zerner diode

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4.5.1. General breakdown characteristics

The maximum reverse bias voltage that can be applied to a p-n diode is limited by breakdown. Breakdow

is characterized by the rapid increase of the current under reverse bias. The corresponding applied voltagreferred to as the breakdown voltage.

The breakdown voltage is a key parameter of power devices. The breakdown of logic devices is equallyimportant as one typically reduces the device dimensions without reducing the applied voltages, thereby

increasing the internal electric field.

Two mechanisms can cause breakdown, namely avalanche multiplication and quantum mechanical

tunneling of carriers through the bandgap. Neither of the two breakdown mechanisms is destructive.

However heating caused by the large breakdown current and high breakdown voltage causes the diode to

destroyed unless sufficient heat sinking is provided.

Breakdown in silicon at room temperature can be predicted using the following empirical expression for

electric field at breakdown.

(4.5.1)

Assuming a one-sided abrupt p-n diode, the corresponding breakdown voltage can then be calculated,yielding:

(4.5.2)

The resulting breakdown voltage is inversely proportional to the doping density if one ignores the weak

doping dependence of the electric field at breakdown. The corresponding depletion layer width equals:

(4.5.3)

4.5.2. Edge effects

Few p-n diodes are truly planar and typically have higher electric fields at the edges. Since the diodes w

break down in the regions where the breakdown field is reached first, one has to take into account the ra

of curvature of the metallurgical junction at the edges. Most doping processes including diffusion and io

implantation yield a radius of curvature on the order of the junction depth,xj. The p-n diode interface cathen be approximated as having a cylindrical shape along a straight edge and a spherical at a corner of a

rectangular pattern. Both structures can be solved analytically as a function of the doping density,N, and

the radius of curvature,xj.

The resulting breakdown voltages and depletion layer widths are plotted below as a function of the dopin

density of an abrupt one-sided junction.
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Figure 4.5.1 : Breakdown voltage and depletion layer width at breakdown versus doping density of anabrupt one-sided p-n diode. Shown are the voltage and width for a planar (top curves),

cylindrical (middle curves) and spherical (bottom curves) junction with 1 m radius ofcurvature.

4.5.3. Avalanche breakdown

Avalanche breakdown is caused by impact ionization of electron-hole pairs. This process was described

previously in section2.8.When applying a high electric field, carriers gain kinetic energy and generate

additional electron-hole pairs through impact ionization. The ionization rate is quantified by the ionizatio

constants of electrons and holes, nand p. These ionization constants are defined as the change of the

carrier density with position divided by the carrier density or:

(4.5.4)

The ionization causes a generation of additional electrons and holes. Assuming that the ionizationcoefficients of electrons and holes are the same, the multiplication factorM, can be calculated from:

(4.5.5)

The integral is taken betweenx1andx2, the region within the depletion layer where the electric field isassumed constant and large enough to cause impact ionization. Outside this range, the electric field is

assumed to be too low to cause impact ionization. The equation for the multiplication factor reaches infi
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if the integral equals one. This condition can be interpreted as follows: For each electron coming to the h

field at pointx1one additional electron-hole pair is generated arriving at pointx2. This hole drifts in the

opposite direction and generates an additional electron-hole pair at the starting pointx1. One initial electtherefore yields an infinite number of electrons arriving atx2, hence an infinite multiplication factor.

The multiplication factor is commonly expressed as a function of the applied voltage and the breakdown

voltage using the following empirical relation:

(4.5.6)

4.5.4. Zener breakdown

Quantum mechanical tunneling of carriers through the bandgap is the dominant breakdown mechanism f

highly doped p-n junctions. The analysis is identical to that of tunneling in a metal-semiconductor juncti(section3.4.4.3)where the barrier height is replaced by the energy bandgap of the material.

The tunneling probability equals:

(4.5.7)

where the electric field equals =Eg/(qL).

The tunneling current is obtained from the product of the carrier charge, velocity and carrier density. Th

velocity equals the Richardson velocity, the velocity with which on average the carriers approach the ba

while the carrier density equals the density of available electrons multiplied with the tunneling probabiliyielding:

(4.5.8)

The tunneling current therefore depends exponentially on the bandgap energy to the 3/2 power.

Boulder, December 2
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Zener diode

From Wikipedia, the free encyclopediaZener diode

Zener diode

Type Passive

Working principle Zener breakdown

First production Clarence Zener(1934)

Pin configuration anodeandcathode

Electronic symbol

A Zener diodeis adiodewhich 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 valueknown as thebreakdown voltage,"Zener knee voltage", "Zener voltage", "avalanche point", or "peak

inverse voltage".

The device was named afterClarence Zener,who discovered this electrical property. Strictly speaking, a

Zener diode is one in which the reverse breakdown is due to electronquantum tunnellingunder high ele

field strengththeZener effect.However, many diodes described as "Zener" diodes rely instead on

avalanche breakdownas the mechanism. Both types are used with the Zener effect predominating underV andavalanche breakdownabove. Common applications include providing a reference voltage forvolt

regulators,or to protect other semiconductor devices from momentary voltage pulses.

Contents 1 Operation

o 1.1 Waveform clipper

o 1.2 Voltage shifter

o 1.3 Voltage regulator 2 Construction

o 2.1 Surface Zeners
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o 2.2 Subsurface Zeners 3 Uses 4 References

5 Further reading

Operation

Current-voltage characteristic of a Zener diode with a breakdown voltage of 17 volts. Notice the changevoltage scale between the forward biased (positive) direction and the reverse biased (negative) direction

TC depending on Zener voltage

A conventional solid-state diode allows significant current if it isreverse-biasedabove its reverse

breakdown voltage.When the reverse bias breakdown voltage is exceeded, a conventional diode is subjeto high current due toavalanche breakdown.Unless this current is limited by circuitry, the diode may be

permanently damaged due to overheating. A Zener diode exhibits almost the same properties, except the

device is specially designed so as to have a reduced breakdown voltage, the so-called Zener voltage. By

contrast with the conventional device, a reverse-biased Zener diode exhibits a controlled breakdown and

allows the current to keep the voltage across the Zener diode close to the Zener breakdown voltage. Forexample, a diode with a Zener breakdown voltage of 3.2 V exhibits a voltage drop of very nearly 3.2 V

across a wide range of reverse currents. The Zener diode is therefore ideal for applications such as the

generation of areference voltage(e.g. for anamplifierstage), or as a voltage stabilizer for low-currentapplications.[1]

Another mechanism that produces a similar effect is the avalanche effect as in theavalanche diode.[1]Th

two types of diode are in fact constructed the same way and both effects are present in diodes of this typ
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In silicon diodes up to about 5.6 volts, theZener effectis the predominant effect and shows a markednegativetemperature coefficient.Above 5.6 volts, theavalanche effectbecomes predominant and exhibipositive temperature coefficient.[2]

In a 5.6 V diode, the two effects occur together, and their temperature coefficients nearly cancel each oth

out, thus the 5.6 V diode is useful in temperature-critical applications. An alternative, which is used forvoltage references that need to be highly stable over long periods of time, is to use a Zener diode with a

temperature coefficient of +2 mV/C (breakdown voltage 6.26.3 V) connected in series with a forwardbiased silicon diode (or a transistor B-E junction) manufactured on the same chip.[3]The forward-biased

diode has a temperature coefficient of 2 mV/C, causing the TCs to cancel out.

Modern manufacturing techniques have produced devices with voltages lower than 5.6 V with negligibl

temperature coefficients,[citation needed]but as higher-voltage devices are encountered, the temperature

coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode.

Zener and avalanche diodes, regardless of breakdown voltage, are usually marketed under the umbrella t

of "Zener diode".

Waveform clipper

Examples of a Waveform Clipper

Two Zener diodes facing each other in series will act to clip both halves of an input signal.Waveformclipperscan be used to not only reshape a signal, but also to prevent voltage spikes from affecting circuithat are connected to the power supply.[4]

Voltage shifter

Examples of a Voltage Shifter
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A Zener diode can be applied to a circuit with a resistor to act as a voltage shifter. This circuit lowers theinput voltage by a quantity that is equal to the Zener diode's breakdown voltage.

Voltage regulator

Examples of a Voltage Regulator

A Zener diode can be applied to a circuit to regulate the voltage applied to a load.

Construction

The Zener diode's operation depends on the heavydopingof itsp-n junction.The depletion region formein the diode is very thin (


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The emitter-base Zener diodes can handle only smaller currents as the energy is dissipated in the basedepletion region which is very small. Higher amount of dissipated energy (higher current for longer timea short very high current spike) causes thermal damage to the junction and/or its contacts. Partial damag

the junction can shift its Zener voltage. Total destruction of the Zener junction by overheating it and

causing migration of metallization across the junction ("spiking") can be used intentionally as a Zener z

antifuse.[5]

Subsurface Zeners

A subsurface Zener diode, also called buried Zener, is a device similar to the Surface Zener, but with th

avalanche region located deeper in the structure, typically several micrometers below the oxide. The hot

carriers then lose energy by collisions with the semiconductor lattice before reaching the oxide layer andcannot be trapped there. The Zener walkout phenomenon therefore does not occur here, and the buried

Zeners have voltage constant over their entire lifetime. Most buried Zeners have breakdown voltage of 5

volts. Several different junction structures are used.[6]

Uses

Zener diode shown with typical packages.Reversecurrent is shown.

Zener diodes are widely used as voltage references and asshuntregulatorsto regulate the voltage acrosssmall circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Ze

diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point on, the

relatively low impedance of the diode keeps the voltage across the diode at that value.[7]
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In this circuit, a typical voltage reference or regulator, an input voltage, UIN, is regulated down to a stabloutput voltage UOUT. The breakdown voltage of diode D is stable over a wide current range and holds U

relatively constant even though the input voltage may fluctuate over a fairly wide range. Because of the

impedance of the diode when operated like this, resistor R is used to limit current through the circuit.

In the case of this simple reference, the current flowing in the diode is determined using Ohm's law and known voltage drop across the resistor R;

The value ofRmust satisfy two conditions :

1. Rmust be small enough that the current through D keeps D in reverse breakdown. The value of t

current is given in the data sheet for D. For example, the common BZX79C5V6[8]device, a 5.6 V

0.5 W Zener diode, has a recommended reverse current of 5 mA. If insufficient current existsthrough D, then UOUTis unregulated and less than the nominal breakdown voltage (this differs tovoltage-regulator tubeswhere the output voltage will be higher than nominal and could rise as hi

as UIN). When calculatingR, allowance must be made for any current through the external load, n

shown in this diagram, connected across UOUT.2. Rmust be large enough that the current through D does not destroy the device. If the current thro

D isID, its breakdown voltage VBand its maximum power dissipationPMAXcorrelate as such:

.

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.

Shunt regulators are simple, but the requirements that the ballast resistor be small enough to avoid excesvoltage drop during worst-case operation (low input voltage concurrent with high load current) tends to

leave a lot of current flowing in the diode much of the time, making for a fairly wasteful regulator with h

quiescent power dissipation, only suitable for smaller loads.

These devices are also encountered, typically in series with a base-emitter junction, in transistor stageswhere selective choice of a device centered around the avalanche or Zener point can be used to introduce

compensating temperature co-efficient balancing of the transistorpn junction.An example of this kind

use would be a DCerror amplifierused in aregulated power supplycircuit feedback loop system.

Zener diodes are also used insurge protectorsto limit transient voltage spikes.

Another notable application of the Zener diode is the use ofnoisecaused by itsavalanche breakdowninrandom number generator.
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Backward diode
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zerner diode

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