Schottky diode process and principle

Rectifying junctions can also be formed between semiconductors and metals. This type of junction is called Schottky barriers. Schottky barriers are somewhat similar to pn junctions. For example, Schottky barriers can be used to make Schottky diodes, which are similar to pn diodes. Schottky barriers can also be formed in the contact areas of integrated circuit interconnect systems.

The work function of a substance is equivalent to the energy required to remove an electron from it. The unique work function of each substance depends on its crystal structure and composition. When two substances with different work functions come into contact with each other, the electrons in each substance have different initial energies. Therefore, there is a voltage difference between these two substances, called contact voltage. Consider the situation of pn junction. The semiconductors on both sides of the junction have the same crystal structure. The contact voltage of a pn junction, or its internal electric field, depends only on the doping situation. In Schottky barriers, the different crystal structures of metals and semiconductors also have an impact on the contact voltage.

When aluminum encounters lightly doped n-type silicon, a typical rectifying Schottky barrier is formed (Figure 1.14b). In order to balance the contact voltage, charge carriers must be redistributed. Electrons diffuse from semiconductors to metals, where they pile up to form a negatively charged thin film. The large number of electrons leaving silicon creates a depletion region formed by ionized impurity atoms (Figure 1.14a). The electric field in the depletion region pulls electrons back from the metal to the semiconductor. Balance is only established when the diffusion current and drift current are equal. Now the voltage difference along the Schottky barrier is equal to the contact voltage. There are only a few minority carriers on the semiconductor side of the Schottky barrier, so Schottky diodes are also known as majority carrier devices. Figure 1.14 shows the concentration of excess charge carriers on both sides of the Schottky barrier (a) and the corresponding cross-sectional view of the Schottky structure (b).

The performance of Schottky diodes under bias conditions can be analyzed using similar methods. N-type silicon is the cathode of the diode, while the metal region is the anode. The Schottky diode with zero bias is the same as the Schottky barrier in the equilibrium state analyzed earlier. The reverse biased Schottky diode has its semiconductor terminal connected to the positive electrode and its metal terminal connected to the negative electrode. The final applied voltage difference strengthened the contact voltage. In order to balance the increased voltage difference, the depletion region also widened, and finally balance was established, with only a small current in the diode.

A forward biased Schottky diode has its semiconductor terminal connected to the negative electrode and its metal terminal connected to the positive electrode. The applied voltage difference along the junction weakens the contact voltage and narrows the width of the depletion region. The final contact voltage is completely cancelled out, attempting to establish a depletion region at the metal end of the junction. But the metal region is a conductor and cannot support an electric field, so a depletion region cannot be established to counteract the applied voltage. This voltage starts pushing electrons back from the semiconductor to the metal along the junction, and there is a current in the diode.

The current voltage characteristics of Schottky diodes are similar to those of pn diodes (Figure 1.13). Schottky diodes also have leakage currents caused by minority carriers injected from metals into semiconductors. High temperature will exacerbate this conduction mechanism, which, like pn diodes, has a temperature property.

Despite many similarities, Schottky diodes and pn junction diodes still have some essential differences. Due to the fact that Schottky diodes mainly rely on majority carriers for conduction, they are majority carrier devices. At high current densities, there are indeed some holes flowing from the metal to the semiconductor, but these only account for a small portion of the total current. Schottky diodes do not support a large excess of minority carriers. Due to the fact that the switching speed of diodes is a function of the time required for excess minority carrier recombination, Schottky diodes can switch quickly. Some Schottky diodes have a lower forward bias voltage than pn diodes. The lower forward bias voltage and efficient switching make Schottky diodes very useful.

Schottky diodes can also be made of p-type silicon, but the forward bias voltage is usually very low. This makes the leakage current of p-type Schottky diodes quite severe, so they are rarely used. For example, to compare the difference in work function between n-type silicon platinum (0.85V) and p-type silicon platinum (0.25V): rs.muller and t.i.kamins, device electronics for integrated circuits, 2nd ed.(new york: john wiley  Most practical Schottky diodes are synthesized using lightly doped n-type silicon and a substance called silicates. These substances are composed of silicon and certain metals such as platinum and palladium. Silicides have stable work functions, resulting in Schottky diodes with stable and reproducible characteristics.