Rectifying junctions can also form between semiconductors and metals. Such junctions are called Schottky barriers. Schottky barriers are somewhat similar to PN junctions. For example, Schottky barriers can be used to make Schottky diodes, which are very similar to PN diodes. Schottky barriers can also form in the contact regions of the interconnection systems of integrated circuits.
The work function of a substance is equivalent to the energy required to remove an electron from it. The specific work function of each substance depends on its crystal structure and its 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 the two substances, which is called the contact voltage. Consider the case of a 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 the Schottky barrier, the different crystal structures of the metal and the semiconductor also affect the contact voltage.
A typical rectifying Schottky barrier is formed when aluminum meets lightly doped N-type silicon (Figure 1.14B). To balance the contact voltage, the carriers must be redistributed. Electrons diffuse from the semiconductor into the metal, where they pile up to form a negatively charged film. The mass departure of electrons from the silicon creates a region of ionized impurity atoms - the depletion region (Figure 1.14A). The electric field in the depletion region pulls electrons from the metal back into the semiconductor. Equilibrium is established only when the diffusion current and the drift current are equal. The voltage difference across the Schottky barrier is now 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 called majority carrier devices.
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Figure 1.14 Excess carrier concentration on both sides of the Schottky barrier (A) and a cross-sectional view of the corresponding Schottky structure (B).
The behavior of a Schottky diode under bias can be analyzed in a similar way. The N-type silicon is the cathode of the diode, while the metal region is the anode. The zero-biased Schottky diode is the same as the equilibrium Schottky barrier analyzed above. The reverse-biased Schottky diode has the semiconductor terminal connected to the anode and the metal terminal connected to the cathode. The resulting applied voltage difference increases the contact voltage. The depletion region also widens to balance the increased voltage difference, and eventually equilibrium is established, with only a small current flowing in the diode.
The semiconductor end of the forward-biased Schottky diode is connected to the negative terminal, and the metal end is connected to the positive terminal. The voltage difference applied along the junction weakens the contact voltage, and the width of the depletion region is also narrowed. Eventually, the contact voltage is completely offset, and a depletion region is attempted to be established at the metal end of the junction. However, the metal region is a conductor, and it cannot support an electric field, so the depletion region to offset the applied voltage cannot be established. This voltage begins to push electrons back from the semiconductor to the metal along the junction, and current flows in the diode.
The current-voltage characteristics of a Schottky diode are similar to those of a PN diode (Figure 1.13). Schottky diodes also have leakage currents caused by minority carriers injected from the metal into the semiconductor. High temperatures exacerbate this conduction mechanism, and it has a temperature property like a PN diode.
Despite their many similarities, Schottky diodes and PN junction diodes have some fundamental differences. Since Schottky diodes rely primarily on majority carriers to conduct, they are majority carrier devices. At high current densities, some holes do flow from the metal to the semiconductor, but these are only a small fraction of the total current. Schottky diodes do not support large excess minority carriers. Since the switching speed of the diode is a function of the time it takes for the excess minority carriers to recombine, 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 from P-type silicon, but the forward bias voltage is usually very low. This makes the leakage current of P-type Schottky diodes quite high, so they are rarely used. (7 For example, compare the difference in work function between N-type silicon platinum (0.85V) and P-type silicon platinum (0.25V): RS Muller and TI Kamins, Device Electronics for Integrated Circuits, 2nd ed. (New York: John Wiley and Sons, 1986), p.157.) Most practical Schottky diodes are made from lightly doped N-type silicon and a material called silicides. These materials are made of silicon combined with a metal such as platinum and palladium. Silicides have very stable work functions and thus the resulting Schottky diodes have stable and repeatable characteristics.
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