Reverse breakdown avalanche

Zener diode - A control device utilizing a p-n junction with a well defined reverse-bias avalanche breakdown voltage. 

Lp Pi 50 pm. and the reverse saturation current would be 17 x 10 = 17 pA for a square centimeter of junction area. Typical reverse saturation currents are about one thousand times greater as a result of generation—recombination currents in the depletion region (9). As the reverse voltage bias increases, the field increases in the depletion region until avalanche breakdown occurs, resulting in the characteristic shown in Figure 7. 

The formation of pores during anodization of an initially flat silicon electrode in HF affects the I-V characteristics. While this effect is small for p-type and highly doped n-type samples, it becomes dramatic for moderate and low doped n-type substrates anodized in the dark. In the latter case a reproducible I-V curve in the common sense does not exist. If, for example, a constant potential is applied to the electrode the current density usually increases monotonically with anodization time (Thl, Th2]. Therefore the I-V characteristic, as shown in Fig. 8.9, is sensitive to scan speed. The reverse is true for application of a certain current density. In this case the potential jumps to values close to the breakdown bias for the flat electrode and decreases to much lower values for prolonged anodization. These transient effects are caused by formation of pores in the initially flat surface. The lowering of the breakdown bias at the pore tips leads to local breakdown either by tunneling or by avalanche multiplication. The prior case will be discussed in this section while the next section focuses on the latter. 

In contrast to p-type electrodes, an n-type electrode is under reverse conditions in the anodic regime. This has several consequences for pore formation. Significant currents in a reverse biased Schottky diode are expected under breakdown conditions or if injected or photogenerated minority carriers can be collected. Breakdown at the pore tip due to tunneling generates mainly mesopores, while avalanche breakdown forms larger etch pits. Both cases are discussed in Chapter 8. Macropore formation by collection of minority carriers is understood in detail and a quantitative description is possible [Le9], which is in contrast to the pore formation mechanisms discussed so far. 

Avalanche Photodiode (APD)—A photodiode designed to take advantage of avalanche multiplication of photocurrent. As the reverse-bias voltage approaches the breakdown voltage, hole-electron pairs created by absorbed photons acquire sufficient energy to create additional hole electron pairs when they collide with ions thus a multiplication or signal gain is achieved. 

Breakdown of a semiconductor electrode occurs when the limiting current at reverse bias sharply increases with increasing potential. At breakdown the electrode loses its insulating character and becomes conductive. Two types of breakdown may occur in a semiconductor at high field Zener breakdown and avalanche breakdovra." " ... 

The field required for breakdown to occur and the mode of breakdown depend on doping level. As the dopant concentration increases, the width of the space charge layer decreases and the probability of tunneling increases rapidly so that Z mr breakdown becomes more likely than avalanche breakdown. Zener breakdown is, in general, involved in the electrode processes on p and n materials under a reverse bias. 

TTS exists also in single photon avalanche photodiodes (SPADs). The source of TTS in SPADs is the different depth at which the photons are absorbed, and the nonuniformity of the avalanche multiplication efficiency. This results in differing delays in the build-up of the carrier avalanche and in different avalanche transit times. Consequently the TTS depends on the wavelength and the voltage. Moreover, if a passive quenching circuit is used, the reverse voltage may not have completely recovered from the breakdown of the previous photon. The result is an increase of the TTS width or a shift of the TTS with the count rate. 

The minimum pulse width delivered by a PIN or avalanehe photodiode is given by the produet of the junetion eapacitance, Cj, and the load resistance of 50 Ohm. A small Cj is aehieved only if the 1 region of a PIN diode or the avalanche region of an APD is fully depleted. This requires PIN diodes to be operated close to their maximum permissible reverse voltage. APDs should be used at 30% or more of their breakdown voltage. 

---