Electronics & Communication question/topics for PI

This is a thread to help ECE people with the academic questions for interviews. let's begin!

Let's us start with the very basic.


A diode is a two-terminal device, having two active electrodes, between which it allows the transfer of current in one direction only. Diodes are known for their unidirectional current property, wherein, the electric current is allowed to flow in one direction.

Most diodes are made from semiconductors such as silicon, however, germanium is also used sometimes.

Different types of diodes :

P-N junciton Diode, Zener Diode,Light Emitting Diode, Avalanche Diode, Schottky Diode, Photo Diode andGunn Diode.

Last year in XLRI interview, i was asked about the optical fibres and the multiplexing thing. Could not explain it very well though.

P-N junciton construction, forward/reverse bias  and avalanche breakdown.


In PI will they ask ques related to ECE only and not C/C++. I am asking this as I have studied OOPS, Data Structures, DBMS etc. too and i suck at programming. 

Diodes contd.


 It is one of the most popular type of diodes and when this diode permits the transfer of electric current between the electrodes, light is produced.

When the diode is switched on or forward biased, the electrons recombine with the holes and release energy in the form of light (electroluminescence). The color of light depends on the energy gap of the semiconductor.Operating Votage - 1.8 -3.3 volts.

Photo diode- Photodiodes are used to detect light and feature wide, transparent junctions. Generally, these diodes operate in reverse bias, wherein even small amounts of current flow, resulting from the light, can be detected with ease. Photodiodes can also be used to generate electricity, used as solar cells and even in photometry. 

Avalanche Diode-

This type of diode operates in the reverse bias, and used avalanche effect for its operation. The avalanche breakdown takes place across the entire PN junction, when the voltage drop is constant and is independent of current. Generally, the avalanche diode is used for photo-detection, wherein high levels of sensitivity can be obtained by the avalanche process. Operating voltage- as high as 100-200 V for avalanche photodiodes.

Zener Diode-

A zener diode is a diode which allows current to flow in the forward direction in the same manner as an ideal diode, but also permits it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "Zener knee voltage", "Zener voltage".The diode runs in reverse bias, and breaks down on the arrival of a certain voltage. A stable voltage is produced, if the current through the resistor is limited

operating voltage 5-5.5 V.

Used as voltage regulator,waveform clipper.

Schottky Diodes- 

These diodes feature lower forward voltage drop as compared to the ordinary silicon PN junction diodes. The voltage drop may be somewhere between 0.15 and 0.4 volts at low currents, as compared to the 0.6 volts for a silicon diode.

 This lower voltage drop can be used to give higher switching speeds and better system efficiency.

In order to achieve this performance, these diodes are constructed differently from normal diodes, with metal to semiconductor contact. Schottky diodes are used in RF applications, rectifier applications and clamping diodes.

GUNN Diode-

The Gunn diode is a unique component - even though it is called a diode, it does not contain a PN diode junction. The Gunn diode or transferred electron device can be termed a diode because it does have two electrodes. It depends upon the bulk material properties rather than that of a PN junction. The Gunn diode operation depends on the fact that it has a voltage controlled negative resistance.

Gunn diodes are fabricated from a single piece of n-type semiconductor. The most common materials are gallium Arsenide, GaAs and Indium Phosphide, InP. However other materials including Ge, CdTe, InAs, InSb, ZnSe and others have been used. The device is simply an n-type bar with n+ contacts. It is necessary to use n-type material because the transferred electron effect is only applicable to electrons and not holes found in a p-type material.

P.S.- Additions this are welcome.


transistor is a semiconductor device used to amplify and switch electronics signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal.

The bipolar junction transistor

can be of two types : n-p-n (one p region sandwiched between two n regions) and p-n-p (one n region sandwiched between two p regions) . n-p-n type transistor is the more common transistor. The two n regions are called emitter and controller and the thin p region in between is called as base.

The transistor action is such that if the electric potentials on the segments are properly determined, a small current between the base and emitter connections results in a large current between the emitter and collector connections, thus producing current amplification. Some circuits are designed to use the transistor as a switching device; current in the base-emitter junction creates a low-resistance path between the collector and emitter. The p-n-p junction transistor, consisting of a thin layer of n -type semiconductor lying between two p -type semiconductors, works in the same manner, except that all polarities are reversed. They can be used in 3 configurations i.e. common base,common emitter and common controller. In all these configuration base,emitter and controller terminals are grounded respectively.

Field Effect Transirtor (FET) 

A very important type of transistor developed after the junction transistor is the field-effect transistor (FET). It draws virtually no power from an input signal, overcoming a major disadvantage of the junction transistor. An n -channel FET consists of a bar (channel) of n -type semiconductor material that passes between and makes contact with two small regions of p -type material near its center. The terminals attached to the ends of the channel are called the source and the drain; those attached to the two p -type regions are called gates. A voltage applied to the gates is directed so that no current exists across the junctions between the p - and n -type materials; for this reason it is called a reverse voltage. Variations of the magnitude of the reverse voltage cause variations in the resistance of the channel, enabling the reverse voltage to control the current in the channel. A p -channel device works the same way but with all polarities reversed.


The metal-oxide semiconductor field-effect transistor (MOSFET) is a variant in which a single gate is separated from the channel by a layer of metal oxide, which acts as an insulator, or dielectric. The electric field of the gate extends through the dielectric and controls the resistance of the channel. In this device the input signal, which is applied to the gate, can increase the current through the channel as well as decrease it.

Someone please put relevant stuff here... 

I have completely forgotten all this. 

Can someone post in this ?