Zener diode From Wikipedia, the free encyclopedia
Zener diode
Zener diode
Type
Passive
Working principle
Zener breakdown
Pin confguration
anode and anode and cathode
Electronic symbol
A Zener diode is a diode which allows current to flow in the forward direction in the same manner as an ideal diode, but also permits it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, voltage, "Zener knee voltage", "Zener voltage", "avalanche point", or "peak inverse voltage". The device was named after larence Zener , who discovered this electrical property. !trictly speaking, a Zener diode is one in which the reverse breakdown is due to electron uantum tunnelling under tunnelling under high electric field strength#the Zener effect. effect. $owever, many diodes described as "Zener" diodes rely instead on avalanche breakdown as breakdown as the mechanism. %oth types are used with the Zener effect predominating under &.' ( and avalanche breakdown above. breakdown above. ommon applications include providing a reference voltage for voltage for voltage regulators, regulators, or to protect other semiconductor devices from momentary voltage pulses.
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Contents
) *peration
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o
).) Waveform clipper
o
).+ (oltage shifter
o
). (oltage regulator + onstruction
•
o
+.) !urface Zeners
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+.+ !ubsurface Zeners
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-ses
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!ee also
•
& /eferences
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' Further reading
Operation0edit1
urrent2voltage characteristic of a Zener diode with a breakdown voltage of )3 volts. 4otice the change of voltage scale between the forward biased 5positive6 direction and the reverse biased 5negative6 direction.
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T depending on Zener voltage
A conventional solid2state diode allows significant current if it is reverse2biased above its reverse breakdown voltage. When the reverse bias breakdown voltage is e7ceeded, a conventional diode is sub8ect to high current due to avalanche breakdown. -nless this current is limited by circuitry, the diode may be permanently damaged due to overheating. A Zener diode e7hibits almost the same properties, e7cept the device is specially designed so as to have a reduced breakdown voltage, the so2called Zener voltage. %y contrast with the conventional device, a reverse2biased Zener diode e7hibits a controlled breakdown and allows the current to keep the voltage across the Zener diode close to the Zener breakdown voltage. For e7ample, a diode with a Zener breakdown voltage of .+ ( e7hibits a voltage drop of very nearly .+ ( across a wide range of reverse currents. The Zener diode is therefore ideal for applications such as the generation of a reference voltage 5e.g. for an amplifier stage6, or as a voltage stabili9er for low2current applications. 0)1 Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. 0)1 The two types of diode are in fact constructed the same way a nd both effects are present in diodes of this type. :n silicon diodes up to about &.' volts, the Zener effect is the predominant effect and shows a marked negative temperature coefficient. Above &.' volts, the avalanche effect becomes predominant and e7hibits a positive temperature coefficient. 0+1 :n a &.' ( diode, the two effects occur together, and their temperature coefficients nearly cancel each other out, thus the &.' ( d iode is useful in temperature2critical applications. An alternative, which is used for voltage references that need to be highly stable over long periods of time, is to use a Zener diode with a temperature coefficient of ;+ m(<= 5breakdown voltage '.+>'. (6 connected in series with a forward2biased silicon diode 5or a transistor %2? 8unction6 manufactured on the same chip. 01 The forward2biased diode has a temperature coefficient of @+ m(<=, causing the Ts to cancel out. odern manufacturing techniues have produced devices with voltages lower than &.' ( with 1 negligible temperature coefficients,0 but as higher2voltage devices are e ncountered, the temperature coefficient rises dramatically. A 3& ( diode has )B times the coefficient of a )+ ( diode. citation needed
Zener and avalanche diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of "Zener diode".
Waveform clipper 0edit1
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?7amples of a Waveform lipper
Two Zener diodes facing each other in series will act to clip both halves of an input signal. Waveform clippers can be used to not only reshape a signal, but also to prevent voltage spikes from affecting circuits that are connected to the power supply. 01
(oltage shifter 0edit1
?7amples of a (oltage !hifter
A Zener diode can be applied to a circuit with a resistor to act as a voltage shifter. This circuit lowers the input voltage by a uantity that is eual to the Zener diodeCs breakdown voltage.
(oltage regulator 0edit1
?7amples of a (oltage /egulator
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A Zener diode can be applied to a circuit to regulate the voltage applied to a load, such as in a linear regulator .
Construction0edit1 The Zener diodeCs operation depends on the heavy doping of its p2n 8unction. The depletion region formed in the diode is very thin 5D) Em6 and the electric field is conseuently very high 5about &BB k(
!urface Zeners0edit1 The emitter2base 8unction of a bipolar 4H4 transistor behaves as a Zener di ode, with breakdown voltage at about '.I ( for common bipolar processes and about )B ( for lightly doped base regions in %i*! processes. *lder processes with poor control of doping characteristics had the variation of Zener voltage up to J) (, newer processes using ion implantation can achieve no more than JB.+& (. The 4H4 transistor structure can be employed as a surface Zener diode , with collector and emitter connected together as its cathode and base regi on as anode. :n this approach the base doping profile usually narrows towards the surface, creating a region with intensified electric field where the avalanche breakdown occurs. The hot carriers produced by acceleration in the intense field sometime shoot into the o7ide layer above the 8unction and become trapped there. The accumulation of trapped charges can then cause CZener walkoutC, a corresponding change of the Ze ner voltage of the 8unction. The same effect can be achieved by radiation damage. The emitter2base Zener diodes can handle only smaller currents as the energy is dissipated in the base depletion region which is very small. $igher amount of dissipated energy 5higher current for longer time, or a short very hi gh current spike6 causes thermal damage to the 8u nction and
!ubsurface Zeners0edit1 A subsurface Zener diode, also called Cburied ZenerC, is a device similar to the !urface Zener, but with the avalanche region located deeper in the structure, typically several micrometers below the o7ide. The hot carriers then lose energ y by collisions with the semiconductor lattice before reaching the o7ide layer and cannot be trapped there. The Zener walkout phenomenon therefore does n ot occur
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here, and the buried Zeners have voltage constant over their entire lifetime. ost buried Zeners have breakdown voltage of &>3 volts. !everal different 8unction structures are used. 0'1
Uses0edit1
Zener diode shown with typical packages. Reverse current
is shown.
Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diodeCs reverse breakdown voltage. From that point on, the relatively lo w impedance of the diode keeps the voltage across the diode at that value.031
:n this circuit, a typical voltage reference or regulator, an input voltage, - :4, is regulated down to a stable output voltage - *-T. The breakdown voltage of diode K is stable over a wide current range and holds -*-T relatively constant even though the input voltage may fluctuate over a fairly wide rang e. %ecause of the low impedance of the diode when operated like this, resistor / is used to limit current through the circuit. :n the case of this simple reference, the current flowing in the diode is determined using *hmCs law and the known voltage drop across the resistor /
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The value of
R must
).
R must
+.
R must
satisfy two conditions L
be small enough that the current through K keeps K in reverse breakdown. The value of this current is given in the data sheet for K. For e7ample, the common %ZM3N&('0I1 device, a &.' ( B.& W Zener diode, has a recommended reverse current of & mA. :f insufficient current e7ists through K, then - *-T is unregulated and less than the nominal breakdown voltage 5this differs to voltage2regulator tubes where the output voltage will be higher than nominal and could rise as high as - :46. When calculating R , allowance must be made for any current through the e7ternal load, not shown in this diagram, connected across - *-T. be large enough that the current through K does not destroy the device. :f the current through K is I K, its breakdown voltage V % and its ma7imum power dissipationP AM correlate as suchL
.
A load may be placed across the diode in this reference circuit, and as long as the Zener stays in reverse breakdown, the diode provides a stable voltage source to the load. Zener diodes in this configuration are often used as stable references for more advanced voltage regulator circuits. !hunt regulators are simple, but the reuirements that the ballast resistor be small enough to avoid e7cessive voltage drop during worst2case operation 5l ow input voltage concurrent with high load current6 tends to leave a lot of current flowing in the di ode much of the time, making for a fairly wasteful regulator with high uiescent power dissipation, only suitable for smaller loads. These devices are also encountered, typically in series with a base2emitter 8unction, in transistor stages where selective choice of a device centered around the avalanche or Zener point can be used to introduce compensating temperature co2efficient balancing of the transistor p>n 8unction. An e7ample of this kind of use would be a K error amplifier used in a regulated power supply circuit feedback loop system. Zener diodes are also used in surge protectors to limit transient voltage spikes. Another application of the Zener diode is the use of noise caused by its avalanche breakdown in a random number generator .
See also0edit1 •
%ackward diode
•
Transient voltage suppression diode
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