

Ids

Idg

PN junction

Avalanche region

Multiplied carriers

KAPDC0011EA

- -

The surface leakage current is not multiplied because

it does not pass through the avalanche layer, but the

generated current is because it does pass through.

Thus, the total dark current (I

D) is expressed by

equation (2-1).

ID = Ids + M Idg

M: gain

............

(2-1)

Idg, the dark current component that is multiplied,

greatly affects the noise characteristics.

Gain vs. reverse voltage characteristics

2 - 2

The APD gain is determined by the ionization rate,

and the ionization rate depends strongly on the

electric field across the depletion layer. In the normal

operating range, the APD gain increases as reverse

voltage increases. If the reverse voltage is increased

even higher, the reverse voltage across the APD PN

junction decreases due to the voltage drop caused by

the series resistance component including the APD

and circuit, and the gain begins to decrease.

When an appropriate reverse voltage is applied to the

PN junction, the electric field in the depletion layer

increases so avalanche multiplication occurs. As the

reverse voltage is increased, the gain increases and the

APD eventually reaches the breakdown voltage. Figure

2-2 shows the relation between the gain and reverse

voltage for Hamamatsu Si APD S12023-05.

KAPDC0011EA