Type of photodetector device structure

Type of photodetector device structure
Photodetector is a device that converts optical signal into electrical signal, ‌ its structure and variety, ‌ can be mainly divided into the following categories: ‌
(1) Photoconductive photodetector
When photoconductive devices are exposed to light, the photogenerated carrier increases their conductivity and decreases their resistance. The carriers excited at room temperature move in a directional manner under the action of an electric field, thus generating a current. Under the condition of light, electrons are excited and transition occurs. At the same time, they drift under the action of an electric field to form a photocurrent. The resulting photogenerated carriers increase the conductivity of the device and thus reduce the resistance. Photoconductive photodetectors usually show high gain and great responsiveness in performance, but they cannot respond to high-frequency optical signals, so the response speed is slow, which limits the application of photoconductive devices in some aspects.

(2) PN photodetector
PN photodetector is formed by the contact between P-type semiconductor material and N-type semiconductor material. Before the contact is formed, the two materials are in a separate state. The Fermi level in P-type semiconductor is close to the edge of the valence band, while the Fermi level in N-type semiconductor is close to the edge of the conduction band. At the same time, the Fermi level of the N-type material at the edge of the conduction band is continuously shifted downward until the Fermi level of the two materials is in the same position. The change of the position of conduction band and valence band is also accompanied by the bending of the band. The PN junction is in equilibrium and has a uniform Fermi level. From the aspect of charge carrier analysis, most of the charge carriers in P-type materials are holes, while most of the charge carriers in N-type materials are electrons. When the two materials are in contact, due to the difference in carrier concentration, the electrons in N-type materials will diffuse to P-type, while the electrons in N-type materials will diffuse in the opposite direction to the holes. The uncompensated area left by the diffusion of electrons and holes will form a built-in electric field, and the built-in electric field will trend carrier drift, and the direction of drift is just opposite to the direction of diffusion, which means that the formation of the built-in electric field prevents the diffusion of carriers, and there are both diffusion and drift inside the PN junction until the two kinds of motion are balanced, so that the static carrier flow is zero. Internal dynamic balance.
When the PN junction is exposed to light radiation, the energy of the photon is transferred to the carrier, and the photogenerated carrier, that is, the photogenerated electron-hole pair, is generated. Under the action of the electric field, the electron and hole drift to the N region and the P region respectively, and the directional drift of the photogenerated carrier generates photocurrent. This is the basic principle of PN junction photodetector.

(3) PIN photodetector
Pin photodiode is a P-type material and N-type material between the I layer, the I layer of the material is generally an intrinsic or low-doping material. Its working mechanism is similar to the PN junction, when the PIN junction is exposed to light radiation, the photon transfers energy to the electron, generating photogenerated charge carriers, and the internal electric field or the external electric field will separate the photogenerated electron-hole pairs in the depletion layer, and the drifted charge carriers will form a current in the external circuit. The role played by layer I is to expand the width of the depletion layer, and the layer I will completely become the depletion layer under a large bias voltage, and the generated electron-hole pairs will be rapidly separated, so the response speed of the PIN junction photodetector is generally faster than that of the PN junction detector. Carriers outside the I layer are also collected by the depletion layer through diffusion motion, forming a diffusion current. The thickness of the I layer is generally very thin, and its purpose is to improve the response speed of the detector.

(4) APD photodetector avalanche photodiode
The mechanism of avalanche photodiode is similar to that of PN junction. APD photodetector uses heavily doped PN junction, the operating voltage based on APD detection is large, and when a large reverse bias is added, collision ionization and avalanche multiplication will occur inside APD, and the performance of the detector is increased photocurrent. When APD is in the reverse bias mode, the electric field in the depletion layer will be very strong, and the photogenerated carriers generated by light will be quickly separated and quickly drift under the action of the electric field. There is a probability that electrons will bump into the lattice during this process, causing the electrons in the lattice to be ionized. This process is repeated, and the ionized ions in the lattice also collide with the lattice, causing the number of charge carriers in the APD to increase, resulting in a large current. It is this unique physical mechanism inside APD that APD-based detectors generally have the characteristics of fast response speed, large current value gain and high sensitivity. Compared with PN junction and PIN junction, APD has a faster response speed, which is the fastest response speed among the current photosensitive tubes.

(5) Schottky junction photodetector
The basic structure of the Schottky junction photodetector is a Schottky diode, whose electrical characteristics are similar to those of the PN junction described above, and it has unidirectional conductivity with positive conduction and reverse cut-off. When a metal with a high work function and a semiconductor with a low work function form contact, a Schottky barrier is formed, and the resulting junction is a Schottky junction. The main mechanism is somewhat similar to the PN junction, taking N-type semiconductors as an example, when two materials form contact, due to the different electron concentrations of the two materials, the electrons in the semiconductor will diffuse to the metal side. The diffused electrons accumulate continuously at one end of the metal, thus destroying the original electrical neutrality of the metal, forming a built-in electric field from the semiconductor to the metal on the contact surface, and the electrons will drift under the action of the internal electric field, and the carrier’s diffusion and drift motion will be carried out simultaneously, after a period of time to reach dynamic equilibrium, and finally form a Schottky junction. Under light conditions, the barrier region directly absorbs light and generates electron-hole pairs, while the photogenerated carriers inside the PN junction need to pass through the diffusion region to reach the junction region. Compared with PN junction, the photodetector based on Schottky junction has a faster response speed, and the response speed can even reach ns level.