Types of the diode – The Schottky Diode and Characteristics
The Schottky Diode
A Schottky Diode is a metal-semiconductor junction diode with a small forward voltage drop and a very fast switching speed. Schottky diode is also named as a hot carrier diode. When a current flows through the Schottky diode, there is a minor voltage drop across the diode terminals. In a normal p-n junction diode, the voltage drop is between 0.6 to 1.7 volts, whereas in a Schottky diode the voltages drop normally lying between 0.15 and 0.45volts. This low voltage drop property provides higher switching speed and better system efficiency.
The N-type semiconductor side acts as a cathode and the metal side acts as the anode of the diode.
Schottky Diode Construction and working
It’s a metal-semiconductor junction formed between a metal and a semiconductor. Typical metals used are platinum, molybdenum, tungsten or chromium, and certain silicides (e.g, palladium silicide and platinum silicide), whereas the semiconductor material used typically be n-type silicon, because of high mobility of n-type semiconductor. The Direction of conventional current flows across the junction is from the metal side to the semiconductor side, but not in the opposite direction. This Schottky barrier diode provides very fast switching and low forward voltage drop.
Fig. Schottky diode
The choice of the combination of the metal and semiconductor determines the forward voltage drop of the diode.
Both n and p-type semiconductors can be used in the making of Schottky barriers. But, the p-type has typically much lower forward voltage. The reverse leakage current increases with the lowering of the forward voltage, it cannot be too low, so the usually employed range is about 0.5–0.7 V, and p-type semiconductors are employed rarely.
With the increased doping profile of the semiconductor, the width of the depletion region drops. Below a definite width, the charge carriers can tunnel through the depletion region.
Biasing of schottky diode
When the metal-semiconductor junction is formed. Conduction band free electrons in the n-type semiconductor will start moving from n-side semiconductor to the metal side until an equilibrium condition is reached.
Fig .Unbiased schottky diode
The conduction band electrons that are crossing the junction will provide extra electrons to the atoms in the metal side. As a result, metal side atoms gain extra electrons and the atoms at the n-side junction lose their electrons. Due to the recombination of free electron and holes across the junction, positive ions are created the n-side junction and negative ions are created at the metal side of the junction. This region of immobile ions creates a depletion region Since the metal has an ocean of free electrons, the width of the depletion region in the metal side negligible as compared to the width of the depletion region inside the n-type semiconductor. So the built-in-potential is primarily present in the n-type semiconductor. The built-in-potential works as the barrier seen by the conduction band electrons of the n-type semiconductor when trying to move into the metal. To overwhelm this barrier, free electrons need external energy greater than the built-in-voltage. In unbiased condition, only a small number of electrons will flow from n-type semiconductor to metal. The built-in-potential prevents the further transfer of electron flow from the semiconductor conduction band into the metal.
Forward biased Schottky diode
When the +ve terminal of the battery is connected to the metal side and negative terminal of the battery is connected to the n-type semiconductor, the Schottky diode is said to be in forwarding biased condition. When the forward bias voltage is applied to the diode, a large number of free electrons are generated in the n-type semiconductor and metal. However, the free electrons in n-type semiconductors and metal cannot cross the junction unless the applied voltage is greater than 0.2 volts.
Fig. forward biased schottky diode
If the applied voltage is greater than 0.2 volts, the free electrons gain sufficient kinetic energy and overcome the built-in-potential barrier of the depletion region. Hence, the electric current starts flowing through the diode. If the applied voltage largely increased, the depletion region becomes very thin and finally disappears.
Reverse bias Schottky diode
If the -ve terminal of the battery is connected to the metal side and +ve terminal of the battery is connected to the n-type semiconductor, the Schottky diode is said to be in reverse biased condition. When a reverse bias is applied to the Schottky diode, the depletion layer width increases. As a result, the electric current stops flowing. However, a small leakage current flows due to the thermally excited electrons in the metal.
Fig. Reverse bias schottky diode
If the reverse bias voltage is constantly increased, the electric current gradually increases due to the weak barrier. If the reverse bias voltage is increased continuously, a sudden rise in electric current takes place. This sudden rise in electric current causes the depletion region to break down which may cause permanent damage to the device.
V-I characteristics of Schottky diode
The V-I (Voltage-Current) characteristics of the Schottky diode is shown here. The vertical linin the figure represents the current flow in the Schottky diode and the horizontal line represents the applied voltage across the Schottky diode. The V-I characteristics of the Schottky diode are almost similar to the P-N junction diode. As compared to the P-N junction diode, the forward voltage drop of the Schottky diode is very low.
The forward voltage drop of the Schottky diode is 0.2 to 0.3 volts as compared to the silicon P-N junction diode, which is 0.6 to 0.7 volts.
If the forward bias voltage is greater than 0.2 or 0.3 volts, the electric current starts flowing through the diode.
In the Schottky diode, the reverse saturation current occurs at a very low voltage as compared to the silicon diode.
fig.V-I characteristics of schottky diode