Normally, a PN junction’s reverse-bias blocking action allows a small 'leakage' current to flow. However, if an adequately large reverse-bias is applied, the type of junction phenomena that developsand dominate the leakage current, allows much larger reverse-bias currents.
This 'breakdown' part of the diode characteristichere refers to 'breakdown'which implies that some other phenomenon are overshadowing of the semiconductor junction behavior and not to a destructive effect.
Although all diodes display reverse-bias breakdown phenomenon,Zener diodes, in particular, are specifically manufactured for operating in the breakdown condition with guaranteed specifications. During manufacturing, breakdown parameters of these Zener (or voltage reference) diodes get special processing attention.
Two discrete phenomena, that acts individually or concurrently depending on details of the diode, is involved in the breakdown. The first mechanism is associated with the acceleration of carriers across the strong junction electric field. If the kinetic energy that is gained by an accelerated carrier isadequately great, it can result in additional impurity atom ionization when collides with the atom. Then, each additional carrier is also accelerated and result is some more additional ionization; which grows exponentially. This is known as the ‘avalanche effect’, called so, as it resembles the initiation of a massive snow slide by a trivial snowball.
The second mechanism that is known as quantum mechanical effect is rather difficult to describe by any of the familiar analogies. Quantum mechanics foretells that there is a possibility of aunprompted crossing of a semiconductor junction by carriers that are subjected to a strong electric field. This is known as the ‘tunnel effect’; because it is not
related to ordinary mechanics, however, it was suggested frivolously that some metaphysical tunnel existed through which carriers traveled out of ordinary sight in some manner.
These breakdown characteristics for the two phenomena are not exactly identical but they are close enough, in general. Thus, while designing circuits, the distinction between these two otherwise different phenomenon’s may be largely ignored. The Zener effect was originally used for the quantum mechanical phenomena but the label Zener diode is applied universally,irrespective of the details of the breakdown mechanism.
To the left, an illustration of breakdown characteristic is drawn; and the scale is exaggerated for better clarity. The specified 'test' current IZT flows as specified by the manufacturer at nominal Zener reference voltage of the diode i.e. is the reverse-bias voltage, and it typically represents rated maximum diode current. In general,Zener voltage is a modest function of temperature.A more definite representative temperature specification is 0.1 % change every °C change.
For a diode with a reference voltage below 5 volts, coefficient is negative; otherwise it is positive. (This is associated with the dominance of one of the two phenomena that produces same terminal breakdown characteristics.)
The inverse of the slope of the diode characteristic (at the test point) is known as the 'dynamic resistance' of the diode. This parameter is noted in the manufacturers' specifications. The slope of the characteristic does not greatly vary for currents ranging (roughly) between 0.1 IZT and IZT, i.e. the usual range of operation of a Zener diode.
The inevitability of operation above the knee, i.e., in the breakdown region conditions the minimum usable current and so also the wish to generally avoid rapid change of slope in the closeproximity of the knee.
NOTE:-
"Either or both breakdown mechanisms may be present in a 'Zener' diode. The avalanche mechanism dominates at low doping levels and higher voltages, while at heavy doping levels and lower voltages, the Zener mechanism dominates.
Y At a certain doping level and around 6 V for Si, both mechanisms are present and there, the temperature coefficients just cancel. It is also possible to create Zener diodes with very small temperature coefficients Y.
This 'breakdown' part of the diode characteristichere refers to 'breakdown'which implies that some other phenomenon are overshadowing of the semiconductor junction behavior and not to a destructive effect.
Although all diodes display reverse-bias breakdown phenomenon,Zener diodes, in particular, are specifically manufactured for operating in the breakdown condition with guaranteed specifications. During manufacturing, breakdown parameters of these Zener (or voltage reference) diodes get special processing attention.
Two discrete phenomena, that acts individually or concurrently depending on details of the diode, is involved in the breakdown. The first mechanism is associated with the acceleration of carriers across the strong junction electric field. If the kinetic energy that is gained by an accelerated carrier isadequately great, it can result in additional impurity atom ionization when collides with the atom. Then, each additional carrier is also accelerated and result is some more additional ionization; which grows exponentially. This is known as the ‘avalanche effect’, called so, as it resembles the initiation of a massive snow slide by a trivial snowball.
The second mechanism that is known as quantum mechanical effect is rather difficult to describe by any of the familiar analogies. Quantum mechanics foretells that there is a possibility of aunprompted crossing of a semiconductor junction by carriers that are subjected to a strong electric field. This is known as the ‘tunnel effect’; because it is not
related to ordinary mechanics, however, it was suggested frivolously that some metaphysical tunnel existed through which carriers traveled out of ordinary sight in some manner.
These breakdown characteristics for the two phenomena are not exactly identical but they are close enough, in general. Thus, while designing circuits, the distinction between these two otherwise different phenomenon’s may be largely ignored. The Zener effect was originally used for the quantum mechanical phenomena but the label Zener diode is applied universally,irrespective of the details of the breakdown mechanism.
To the left, an illustration of breakdown characteristic is drawn; and the scale is exaggerated for better clarity. The specified 'test' current IZT flows as specified by the manufacturer at nominal Zener reference voltage of the diode i.e. is the reverse-bias voltage, and it typically represents rated maximum diode current. In general,Zener voltage is a modest function of temperature.A more definite representative temperature specification is 0.1 % change every °C change.
For a diode with a reference voltage below 5 volts, coefficient is negative; otherwise it is positive. (This is associated with the dominance of one of the two phenomena that produces same terminal breakdown characteristics.)
The inverse of the slope of the diode characteristic (at the test point) is known as the 'dynamic resistance' of the diode. This parameter is noted in the manufacturers' specifications. The slope of the characteristic does not greatly vary for currents ranging (roughly) between 0.1 IZT and IZT, i.e. the usual range of operation of a Zener diode.
The inevitability of operation above the knee, i.e., in the breakdown region conditions the minimum usable current and so also the wish to generally avoid rapid change of slope in the closeproximity of the knee.
NOTE:-
"Either or both breakdown mechanisms may be present in a 'Zener' diode. The avalanche mechanism dominates at low doping levels and higher voltages, while at heavy doping levels and lower voltages, the Zener mechanism dominates.
Y At a certain doping level and around 6 V for Si, both mechanisms are present and there, the temperature coefficients just cancel. It is also possible to create Zener diodes with very small temperature coefficients Y.
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