Draw a neat diagram showing forward biasing of a p-n junction.

Here, the positive terminal is connected to p terminal of diode and negative terminal of battery is connected to n terminal of diode. So, the p-n junction diode is in forward bias as the width of depletion region decreases.

With the help of a circuit diagram, show how a Zener diode can be used as a voltage regulator.

A Zener diode is a heavily doped semiconductor diode that is designed to operate in reverse bias and also referred to be breakdown diode. In forward bias the Zener diode behaves like a normal p-n junction diode. But when reverse voltage is applied across Zener Diode upto reverse breakdown voltage, this voltage will be constant for a wide range of currents. So, by connecting Zener diode parallelly to the load so that the applied voltage will reverse bias the zener diode. If the reverse bias voltage across the zener diode exceeds the knee voltage, the voltage across the load will be constant. Thus, Zener Diodes are widely used as Shunt Voltage Regulators to regulate voltage across small loads.

Discuss the working of a photodiode. Why a photodiode is operated in reverse bias mode?

A photodiode is a light detector that produces voltage or current on the base of mode of operation in response to incident light. When a certain energy of photon is incident on photodiode this makes electron-hole pair. After the generation of electron-hole pair, hole move towards the anode and electron move towards cathode as a result photocurrent is generated. Photodiode operates in three different modes: Photovoltaic mode: In this mode voltage is produced by the lightened photodiode. This is also referred as zero-bias mode. Photo conductive mode: In this mode photodiode is kept in reverse bias.This increases the depletion layer's width that decreases the response time and junction capacitance. This is also referred as reverse bias mode. When the photodiode is operated in reverse bias condition, this increases the depletion layer width. This, reduces the capacitance of junction and relaxation time. As a result the response time of photodiode is much quicker.

Mention one practical application of logic gates.

Logic gates are used in the form or transistors in microprocessors and microcontrollers where NAND and NOR gates are used, since they are universal gates so any of the operations of basic gates like AND, OR, and NOT gates can be done using universal gates.

A semiconductor has equal electron and hole concentration 6 × 10 8 m - 3 . On doping with a certain impurity electron concentration increases to 9 × 10 12 m - 3 . Calculate the new hole concentration. Also identify the new semiconductor.

Initially, the electron concentration n e is equal to the hole concentration n h . This relation is written as: n i = 6 × 10 8 m - 3 Given: n e = 9 × 10 12 m - 3 For the given semiconductor, we may write: n e × n h = n i 2 ......(iii) Using(ii) and(iii) we get: n h × 9 × 10 12 = 6 × 10 8 2 n h = 36 × 10 16 9 × 10 12 n h = 4 × 10 4 m - 3 As, after doping n e > n h , so the new semiconductor is n - type.