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Semiconductor Avalanche Photodiode: Working Method And Advantages

Semiconductor avalanche photodiode is a semiconductor optoelectronic device having an internal photocurrent gain, also referred to as a solid photomultiplier tube. It uses the impact ionization effect of photogenerated carriers in the diode depletion layer to obtain an avalanche multiplication of the photocurrent. This device has the advantages of small size, sensitivity, and speed. It is suitable for detecting and receiving weak optical signals, and is widely used in optical fiber communication, laser ranging, and other photoelectric conversion data processing systems.  


Working Principle

An avalanche photodiode is a p-n junction type photodetector diode in which an avalanche multiplication effect of carriers is used to amplify photo signals to increase detection sensitivity. Its basic structure often uses Read diode structure (ie, N+PIP+ type structure, P+ one-side receiving light) which is easy to produce avalanche multiplication effect, and when it is working, a large reverse bias is applied, so that it reaches an avalanche multiplication state; its light absorption The area is basically the same as the multiplication area (P area and I area where there is a high electric field).

The PN junction is coupled with a suitable high reverse bias voltage, so that photocarriers in the depletion layer are accelerated by a strong electric field to obtain a sufficiently high kinetic energy, and they are ionized with a lattice to generate a new electron-hole pair. It continues to cause new collisional ionization, causing the avalanche of carriers to multiply and gain current gain. In the 0.6-0.9 μm band, silicon APDs have near-ideal performance. InGaAs (indium gallium arsenide) / InP (indium phosphorus) APD is an ideal photodetector for long-wavelength (1.3μn, 1.55μm) band fiber optic communications. The optimized structure is shown in the figure. The light absorption layer is made of InGaAs material, which has a high absorption coefficient for light of 1.3 μm and 1.55 μn. To avoid the breakdown of InGaAs tunnels prior to avalanche breakdown, the avalanche region is Absorption is differentiated, ie PN junctions are made in the InP window layer. In view of the fact that the hole ionization coefficient in InP material is larger than the electron ionization coefficient, n-type InP is used in the avalanche region, and the valence band barrier exists between the n-InP and n-InGaAs heterointerfaces, which easily causes the sinking of the light-generating holes. Into the bandgap graded InGaAsP (indium gallium arsenite) transition zone, the formation of SAGM (absorption, classification and doubling) structure.

In the manufacture of APD, it is necessary to add a protection ring on the surface of the device to improve the reverse withstand voltage performance; the semiconductor material is superior to Si (widely used to detect light below 0.9um), but when detecting long wavelength light above 1um Commonly used Ge and InGaAs (large noise and dark current). The disadvantage of this APD is that there is a tunnel current multiplication process, which will produce a large shot noise (reducing the doping of the p-region can reduce the tunneling current, but the avalanche voltage will increase). An improved structure is the so-called SAM-APD: a material with a wider bandgap width (so that it does not absorb light) in the doubling area and a material with a narrow band gap in the light absorption area; here, due to the use of a heterojunction, The doping concentration of the doubling region is reduced without affecting the light absorption region, so that the tunneling current is reduced (if it is a mutated heterojunction, because the presence of ΔEv will cause the accumulation of photogenerated holes and affect the device Response speed, then a gradual layer can be inserted in the middle of the mutant heterojunction to reduce the effect of ΔEv).


Avalanche diode invention 

In 1965, K.M. Johnson and L.K. Anderson et al. reported a uniform breakdown semiconductor avalanche photodiode that still had a relatively high photocurrent gain at microwave frequencies. Since then, avalanche photodiodes as a new type of high-speed, sensitive solid-state photodetector devices gradually gain attention.

The average photocurrent gain of a good avalanche photodiode can reach tens, hundreds, or even more. The collisional ionization ability of two carriers in the semiconductor may be different, so that the injection of carriers with higher ionization capacity into the depletion region is favorable to obtain a higher avalanche multiplication under the same electric field conditions. However, this avalanche multiplication of photocurrents is not absolutely ideal. On the one hand, because the germanium decreases with the increase of injected light intensity, the linear range of the avalanche photodiode is limited, and more importantly, the impact ionization of carriers is a random process, ie, The avalanche gain of each individual carrier in the depletion layer can have a very wide probability distribution, so the doubling photocurrent I has a greater random fluctuation than the photocurrent I0 before the doubling, ie, the photocurrent. There is an additional increase in noise. Compared with vacuum photomultipliers, this kind of fluctuation is more serious because of the ionization ability of both carriers in the semiconductor.


Since F is greater than 1 and increases with the increase, only when the noise of a receiving system (including detector elements, ie avalanche photodiodes, load resistance, and preamplifier) is mainly determined by the load resistance and thermal noise of the amplifier, Increasing the avalanche gain can effectively improve the signal-to-noise ratio of the system, thereby improving the detection performance of the system. Conversely, when the noise of the system is mainly determined by the noise of the photocurrent, increasing the value of the system can no longer improve the performance of the system. What plays a major role here is the size of the excess noise factor F. In order to obtain a smaller F-value, two materials with large carrier ionization capacities should be used, so that carriers with higher ionization capacity can be injected into the depletion layer, and the device structure can be designed rationally.


Factors Affecting Speed of Response Editing

The greater the avalanche gain of carriers in the depletion layer, the longer the avalanche doubling process takes. Therefore, the avalanche multiplication process is limited by the "gain-bandwidth product". In the case of high avalanche gain, this limitation may be one of the main factors affecting the response speed of the avalanche photodiode. However, at a moderate gain, compared with other factors that affect the response speed of the photodiode, this restriction often does not play a major role, so the avalanche photodiode can still obtain a high response speed. The gain-bandwidth product of modern avalanche photodiodes has reached several hundred GHz.

As with typical semiconductor photodiodes, the spectral sensitivity range of avalanche photodiodes depends largely on the bandgap of the semiconductor material. Materials for preparing avalanche photodiodes include III-V compounds such as silicon, germanium, gallium arsenide, and indium phosphide, and their ternary and quaternary solid solutions. According to the method of forming a depletion layer, an avalanche photodiode has a PN junction type (homogeneous or heterostructured PN junction. Among them, there are general PN junctions, PIN junctions, and special structures such as N+PπP+ junctions), Schottky barrier metal-based and metal-oxide-semiconductor structures.

Advantages

Compared with the vacuum photomultiplier tube, the avalanche photodiode has the advantages of small size and does not require high voltage power supply, so it is more suitable for practical applications; compared with the general semiconductor photodiode, the avalanche photodiode has the advantages of high sensitivity, high speed, etc. When the system bandwidth is relatively large, the detection performance of the system can be greatly improved.



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