A normal diode will have a voltage drop between 0. This lower voltage drop provides better system efficiency and higher switching speed. In a Schottky diode, a semiconductor—metal junction is formed between a semiconductor and a metal, thus creating a Schottky barrier. The N-type semiconductor acts as the cathode and the metal side acts as the anode of the diode. This Schottky barrier results in both a low forward voltage drop and very fast switching.
Schottky diodes are high-current diodes used primarily in high-frequency and fast-switching applications. They are also known as hot-carrier diodes. The term hot-carrier is derived from the higher energy level of electrons in the n region compared to those in the metal region.
A Schottky diode symbol is shown in Above Figure. 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, as shown in Below Figure. The forward voltage drop is typically around 0. The Schottky diode operates only with majority carriers. There are no minority carriers and thus no reverse leakage current as in other types of diodes. The metal region is heavily occupied with conduction-band electrons, and the n-type semiconductor region is lightly doped.
So the depletion region is present in the non-ohmic contact diode. The non-rectifying metal-semiconductor junction ohmic contact offers very low resistance to the electric current whereas the rectifying metal-semiconductor junction offers high resistance to the electric current as compared to the ohmic contact. The rectifying schottky barrier is formed when a metal is in contact with the lightly doped semiconductor, whereas the non-rectifying barrier is formed when a metal is in contact with the heavily doped semiconductor.
The ohmic contact has a linear current-voltage I-V curve whereas the non-ohmic contact has a non-linear current-voltage I-V curve. The energy band diagram of the N-type semiconductor and metal is shown in the below figure. The vacuum level is defined as the energy level of electrons that are outside the material. The work function is defined as the energy required to move an electron from Fermi level E F to vacuum level E 0.
The work function is different for metal and semiconductor. The work function of a metal is greater than the work function of a semiconductor. Therefore, the electrons in the n-type semiconductor have high potential energy than the electrons in the metal. The energy levels of the metal and semiconductor are different. The Fermi level at N-type semiconductor side lies above the metal side.
We know that electrons in the higher energy level have more potential energy than the electrons in the lower energy level. So the electrons in the N-type semiconductor have more potential energy than the electrons in the metal. The energy band diagram of the metal and n-type semiconductor after contact is shown in the below figure. When the metal is joined with the n-type semiconductor, the conduction band electrons free electrons in the n-type semiconductor will move from n-type semiconductor to metal to establish an equilibrium state.
We know that when a neutral atom loses an electron it becomes a positive ion similarly when a neutral atom gains an extra electron it becomes a negative ion. The conduction band electrons or free electrons that are crossing the junction will provide extra electrons to the atoms in the metal.
As a result, the atoms at the metal junction gains extra electrons and the atoms at the n-side junction lose electrons. The atoms that lose electrons at the n-side junction will become positive ions whereas the atoms that gain extra electrons at the metal junction will become negative ions. Thus, positive ions are created the n-side junction and negative ions are created at the metal junction.
These positive and negative ions are nothing but the depletion region. Since the metal has a sea of free electrons, the width over which these electrons move into the metal is negligibly thin as compared to the width inside the n-type semiconductor.
So the built-in-potential or built-in-voltage is primarily present inside the n-type semiconductor. The built-in-voltage is the barrier seen by the conduction band electrons of the n-type semiconductor when trying to move into the metal. To overcome this barrier, the free electrons need energy greater than the built-in-voltage. In unbiased schottky diode, only a small number of electrons will flow from n-type semiconductor to metal.
The built-in-voltage prevents further electron flow from the semiconductor conduction band into the metal. The transfer of free electrons from the n-type semiconductor into metal results in energy band bending near the contact. If the positive terminal of the battery is connected to the metal and the negative terminal of the battery is connected to the n-type semiconductor, the schottky diode is said to be forward biased.
When a forward bias voltage is applied to the schottky diode, a large number of free electrons are generated in the n-type semiconductor and metal. However, the free electrons in n-type semiconductor and metal cannot cross the junction unless the applied voltage is greater than 0. If the applied voltage is greater than 0. As a result, electric current starts flowing through the schottky diode. If the applied voltage is continuously increased, the depletion region becomes very thin and finally disappears.
If the negative terminal of the battery is connected to the metal and the positive terminal of the battery is connected to the n-type semiconductor, the schottky diode is said to be reverse biased. When a reverse bias voltage is applied to the schottky diode, the depletion 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. If the reverse bias voltage is continuously increased, the electric current gradually increases due to the weak barrier.
If the reverse bias voltage is largely increased, a sudden rise in electric current takes place. This sudden rise in electric current causes depletion region to break down which may permanently damage the device. The V-I Voltage-Current characteristics of schottky diode is shown in the below figure.
The vertical line in the below figure represents the current flow in the schottky diode and the horizontal line represents the voltage applied across the schottky diode. The V-I characteristics of schottky diode is almost similar to the P-N junction diode. However, the forward voltage drop of schottky diode is very low as compared to the P-N junction diode. The forward voltage drop of schottky diode is 0. If the forward bias voltage is greater than 0. Since the Schottky barriers could lead to rectifying characteristics, it is normally used as a diode, which is a single MS junction with rectifying characteristics.
Both n-type and p-type semiconductors can formed the Schottky contact, such as Titanium Silicide, and Platinum Silicide.
The net loss of electrons creates a negative charge in the metal and a positive charge in the semiconductor, which results a depletion region and a growing barrier at the semiconductor surface. As the result, the equilibrium band structure for a metal and a n-type semiconductor is illustrated in Figure 2.
The barrier results a high resistance when there is even a small applied voltage. Since the potential barrier height could be governed by the voltage bias, which has a significant influence of the electrical current flowing through the barrier, the effect of forward bias and reverse bias are interested.
Figure 3 The current is defined to be positive when it flows from the metal to the semiconductor. Consequently, it will be much easier for electrons to pass over the barrier, which makes the electrons diffuse much easier from semiconductor to the metal.
There will be more electrons diffuse from the semiconductor towards the metal than the electrons drifting into the semiconductor, a positive current will be generated across the MS junction. The Fermi energy of metal becomes higher than the Fermi energy in the semiconductor, which results an increasing of the barrier potential across the MS junction.
The large barrier will block the diffusion of electrons from semiconductor to the metal. Under a small reversed bias, only a small amount of electrons in the metal may be able to overcome the potential barrier. In general, the current flowing through the Schottky contact can be defined with the applied voltage, which is very similar to those of pn-junction diode.
Not all MS contact can perform as the rectifying Schottky diode, since there is no potential barrier formed. Under this situation, when the current can be conducted in both directions of the MS contact, the contact is defined as the Ohmic contact. An ideal Ohmic contact is a low resistance, and non-rectifying junction with no potential exists between the the metal-semiconductor interface.
0コメント