Which of the following statements are true for a zener diode ? (a) The voltage across the zener resistance is zero. (b) The diode is used as a constant voltage device. (c) The diode is used as a varactor diode. (d) The diode is formed of heavily doped p and n-type semiconductor. Options :
(a), (b) and (d) are correct
This question was cancelled by UGC. The official answer key records code 9 against it, meaning every option was credited and all candidates were awarded the two marks whatever they marked. The discussion below therefore gives the best-supported reading of the item rather than a uniquely correct option.
Statement (c) is the one certainly false, and it removes two options at once. A varactor is a quite different device, operated in reverse bias below breakdown and used for its voltage-dependent junction capacitance:
\(C_j\propto\left(V_{bi}-V\right)^{-n}\)
It is the tuning element of a VCO or an FM modulator. A zener works in breakdown and is used for its constant voltage, not its capacitance. The two are never interchangeable, so options 3 and 4 fall.
Statement (b) — correct, and it is the defining application. Beyond the knee the reverse characteristic is almost vertical: a large change in current produces almost no change in voltage. That is what makes the device a voltage reference and a shunt regulator.
Statement (d) — correct. Both sides are heavily doped, which makes the depletion layer very thin. A thin layer means an enormous electric field for a modest applied voltage,
\(E=\dfrac{V}{W}\approx10^{8}\ \text{V/m}\)
strong enough to tear electrons directly out of covalent bonds — the true Zener (tunnelling) mechanism, which gives breakdown voltages below about 5 V. Above roughly 6 V the doping is lighter, the layer wider, and avalanche multiplication takes over instead; commercial "zener" diodes cover both and the name is used loosely. The two mechanisms have opposite temperature coefficients, which is why the 5–6 V parts are the most stable references.
Statement (a) — correct in the idealised model, which is how the question intends it. A real zener has a small dynamic resistance rz, of a few ohms to a few tens of ohms, and the working voltage is
\(V_Z=V_{Z0}+I_Zr_z\)
In the ideal zener model rz is taken as zero, so the voltage dropped across it is zero and the terminal voltage is perfectly constant — which is exactly the assumption that makes statement (b) hold. Statements (a) and (b) therefore stand or fall together.
Assemble. (a), (b) and (d) are correct, which is option 1.
Why rz matters in practice. It sets the regulator's line and load regulation: a ripple \(\Delta I_Z\) passes through to the output as \(\Delta V=r_z\Delta I_Z\), so a small rz is what a good reference is bought for.
Hence, the true statements are (a), (b) and (d).
Similar Questions
An important figure of merit for microwave application of Schottky diode is forward bias cut off frequency fco and is given by :
Assertion (A) : A tunnel diode consists of simple p-n junction mode of degenerate semiconductor. The current voltage characteristics consists of three components and shows the negative differential region over part of the forward characteristics.
Reason (R) : In tunnel diode the p-side is highly doped and n-side is highly doped and also used as detector.
Varacter diodes have the following properties :
A. Junction capacitance varies with bias voltage
B. Junction capacitance varies with time
C. Non linearity of varacter diodes makes them suitable for frequency multipliers
D. Linearity of varactor diode makes them suitable for frequency multipliers.
Choose the correct answer from the options given below:
A TRAPATT diode has the doping concentration NA = 2 x 1015/cm3 and a current density of 20 kA/cm2. The value of avalanche zone velocity is given by :
Assertion (A) : The Gunn diode characteristics shows the region of negative resistance.
Reason (R) : The Gunn diode is made of n-type and p-type degenerate semiconductor.
Select your answer using the codes given below.
Assertion (A) : Tunnel diode is used as rectifier.
Reason (R) : The tunnel diode is made of degenerate semiconductors.
Select your answer using the codes given below.
Assertion (A) : In an IMPATT diode, for a transit angle larger than π and approaching 3π/2, the negative resistance of the diode increases rapidly.
Reason (R) : The IMPATT diode consists of a high doping avalanche region followed by a drift region.
Select your answer using the codes given below.
In a tunnel diode, the degeneracy on the n-side can be expressed as
The static current voltage characteristics of a tunnel diode consist of
(a) Tunneling current and thermal current
(b) Thermal current and excess current
(c) Tunneling current only
(d) Tunneling current, excess current and thermal current
Out of these which are correct ?
Match the following :
List – I List – II a. Gunn diode i. degenerate silicon b. Tunnel diode ii. n-type Ga As c. BJT iii. Silicon d. Diode (P-N junction) iv. \(I_o\left[\exp\left(\dfrac{qV}{kT}\right)-1\right]\) Choose the correct answer from the codes given below:
Important Questions from Types of Diode
What is an advantage of a bridge rectifier over a center-tapped full-wave rectifier?
An important figure of merit for microwave application of Schottky diode is forward bias cut off frequency fco and is given by :
Assertion (A) : A tunnel diode consists of simple p-n junction mode of degenerate semiconductor. The current voltage characteristics consists of three components and shows the negative differential region over part of the forward characteristics.
Reason (R) : In tunnel diode the p-side is highly doped and n-side is highly doped and also used as detector.
Varacter diodes have the following properties :
A. Junction capacitance varies with bias voltage
B. Junction capacitance varies with time
C. Non linearity of varacter diodes makes them suitable for frequency multipliers
D. Linearity of varactor diode makes them suitable for frequency multipliers.
Choose the correct answer from the options given below:
For which of the following diodes when the voltage is increased the current flowing through it decreases?
