ELECTRONICS ENGINEERING DEPARTMENT

ASSIGNMENT OF BASIC ELECTRONICS

DIFFERENT TYPES OF DIODES

Group Members:

Mohsin Meraj 2012-EE-313

Shaheryar Farrukh 2012-EE-314

Moin Aman 2012-EE-336

Submitted to: Miss Sana

AVALANCHE DIODE

Definition:

An avalanche diode is a diode that is designed to break down and conduct at a specified reverse bias voltage.  This is somewhat similar, but not identical to Zener breakdown. 

When avalanche occurs in a typical diode or other semiconductor, it generally causes catastrophic failure.  However, if the diode is designed to control the avalanche phenomenon, the diode can tolerate avalanche caused by overvoltage, and remain undamaged.  Thus, avalanche type diodes are often used in protecting circuits against transient high voltages which otherwise would damage the circuit. 

Explanation:

Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials.  It is a form of electric current multiplication that can allow very large currents to flow within materials which are otherwise good insulators.  Avalanche breakdown can occur within solids, liquids, or gases when the voltage applied across the insulating material is great enough to accelerate free electrons to the point that, when they strike atoms in the material, they can knock other electrons free.  

Avalanche occurs extremely quickly, and as a result, avalanche diodes are the fastest surge-suppression devices.  Applications include protection against transients, such as in high voltage or inductive circuits, and when putting diodes in series.

 Applications for Avalanche Diodes:Avalanche type diodes can result in increased reliability in many applications, particularly those where voltage transients are expected.  Due to its high speed and ability to withstand large numbers of transients, avalanche diodes are used to protect circuits against surges, lightning and other transients.  They are faster than MOV’s, zeners, and gas tubes. 

Avalanche diodes are the diode type of choice in high voltage circuits, such as high voltage multipliers, due to possibility of transients caused by arcs, pulses, etc.

Another usage for rectifiers with avalanche capability is putting individual diodes in series to obtain higher reverse voltage capability.  The voltage normally divides in proportion to the reverse resistance of each diode.  Since there could be large variation in the reverse resistance, the voltage may not divide equally across the series string of diodes.  A transient of sufficient amplitude will drive the voltage across one or more diodes into the breakdown region.  A typical diode will exhibit catastrophic failure when this occurs.  However, avalanche type diodes will cause the voltage to divide much more evenly.  Also, the avalanche diode is capable of protecting itself by handling transient energy, providing it does not exceed the energy rating of the diode.  Thus, whenever diodes are connected in series to increase the voltage rating, avalanche diodes should be used.

Inductive loads, upon interruption, generate voltage transients.  Particular care should be exercised when specifying rectifiers for these applications, especially for the freewheeling diode placed across and inductive load.  Motors, relays, solenoids and motor starters are typical of equipment, which are capable of rupturing ordinary silicon rectifiers.  Unless it can be clearly shown that the peak inductive transient voltage cannot exceed the voltage rating of the diode, avalanche diodes are the preferred type for this application. 

 

Summary:

Avalanche diodes are designed to break down at a well-defined reverse voltage without being destroyed.  Diodes designed to control this phenomenon are called avalanche diodes.  They are well suited for use in protective applications, inductive circuits, high voltage circuits and when connecting diodes in series.  In many applications, avalanche diodes will survive while normal diodes will fail.  Thus, use of avalanche diodes in selected applications can result in improved reliability.

Symbol:

Graph:

SCHOTTKY DIODE

Introduction to Schottky Diode:

Schottky didoes are used primarily in high frequency and fast switching applications. Schottky diodes are also known as high carrier diodes. A Schottky diode is formed by joining a doped semiconductor region (usually n-type ) with a metal such as gold, silver, or platinum . Rather than a pn junction, there is a metal to semiconductor junction

Explanation:

The Schottky diode operates only with majority carriers.  There are no minority carriers and thus no reverse leakage current as in other types of didoes.  The metal region is heavily occupied with conduction band electrons, and the n-type semiconductor region is lightly doped when forward biased, the higher energy electrons in the n region are injected into the metal region where they give up their excess energy very rapidly.  Since there are no minority carriers, as i fast switching diode, and most of its applications and in many digital circuits to decrease switching times.

Internal Structure of Schottky Diode:

Symbol of Schottky Diode:

VARACTOR DIODE

Introduction to Varactor Diode:

Varactor diodes are also known as variable capacitance diodes because the junction capacitance varies with the amount of reverse-bias voltage. Varactor diodes are specifically designed to take advantage of this variable capacitance characteristic.  These devices are commonly used in electronics tuning circuits used in communications systems.

Characteristics of Varactor Diode:

A varactor diode that always operates in reverse bias and is doped to maximize the inherent capacitance of the depletion region.  The depletion region, widened by the reverse bias, acts as a capacitor dielectric because of its non conductive characteristic.  The p and n regions are conductive and acts as the capacitor plates, as shown in below circuit.

The reverse biased varactor diode acts as a variable capacitor

Basic Operation of Varactor Diode:

As the reverse bias voltage increase, the depletion region widens, effectively increasing the plate separation and the dielectric thickness and thus decreasing the capacitance, when the reverse bias voltage decreases, the depletion region narrows, thus increasing the capacitance.  This action is shown below diagrams (a) and (b).

Varactor diode capacitance varies with reverse biasing

 In a varactor diode, these capacitance parameters are controlled by the method of doping near the pn junction and the size and geometry of the diode’s construction.  Normal varactor capacitances are typically available from a few picofarads to several hundred picofarads.

Varactor Diode Symbol:

Applications of Varactor Diode:

Varactor diodes are used in electronic tuning circuits and modern communications systems

Power Diode

Definition:

A power diode is a two terminal p-n junction device and a p-n junction normally formed by allowing diffusion and epitaxial growth structure of a power diode and symbol are shown in figure below.

High power diodes are silicon-rectifiers that can operate at high junction temperatures.Power diodes have larger Power,Voltage and Current handling capabilities than ordinary signal diodes.In addition,the switching frequencies of power diodes are low as compared to signal diodes.

The voltage current characteristics of power diodes is shown in figure below

Explanation:

When the anode potential is positive with respect to cathode,the diode is said to be forward biased,the diode conducts and behaves essentially as a closed switch.A conducting diode has a relatively small forward voltage drop across it and the magnitude of the drop would depend on the manufacturing process and temperature.When cathode potential is positive with respect to anode,the diode is said to be reversed.It behaves essentially as an open circuit.Under reverse biased condition,a small reverse current is known as leakage current in the range of μA or mA flows and leakage current increases slowly in magnitude with the reverse voltage until the avalanche voltage is reached .The forward voltage drop when it conducts current ,is in the range of 0.8 to 1V.Diodes with ratings as high as 4000V and 2000A are available.

Following the end of forward conduction in diode,a reverse current flows for a short time.The device doesn’t attains its full blocking capability until the reverse current ceases.The reverse current flows in the interval called rectifier recovery time.During this time,charge carriers stored in the diode at the end of forward conduction are removed.The recovery time is in range of a few μs(1-5)μs in a conventional diode to several hundred nanoseconds in fast recovery diodes.This recovery time is of great significance in high frequency operation.The recovery characteristics of conventional and fast recovery diodes are shown in figure below.

Applications:

The application of these devices includes electric traction,battery charging,electro plating,electro metal processing,power supplies,welding ups etc.