Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Transistors
MOSFET: Interface Charge
Introduction:
This section describes the interface charge of the MOSFET in details.
Effects of charges in the oxide at interface:
- In addition to the work function difference, the equilibrium MOS structure is affected by charges in the insulator and at the semiconductor-oxide interface. For example, alkali metal ions (particularly Na ) can be incorporated inadvertently in the oxide during growth or subsequent processing steps.
- Since sodium is a common contaminant, it is necessary to use extremely clean chemicals, water, gases, and processing environment to minimize its effect on dielectric layers.
- Sodium ions introduce positive charges (Qm) in the oxide, which in turn induce negative charges in the semiconductor.
- The effect of such positive ionic charges in the oxide depends upon the number of ions involved and their distance from the semiconductor surface.
- The negative charge induced in the semiconductor is greater if the Na ions are near the interface than if they are farther away.
- The effect of this ionic charge on threshold voltage is complicated by the fact that Na ions are relatively mobile in Si02, particularly at elevated temperatures, and can thus drift in an applied electric field.
- Obviously, a device with VT dependent on its past history of voltage bias is unacceptable. Fortunately, Na contamination of the oxide can be reduced to tolerable levels by proper care in processing. The oxide also contains trapped charges (Qot) due to imperfections in the Si02.
- In addition to oxide charges, a set of positive charges arises from interface states at the Si-Si02 interface.
- These charges, which we will call Qit result from the sudden termination of the semiconductor crystal lattice at the oxide interface.
- Near the interface is a transition layer (SiO2) containing fixed charges (Qf). As oxidation takes place in forming the Si02 layer, Si is removed from the surface and reacts with the oxygen.
- When the oxidation is stopped, some ionic Si is left near the interface. These ions, along with uncompleted Si bonds at the surface, result in a sheet of positive charge Qf near the interface.
- This charge depends on oxidation rate and subsequent heat treatment, and also on crystal orientation. For carefully treated Si-Si02 interfaces, typical charge densities due to Qit and Qf are about 1010 charges/cm2 for samples with {100} surfaces.
- The interface charge density is about a factor of ten higher on {111} surfaces. That is why MOS devices are generally made on {100} Si.
- For simplicity we will include the various oxide and interface charges in an effective positive charge at the interface Qi (C/cm2).
- The effect of this charge is to induce an equivalent negative charge in the semiconductor. Thus an additional component must be added to the flat band voltage:
VFB= Φms - Qi/Ci
- Since the difference in work function and the positive interface charge both tend to bend the bands down at the semiconductor surface, a negative voltage must be applied to the metal relative to the semiconductor to achieve the flat band condition of Fig.(b).