Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Transistors
MOSFET: Control of Threshold Voltage
Introduction:
This section Controls the threshold Voltage of the MOSFET.
Control of Threshold Voltage:
- Since the threshold voltage determines the requirements for turning the MOS transistor on or off, it is very important to be able to adjust VT in designing the device.
- For example, if the transistor is to be used in a circuit driven by a 3-V battery, it is clear that a 4-V threshold voltage is unacceptable.
- Some applications require not only a low value of VT, but also a precisely controlled value to match other devices in the circuit.
- The work function potential difference Φms is determined by choice of the gate conductor material; Φf depends on the substrate doping; Qi can be reduced by proper oxidation methods and by using Si grown in the (100) orientation; Qd can be adjusted by doping of the substrate; and Ci depends on the thickness and dielectric constant of the insulator.
Control of Ci:
- Since a low value of VT and a high drive current is usually desired, a thin oxide layer is used in the gate region to increase Ci = ξi/d.
- We know that increasing Ci makes VT less negative for p-channel devices and less positive for n-channel with -Qd > Qt.
- For practical considerations, the gate oxide thickness is generally 20-100 A (2-10 nm) in modern devices having submicron gate length.
- An example of such a device is shown in Figure (1) given below. The gate oxide, easily observable in this micrograph, is 40 A thick.
- The interfacial layer between the crystalline silicon and the amorphous Si02 is also observable.
- Although a low threshold voltage is desirable in the gate region of a transistor, a large value of VT is needed between devices.
- For example, if a number of transistors are interconnected on a single Si chip, we do not want inversion layers to be formed inadvertently between devices (generally called the field).
- One way to avoid such parasitic channels is to increase VT in the field by using a very thick oxide.
- Figure (2) illustrates a transistor with a gate oxide 10 nm thick and a field oxide of 0.5 μm.
- The value of Ci can also be controlled by varying ξi. A SiO2 layer which has some N incorporated in it, leading to the formation of a silicon oxynitride, is often used.
- Such silicon oxynitrides have slightly higher ξi and Ci than Si02, with excellent interface properties.
- Other high dielectric constant materials such as Ta205, Zr02 and ferroelectrics (e.g., barium-strontium-titanate) are also being investigated as replacements for Si02 as the gate dielectric in MOSFETs in order to increase Ci = ξi/d and, therefore, the drive current of the MOSFET.
- Generally speaking, we cannot use these high dielectric constant materials directly on the Si substrate; a very thin (-0.5 nm) interfacial Si02 layer is needed to achieve a low fast interface state density.
- It is clear from the expression for Ci that for these high dielectric constant materials, a physically thicker layer, d, can be used than for Si02 and still achieve a certain Ci.
- This is very useful for reducing the tunneling leakage current through the gate dielectric.
- A physically thicker layer implies a wider tunneling barrier with a reduced tunneling probability.