Test: Zener Diodes - Electronics and Communication Engineering (ECE) MCQ

Concept:
A Zener diode is a silicon semiconductor device that permits current to flow in either a forward or reverse direction. The diode consists of a special, heavily doped p-n junction, designed to conduct in the reverse direction when a certain specified voltage is reached.
Voltage Vz: The Zener voltage refers to the reverse breakdown voltage.
Current Iz (max.): Maximum current at the rated Zener voltage Vz.
Current Iz (min.): Minimum current required for the diode to break down.
Calculation:
Given;
Vz = 10 V
Open circuit the Zener diode to calculate the voltage appearing across the diode.

V_x = 16 × 4.2/(4.2+1) = 12.92 V
As, V_x > V_z → Zener diode will be in the breakdown region.
V_L = V_Z = 10 V

Zener diodes:

• Zener diodes are a special kind of diode, which permits current to flow not only in the forward direction but also allow current to flow in the reverse direction when the reverse voltage is above a certain value, due to the Zener and avalanche effect.
• The Zener effect is a type of electrical breakdown that occurs in a reverse-biased p - n diode when the electric field due to high reverse bias voltage 4 - 6 V(near 5 V) enables tunneling of electrons from the valence to the conduction band of a semiconductor, leading to a large number of free minority carriers which suddenly increase the reverse current.
• At even higher reverse voltages (> 8 V) the Zener diode minority carriers accelerated by reverse voltage collide with the stationary atoms and break covalent bonds freeing electrons for conduction. This effect that drastically increases current is called avalanche breakdown.

The minimum current flowing through the Zener diode is known as the Zener knee current.

Given, V_Z = 6V 
I_Z = 5 mA = 0.005 A
Applying KVL at node A:


R = 80 Ω 

• The Zener diode is a PN junction diode that operates in the reverse biased condition.
• During the reverse biased condition, if the voltage across the Zener diode is greater than the breakdown voltage, then the Zener diode conducts.
• This breakdown voltage is known as the Zener breakdown voltage.
• The Zener diode is used as the main component in the dc power supply for voltage regulation.

The working of the Zener diode is explained in the below figures.

Calculation:
Given,
Zener voltage Vz = 10 V
Ein = 6 V ⇒ Ein < Vz
Hence zener will be reverse biased and get open-circuited.
Output voltage E0 = 0 V

The Zener diode operates just like the normal diode when in the forward-bias mode, and has a turn-on voltage of between 0.3 and 0.7 V. However when connected in the reverse mode, which is usual in most of its applications, a small leakage current may flow and the voltage across it is kept constant.
V - I characteristic of Zener diode

Zener as a Voltage Limiter:

A resistor, R1 is connected in series with the Zener diode to limit the amount of current flowing through the diode and the input voltage Vin (Which must be greater than the Zener voltage) is connected across as shown in the image, and the output voltage Vout is taken across the Zener diode with Vout = Vz (Zener Voltage).

They are used as Voltage limiter and voltage regulator.

Varactor Diode is a type of diode whose internal capacitance varies with respect to the reverse voltage.
It is used in frequency modulation for FM transmitters and phase-locked loops.

Tunnel diode is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanical effect called tunneling. They areused in oscillator circuits, and in FM receivers.

PIN diode is a diode where an intrinsic layer of high resistivity is sandwiched between the P and N-region of semiconductor material. They are used in attenuators, fast switches, photodetectors

As R decreases current through it will increase.
Current supplied by 10V source is I=(10-6) / 50
I=80 mA
The maximum current through R is 80 - 5 = 75 mA
So Minimum
R = 80 Ω 

Zener diode: A Semiconductor Diode blocks current in the reverse direction, but will suffer from premature breakdown or damage if the reverse voltage applied across becomes too high.
Zener diode is called Voltage regulator diode because it maintains a constant voltage across its terminals called Zener voltage once the breakdown happens.

Zener Diode: 

• Zener diodes are normal PN junction diodes operating in reverse-biased conditions.
• The working of the Zener diode is similar to a PN junction diode in forward biased condition, but the uniqueness lies in the fact that it can also conduct when it is connected in reverse bias above its threshold/breakdown voltage.
• It is operated in a breakdown region.
• A Zener diode is used as a voltage regulator in reverse biased mode.
• The breakdown voltage in the Zener diode when connected in the reverse-biased is called Zener voltage.
• This Zener voltage so steady and constant, it has huge applications in circuits, most importantly, voltage regulation.
• From the above explanation, we can see that the Zener diode is fabricated by heavily doped p-side and lightly doped n-side, which help it in operating in reverse bias.

Symbol: Zener diodes

V-I characteristics of Zener diode:

A transistor is connected as a power switch and is operated in the saturated mode.
Thus, the pulse generator output alternatively turns the switch ON and OFF in the switching regulators.