The structure of a metal-semiconductor junction is shown in Figure 3.2.1. It consists of a metal contacting a piece of semiconductor. An ideal Ohmic contact, a contact such that no potential exists between the metal and the semiconductor, is made to the other side of the semiconductor. The sign convention of the applied voltage and current is also shown on Figure 3.2.1.
Flatband diagram and built-in potential
The barrier between the metal and the semiconductor can be identified on an energy band diagram. To construct such diagram we first consider the energy band diagram of the metal and the semiconductor, and align them using the same vacuum level as shown in Figure 3.2.2 (a). As the metal and semiconductor are brought together, the Fermi energies of the metal and the semiconductor do not change right away. This yields the flatband diagram of Figure 3.2.2 (b).
The barrier between the metal and the semiconductor can be identified on an energy band diagram. To construct such diagram we first consider the energy band diagram of the metal and the semiconductor, and align them using the same vacuum level as shown in Figure 3.2.2 (a). As the metal and semiconductor are brought together, the Fermi energies of the metal and the semiconductor do not change right away. This yields the flatband diagram of Figure 3.2.2 (b).
The barrier height, fB, is defined as the potential difference between the Fermi energy of the metal and the band edge where the majority carriers reside. From Figure 3.2.2 (b) one finds that for an n-type semiconductor the barrier height is obtained from:
Where FM is the work function of the metal and c is the electron affinity. The work function of selected metals as measured in vacuum . For p-type material, the barrier height is given by the difference between the valence band edge and the Fermi energy in the metal:
A metal-semiconductor junction will therefore form a barrier for electrons and holes if the Fermi energy of the metal as drawn on the flatband diagram is somewhere
between the conduction and valence band edge.
In addition, we define the built-in potential, fI, as the difference between the Fermi energy of the metal and that of the semiconductor.
between the conduction and valence band edge.
In addition, we define the built-in potential, fI, as the difference between the Fermi energy of the metal and that of the semiconductor.
Thermal equilibrium
The flatband diagram, shown in Figure 3.2.2 (b), is not a thermal equilibrium diagram, since the Fermi energy in the metal differs from that in the semiconductor.
Electrons in the n-type semiconductor can lower their energy by traversing the junction. As the electrons leave the semiconductor, a positive charge, due to the
ionized donor atoms, stays behind. This charge creates a negative field and lowers the band edges of the semiconductor. Electrons flow into the metal until
equilibrium is reached between the diffusion of electrons from the semiconductor into the metal and the drift of electrons caused by the field created by the ionized
impurity atoms. This equilibrium is characterized by a constant Fermi energy throughout the structure.
The flatband diagram, shown in Figure 3.2.2 (b), is not a thermal equilibrium diagram, since the Fermi energy in the metal differs from that in the semiconductor.
Electrons in the n-type semiconductor can lower their energy by traversing the junction. As the electrons leave the semiconductor, a positive charge, due to the
ionized donor atoms, stays behind. This charge creates a negative field and lowers the band edges of the semiconductor. Electrons flow into the metal until
equilibrium is reached between the diffusion of electrons from the semiconductor into the metal and the drift of electrons caused by the field created by the ionized
impurity atoms. This equilibrium is characterized by a constant Fermi energy throughout the structure.
Forward and reverse bias
Operation of a metal-semiconductor junction under forward and reverse bias is illustrated with Figure 3.2.4. As a positive bias is applied to the metal
(Figure 3.2.4 (a)), the Fermi energy of the metal is lowered with respect to the Fermi energy in the semiconductor. This results in a smaller potential drop across
the semiconductor. The balance between diffusion and drift is disturbed and more electrons will diffuse towards the metal than the number drifting into the
semiconductor. This leads to a positive current through the junction at a voltage comparable to the built-in potential.
Operation of a metal-semiconductor junction under forward and reverse bias is illustrated with Figure 3.2.4. As a positive bias is applied to the metal
(Figure 3.2.4 (a)), the Fermi energy of the metal is lowered with respect to the Fermi energy in the semiconductor. This results in a smaller potential drop across
the semiconductor. The balance between diffusion and drift is disturbed and more electrons will diffuse towards the metal than the number drifting into the
semiconductor. This leads to a positive current through the junction at a voltage comparable to the built-in potential.
As a negative voltage is applied (Figure 3.2.4 (b)),the Fermi energy of the metal is raised with respect to the Fermi energy in the semiconductor. The potential
across the semiconductor now increases, yielding a larger depletion region and a larger electric field at the interface. The barrier, which restricts the electrons
to the metal, is unchanged so that that barrier, independent of the applied voltage, limits the flow of electrons. The metal-semiconductor junction with positive
barrier height has therefore a pronounced rectifying behavior. A large current exists under forward bias, while almost no current exists under reverse bias.
across the semiconductor now increases, yielding a larger depletion region and a larger electric field at the interface. The barrier, which restricts the electrons
to the metal, is unchanged so that that barrier, independent of the applied voltage, limits the flow of electrons. The metal-semiconductor junction with positive
barrier height has therefore a pronounced rectifying behavior. A large current exists under forward bias, while almost no current exists under reverse bias.
Estudiante:
Leonardo Andrés Márquez Fernández.
Electrónica del Estado Sólido (EES).
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