Zener diode is a specially designed PN junction diode. a reverse biased, heavily doped PN junction diode which is operated in the breakdown region is known as zener diode. It is also called a voltage regulator diode or breakdown diode.
Construction of Zener Diode:
Figure 1 shows the symbol of a zener diode. it is similar to the symbol of an ordinary PN junction diode except that its bar is just turned into Z shape. Figure 2 shows a practical equivalent circuit of a zener diode. this circuit shows that a zener diode is equivalent to a battery with voltage (Vz) called zener voltage in series with a resistance (rz) called zener resistance.
Construction
The Zener diode's operation depends on the heavy doping of its p-n junction. The depletion region formed in the diode is very thin (<1 µm) and the electric field is consequently very high (about 500 kV/m) even for a small reverse bias voltage of about 5 V, allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material.In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides.The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.07% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available Zener diodes can vary widely from 1.2 volts to 200 volts.
Surface Zeners
The emitter-base junction of a bipolar NPN transistor behaves as a Zener diode, with breakdown voltage at about 6.8 V for common bipolar processes and about 10 V for lightly doped base regions in BiCMOS processes. Older processes with poor control of doping characteristics had the variation of Zener voltage up to ±1 V, newer processes using ion implantation can achieve no more than ±0.25 V. The NPN transistor structure can be employed as a surface Zener diode, with collector and emitter connected together as its cathode and base region as anode. In 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 oxide layer above the junction and become trapped there. The accumulation of trapped charges can then cause Zener walkout, a corresponding change of the Zener voltage of the junction. The same effect can be achieved by radiation damage.The emitter-base Zener diodes can handle only smaller currents as the energy is dissipated in the base depletion region which is very small. Higher amount of dissipated energy (higher current for longer time, or a short very high current spike) causes thermal damage to the junction and/or its contacts. Partial damage of 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 zap antifuse.
Subsurface Zeners
A subsurface Zener diode, also called buried Zener, is a device similar to the Surface Zener, but with the 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 and cannot 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 5–7 volts. Several different junction structures are used.Working Principle of Zener Diode:
We will see the working of zener diode as two sections like forward bias and reverse bias.
Forward bias:
Figure 3 shows the arrangement for forward bias. The positive terminal of the battery is connected to the anode (A) and the negative terminal of the battery is connected to the cathode (K). When applied voltage is zero no current flows through the zener diode. when the forward biasing voltage is increased the potential barrier is reduced and the current starts flowing in the circuit.
Reverse Bias:
Figure 4 shows the arrangement for reverse bias. The negative terminal of a battery is connected to the anode (A) and positive terminal of the battery is connected to cathode (K). the reverse bias operation is explained in VI characteristic.
V-I characteristic of zener diode.
The forward and reverse V-I characteristic shown in the figure 5. The forward current increases slowly upto the knee voltage. Beyond this voltage the current increases sharply with increase in applied voltage. Thus under forward bias condition zener diode acts like an ordinary PN junction diode.
Under reverse bias condition a small reverse current flows through the zener diode. When a reverse voltage across a zener diode is increased, a critical voltage called breakdown voltage is reached at which the reverse current increases sharply as shown by the curve PQ in the figure 5. This breakdown voltage is called zener breakdown voltage or simply zener voltage. This voltage is almost constant over the operating region. This ability of a diode is called regulating ability and is an important feature of a zener diode. It maintains an essentially a constant voltage across its terminal over a specified range of zener current values.
Vz – Zener breakdown voltage.
Iz (min) – A minimum value of zener current called break over current.
Iz (max) – A maximum value of zener current above which the zener diode may be damaged.
Zener break-down
Zener breakdown takes when both sides of the junction are very heavily doped and the depletion layer is thin. When a small reverse voltage is applied a very strong electric field is set up across the thin depletion layer. This electric field is enough to break the covalent bonds. Now extremely large number of free charge carriers are produced which constitute the zener current. This process is known as zener breakdown. In this process the junction is not damaged. The junction regains its original position when the reverse voltage is removed.
Avalanche breakdown
The avalanche breakdown takes place when both sides of the junction are lightly doped and the depletion layer is large. When the reverse bias voltage is increased, the accelerated free electron collides with the semiconductor atoms in the depletion region. Due to the collision with valence electrons, covalent bonds are broken and electron-hole pairs are generated. These new charge carriers so produced acquire energy from applied potential and in turn produced additional carriers. This forms avalanche multiplication. This avalanche multiplication causes the reverse current to increase rapidly. This leads to avalanche breakdown. Once this breakdown occurs the junction cannot regain its original position. This breakdown occurs at higher reverse voltage as shown in the figure 6.
Applications of zener diode.
- It can be used as a voltage regulator.
- It can be used as a limiter in wave shaping circuits.
- It can be used as a fixed reference voltage in transistor biasing circuits.
- It is used for meter protection against damage from accidental over voltage.
- It can be used as a fixed reference voltage in a network for calibrating voltmeters.
Applications:
Waveform clipper
Examples of a Waveform Clipper
Voltage shifter
Examples of a Voltage Shifter
Voltage regulator
Examples of a Voltage Regulator
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