Photodiodes

Our photodiodes are offered in ceramic and metal types and feature:

• high gain at a low bias voltage
• fast rise time with low capacitance
• high sensitivity

• laser scanners (LIDAR)
• laser range finders
• laser alignment
• analytical instruments
• precision photometry
• medical equipment
• high speed optical communications

Photodiodes utilize the photoelectric effect. They have a p-n junction, in which free electron-hole pairs are generated by incident photons. The photon flux per unit solid angle is referred to as light intensity. By implanting dopands to the Silicon a build-in electric field is created, which seperates and guides the generated carriers to the metal contacts. When reverse biased, a photocurrent in depence of the light intensity can be measured.

Choosing a photodiode depends on following requirements determined by the individual application: the aperture size of the active area, geometry of aperture (circular vs square, rectangle), incident light wavelength, required response time and bandwidth (pulsed incident light source), necessity for amplification (optical signal power level), other opto-electrical parameters (e.g. dark current, capacity, sensitvity).

Applying a negative voltage to a node and positive voltage to cathode is referred to as "reverse bias". The free electrons in the N layer are thus pulled to the positive terminal, and the holes in the P layer are pulled toward the negative terminal.  The reverse bias increases the size of the depletion region and has the result that only current flow caused by incident light is enabled. Due to the higher field strength in the depletion region, the response time is faster.

A photodiode array is a one or two dimensional composition of individual photodiodes (discrete array) or several active areas on a single chip (monolithic array).

Avalanche photodiodes (also known as an APD) have an internal gain mechanism based on the avalanche effect. This gain mechanism makes them suitable for conversion of weak optical signals to measureable electrical current. They can be designed for two main operation regimes: linear and nonlinear dependence on reverse voltage. The nonlinear regime is denoted as Geiger mode operation.

In standard diodes, impinging photons generate electron-hole pairs. These hole pairs provide a measurable photocurrent. In APDs, applied reverse bias voltage triggers an avalanche – it ensures that the electron-hole pairs are accelerated. The resulting impact ionization introduces further electrons to the conduction band. These electrons, in turn, absorb more energy and raise further electrons to the conduction band. This process is called avalanche breakdown and can thus achieve an avalanche multiplication factor of several hundred for the detector.

APDs are also employed for applications with high modulation frequencies. Frequencies of approx. 60 MHz, the noise level increased by the avalanche effect is generally lower than the noise level produced by a combination of a conventional photodiode with external gain electronics. You will typically find APDs in laser scanners and LIDAR systems, analytical instruments, and distance and speed measurement.

A photodiode is a p-n junction in the silicon semiconductor serves as the physical basis for this process. When photons with sufficient energy are absorbed by the detector, this results in the formation of charge carriers (electron-hole pairs), which are separated in the space-charge region and thus generate the photocurrent.

A PIN diode comprises a near-intrinsic semiconductor region – usually the space-charge region – sandwiched between a p-type diode and an n-type substrate. However, the term is also used for components with inverse conductivity, provided that no other non-linear effects are utilized in the component.

A photodetector or hybrid is a combination of a photodiode and a transimpedance amplifier (TIA). Incident light is converted to electrical current by the photodiode. The photocurrent then is converted to a voltage by the TIA. The TIA can also be used to amplify the electrical signal.

While the photodiode has an electrical current as output, the output of a photodetector is a voltage.

A PIN photodiode is a regular photodiode with a p-n junction for a broad range of applications. An APD is a photodiode with an internal gain mechanism used for low light signals, e.g. in LIDAR time of flight measurement.