Avalanche Multiplication Process in Silicon
We will provide you with a general background on multiplication processes in Silicon, and describe some of the characteristics of silicon which make it such a useful material for the fabrication of PIN and Avalanche Photodiodes for visible and near IR applications.
We will describe approaches to the design of APDs, and will provide other information that will be helpful to a potential user of APDs. Finally, a bibliography of a number of useful papers is provided, as a source of further information on subjects that may be treated only briefly in this paper. Ask for the free download to read the whole text.

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Silicon - for VIS to NIR Photodiodes
There is no intention to deal extensively with the subject of multiplication in silicon, since the mathematics involved is beyond the scope of this paper*.
However, in understanding the discussion of the following sections, it is useful to take a look at the multiplication process, and a number of characteristics of silicon that are so important for the fabrication of a multiplying device. In particular, it is important to know that โ in general โ electrons are much more ionizing than holes in silicon 4,5, so that in any useful APD design, electrons - rather than holes - should be swept by the electric field into the high field region where the multiplication takes place.
Thus, there should be a p-type absorbing region of suitable width to absorb the incident radiation and that, as much as possible, the radiation should be able to enter this region without severe loss in any layer which is either n-type (e.g. through the junction in an n-p design), or in which the carrier lifetime is extremely short (e.g. a thick, heavily-doped p+ region). In either case the region would be essentially โdeadโ, and would absorb carriers that would contribute little or nothing to the multiplied signal of the APD.
*For more information on this subject, the reader is referred to the first two references, and in particular to reference 1) entitled โProperties of Avalanche Photodiodesโ, by P. P. Webb, R. J. McIntyre and J. Conradi; RCA Review, Vol. 35; June 1974.


Quantum Efficiency of Avalanche Photodiodes
As noted in the previous paragraph, in designing or selecting an Avalanche Photodiode for a specific application, it is important to know certain characteristics of the silicon. In particular, knowledge of the absorption properties of silicon as a function of the wavelength of the incident radiation is necessary in order to determine the thickness of the active layer (i.e. the absorbing region) needed to obtain acceptable quantum efficiency.
Figure 1 shows the absorption coefficient of silicon as a function of wavelength in the range 400 to 1100 nm.
From figure 1 we note that the absorption coefficient is very high for shorter wavelengths, meaning that good quantum efficiency can be achieved with relatively narrow active layers. On the other hand, the opposite is true for longer wavelengths.
In particular, at 1064 nm, good quantum efficiency can only be achieved with active layers that are at least several hundred micrometers. For a device in which the full thickness of the detector chip is active and is fully depleted by the applied bias voltage, except for narrow โdeadโ regions on either surface, the quantum
efficiency, h, can be determined from the following formula:
Where r1 is the reflectivity at the front surface, r2 is the reflectivity at the back surface, d is the effective thickness of the front โdeadโ layer, w is the thickness of the active layer, and a is the absorption coefficient of the radiation at the wavelength of interest.
From this, it is seen that for short wavelengths, (less than about 800 nm), where a is large, the 2nd and 3rd terms of the numerator, and the denominator all become unity, so that the expression reduces to:
However, at longer wavelengths such as, for example, 1064 nm, the reflection at the back surface becomes important in achieving high quantum efficiency. With a low value of r1 and a highly reflecting back surface, and the value of โdโ small (less than about 1 ยตm at 1064 nm), equation [1] becomes, approximately:
Silicon Reach-Through APD
Chapter 1 - Design
The basic design of a reach-through avalanche photodiode consists of a narrow high-field region where the multiplication takes place, with a much wider low field region in which the incoming radiation is absorbed. The design, impurity concentration, and electric field profiles are shown in figure 2.
For satisfactory operation of the APD design of figure 2(a), the high resistivity p-type substrate must be fully depleted by the applied bias voltage. Generally, this works well provided the substrate wafer is not too thick, and required response times are not less than about 10 ns or so. However, fabrication of APDs on 4-inch wafers, as is now usually the case, will often mean that the substrate is thick, operating voltages are high, and response times slow. These problems can be avoided with the use of an epitaxial version of the design of figure 2.
In this approach, a high-resistivity p-type layer is grown epitaxially on top of a low resistivity p-type substrate. The substrate may be any thickness, and its resistivity is chosen to be low enough that it does not introduce a significant series resistance.
The thickness of the p-type epitaxial layer is typically chosen to be in the range 30 to 50 um, but may be narrower or wider, depending on the requirement of the application. A reach-through structure is achieved by introducing n- and p-layers, as shown in figure 2(a). When bias voltage is applied, the depletion layer stops at the interface between the substrate and the epitaxial layer. Where fast response is a requirement, the narrow active region of this version of the reach-through APD is normally the best option.
The main advantage of the reach-through design is that an APD with moderately wide absorbing region - e.g. up to several hundred micrometers โ can be made which operates perfectly satisfactorily at a few hundred volts bias voltage, whereas the same device without the narrow high field region would require a much higher voltage which, for an APD with a thick active region, would be well in excess of 1000 volts.
One of the main disadvantages of the design is that (often) the APD cannot be operated at low gains, since the device will not operate at voltages below the reach-through voltage, and the gain at that point is usually greater than 1, and often as high as 10 or more.
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Chapter 1: Silicon Reach-Through APD
- Design
- Dead Layers
- Dark Current and Noise
- Excess Noise Factor
- Temperature Effects
- Capacitance
Chapter 2: Signal-to-noise Ratio
References
Product Overview
Silicon Avalanche Photodiodes

