Schottky barrier diode-construction-VI Characteristics-Applications

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Definition of Schottky Diode

A Schottky barrier diode is a metal semiconductor junction formed by bringing metal in contact with a moderately doped n type semiconductor material. A Schottky barrier diode is also called as known as Schottky or hot carrier diode. It is named after its inventor Walter H. Schottky, barrier stands for the potential energy barrier for electrons at the junction. It is a unilateral device conducting currents in one direction (Conventional current flow from metal to semiconductor) and restricting in the other.

History of Schottky diode

The cat’s-whisker detector (sometimes called a crystal detector) is an earliest known point contact Schottky diode. It is an electronic component consisting of a thin wire that lightly touches a crystal of semiconducting mineral (usually galena) to make a crude point-contact rectifier. It was developed by J.C.Bose, G.W.Pickard around 1906 and was used in power applications and wireless.

 Physical construction

A Schottky barrier diode is shown in the figure below

Schottky diode physical structure

Schottky diode physical structure

A metal semiconductor junction is formed at one end, it is a unilateral junction. Another metal semiconductor contact is formed at the other end. It is an ideal Ohmic bilateral contact with no potential existing between metal and semiconductor and is non rectifying. The built-in potential across the open circuited Schottky barrier diode characterizes the Schottky barrier diode. It is a function of temperature and doping. It decreases with increasing temperature and doping concentration in N type semiconductor. The typical metals used in the manufacture of Schottky barrier diode are molybdenum, platinum, chromium, tungsten Aluminium, gold e.t.c and the semiconductor used is N type silicon is used.

Symbol of Schottky Diode

Schottky diode symbol

Schottky diode symbol

A Schottky barrier diode is a two terminal device with metal terminal acting as anode and semiconductor terminal acting as anode. The circuit symbol of Schottky barrier diode is shown in the figure.

VI characteristics of Schottky barrier diode

The VI characteristics of Schottky barrier diode is shown below

schottky curve for varying barriers

schottky curve for varying barrier

Schottky diode v/s normal diode VI charcteristics

Schottky diode v/s normal diode VI characteristics

From the VI characteristics it is obvious that the VI characteristics of Schottky barrier diode is similar to normal PN junction diode with the following exceptions

The forward voltage drop of Schottky barrier diode is low compared to normal PN junction diode. The forward voltage drop of Schottky barrier diode made of silicon exhibits a forward voltage drop of 0.3 volts to o.5 volts.

The forward voltage drop increases with the increasing doping concentration of n type semiconductor.

The VI characteristics of Schottky barrier diode is Steeper compared to VI characteristics of normal PN junction diode due to high concentration of current carriers.

Current components in Schottky diode

In a Schottky barrier diode current conduction is through majority carriers which are electrons in N type semiconductor. The current in Schottky barrier diode current is

                                                IT = IDiffusion+ITunneling+IThermionic emission

Where IDiffusion is diffusion current due to concentration gradient and Diffusion current density Jn= Dn*q* dn/dx for electrons, where Dn is the diffusion constant for electrons, q is electronic charge = 1.6*10-19 Coulombs, dn/dx is the concentration gradient for electrons.

ITunneling is the tunneling current due to quantum mechanical tunneling through the barrier. This current is directly proportional to the probability of tunneling. The probability of tunneling increases with the decreasing barrier or built in potential and decreasing depletion layer width.

IThermionic emission is current due to thermionic emission. Due to thermal agitation some of carriers which have energy equal to or larger than the conduction band energy at the metal-semiconductor interface, contribute to the current flow. This is termed as thermionic emission current.

Since the current conduction through Schottky barrier diode is through majority charge carriers only, it does not exhibit minority carrier storage effects found in normal PN junction diode. Hence it is suitable for high-speed switching applications because the forward voltage is low and the reverse recovery time is short. The reverse recovery time is solely dependent on junction capacitance which will be of the order of Pico farads.

Applications of Schottky Barrier diode

  • Schottky barrier diodes are used in bipolar transistor TTL based 74LS (low power Schottky) and 74S (Schottky) families of logic circuits . The Schottky diode by preventing the BJT going into hard saturation reduces the switching time of BJT from saturation to cut off.

  • Schottky diodes are often used as ant saturation clamps on transistors.
  • They play an important role in GaAs circuits.
  • They are used as rectifiers in high power applications circuits.
  • Schottky Barrier Diodes are used in voltage clamping applications.
  • Schottky Barrier Diodes are used in stand-alone photo voltaic systems
  • They are used in radio frequency applications as mixer (or) detector diode.

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