Thin Insulating Gates
The band diagram for a thin insulating gate is shown in Figure 3-15. As for the ohmic contact, a change in the temperature of the device away from the equilibrium reference results in a shift of the metal and semiconductor Fermi levels (ΔEf) on the absolute energy scale, given by Equation 3-133. An applied potential further shifts the metal Fermi level with respect to the semiconductor Fermi level (which can be defined in regions away from the contact where little current flows).
Figure 3-15: Energy band diagram for a thin insulating gate with n-type (top) and p-type (bottom) semiconductors under different biasing and temperature conditions (left to right). On the left, the semiconductor is in the reference configuration at the equilibrium temperature T0 and at zero bias V0=0. In the center, the temperature of the semiconductor is raised, changing the gap Eg and the electron affinity χ. Correspondingly, the energy of the conduction band edge Ec and of the valence band edge Ev is shifted, as is the Fermi level Ef, in equilibrium regions of the device (indicated to the right of the band diagram). In this configuration the vacuum energy E0 also changes as a result of differences in the space charge distribution in the device. On the right, in addition to the temperature change, a bias V0 is applied to the metal, leading to an inversion layer under the gate.
When the Thin Insulator Gate boundary condition is used, the thin insulating layer is not included in the COMSOL Multiphysics model but its effect is included in the formulation of the boundary condition. The insulator is assumed to be so thin that the electric field (which must be perpendicular to the metal surface) is to a good approximation also perpendicular to the insulator-semiconductor boundary. When this is the case, the normal electric displacement field (D) at the insulator-semiconductor interface can be written as:
where Vg is the potential on the gate, V is the potential at the insulator-semiconductor interface, dins is the thickness of the insulator, εins is the relative dielectric permittivity of the insulator, ε0 is the permittivity of free space, and n is the outward normal of the semiconductor domain. From Figure 3-15, the gate voltage with applied potential V0 is given by:
where Φm is the metal work function. ΔEf is given by Equation 3-133. Therefore:
(3-139)
The electron and hole currents flowing into the insulator must be zero therefore:
(3-140)
The thin insulating gate applies the Neumann boundary conditions given by Equation 3-139 and Equation 3-140.
Traps on the Semiconductor/Insulator Boundary
When traps are present on the gate the traps accumulate charge as described in the Traps section. As a result of the charge on the boundary the electric displacement field is discontinuous across the interface and consequently:
where Q is the surface charge density due to the traps, given by Equation 3-89 or Equation 3-90. Consequently the form of Equation 3-139 is modified so that:
Similarly, in a transient study, the traps can provide a sink for electrons or holes and Equation 3-140 is changed such that:
where Re and Rh are given by Equation 3-88.