Question

A Zener diode
(A) Has a high voltage rating
(B) Has a sharp breakdown at low reverse voltage
(C) Has a negative resistance
(D) None of the above

Answer 1

Hint : The Zener diodes are made to allow current to flow in reverse direction at a certain reverse voltage. In a zener diode, an increase in the voltage (even reverse voltage) brings about an increase in current.

Complete step by step answer
In general, a zener diode is a type of diode specifically designed to allow a current to flow in reverse, at a certain value of reverse voltage connected across it. It is named after its inventor Clarence Melvin Zener.
The zener diode works using a phenomenon known as the zener effect which causes state in a diode called the zener breakdown. Zener breakdown occurs when a reverse electric field is applied, such that it may cause a breakdown of the bond of the electrons bound to the atoms (from p type semiconductor) which hence gain kinetic energy and then get swept to the other side (n-type semiconductor). This process is known as the zener effect. The zener breakdown is sharp at a relatively low voltage.
Another effect known as the avalanche effect may also happen in a zener diode, this process also causes another state known as the avalanche breakdown. The avalanche breakdown occurs when a high reverse voltage is applied such that the number of reverse carriers entering the depletion region, though small, become accelerated hence gain a very high kinetic energy and cause collision with atoms and hence, break the covalent bond bonding the electrons. These electrons get accelerated and break down other electrons which also accelerate and break down others which in total increase the number of free carriers in the diode. The avalanche breakdown is not as sharp as the zener breakdown.
Now, since the zener breakdown is sharp, the zener diode does have a sharp breakdown at low reverse voltage
Hence, the correct option is B.

Note
For clarity, although reverse current may flow through during the zener and avalanche effect, an increase in voltage still brings about an increase in current, and a decrease in voltage causes a decrease in current that is to say higher reverse voltage brings higher reverse current and vice versa (which brings a positive slope). Hence, the zener diode has a positive resistance, and not negative.