MOSFET Threshold Voltage Calculator - Essential Tool for Electronics Engineers
The MOSFET Threshold Voltage Calculator is a useful tool in semiconductor physics and microelectronics, designed to determine the threshold voltage of a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). The threshold voltage (VT0) of a MOSFET is the gate voltage at which the MOSFET starts conducting current. It is a crucial parameter because it determines when the MOSFET will turn on or off, thus it is fundamental in device operation.
The calculator makes use of several input parameters, each of which contributes to the final calculated value of the threshold voltage. These parameters are:
- Capacitance of the insulator (C₀) [F/cm²]: This parameter represents the capacitance per unit area of the insulator layer (usually silicon dioxide) between the gate and the channel in the MOSFET. The insulator capacitance influences the gate's ability to control the channel, thus affecting the threshold voltage. It's typically given in Farads per square centimeter.
- Doping of the substrate (NA): This is the concentration of impurities (dopants) in the substrate, typically given in per cubic centimeter. The type and level of doping in the substrate greatly affect the properties of the MOSFET, including the threshold voltage.
- Relative permittivity (Ɛr): The relative permittivity is a measure of a material's ability to store electrical energy in an electric field. In the case of a MOSFET, it usually refers to the relative permittivity of the semiconductor material forming the channel.
- Surface potential (ϕF) [V]: Surface potential refers to the electric potential difference at the semiconductor-oxide interface. It is a key parameter in determining the charge density at the surface of the semiconductor, which in turn affects the threshold voltage.
- Temperature (T) [K]: The temperature at which the MOSFET operates can have significant effects on its characteristics, including the threshold voltage. This is because the carrier concentration, mobility, and other properties are temperature-dependent.
- Source-body voltage (Vsb) [V]: This is the voltage difference between the source and the body (or substrate) of the MOSFET. An increase in the source-body voltage can lead to an increase in the threshold voltage, a phenomenon known as the body effect.
- Intrinsic concentration (ni) [×10¹⁵/m³]: Intrinsic concentration is the number of free carriers (electrons and holes) in the semiconductor under equilibrium conditions at a given temperature, when no external voltage is applied. It can affect the behavior of the MOSFET, including its threshold voltage.
- Charge (q): In this context, the charge 'q' usually refers to the elementary charge (the charge of a single electron), which is approximately 1.6 × 10^-19 Coulombs. It's a fundamental physical constant used in the calculation.
- Threshold voltage (VT0) [V]: This is the output of the calculator, the voltage at which the MOSFET begins to conduct. If the gate voltage is less than VT0, the MOSFET is off (in cutoff region); if it's greater, the MOSFET is on (in saturation or triode region, depending on other parameters).
All these parameters must be known or estimated to calculate the threshold voltage accurately. With these parameters and the knowledge of the MOSFET structure and operating conditions, the calculator can provide an estimate of the MOSFET's threshold voltage.
The most commonly used formula for the threshold voltage (V_T) of a MOSFET is given by:
V_T = V_T0 + γ(√(2φF + V_SB) - √2φF)
- V_T0 is the zero-bias threshold voltage, the gate voltage necessary for inversion at zero source-bulk voltage
- γ is the body effect parameter or substrate bias factor
- φF is the Fermi potential
- V_SB is the source-bulk voltage
The body effect parameter γ (gamma) can be calculated as:
γ = √(2qε_sε_0NA) / C_ox
- q is the charge of an electron, approximately 1.6 x 10^-19 Coulombs
- ε_s is the relative permittivity of the semiconductor
- ε_0 is the permittivity of free space, approximately 8.85 x 10^-12 F/m
- NA is the doping concentration in the substrate
- C_ox is the oxide capacitance per unit area
The Fermi potential φF is given by:
φF = kT/q * ln(NA/ni)
- k is the Boltzmann constant, approximately 1.38 x 10^-23 J/K
- T is the absolute temperature in Kelvin
- NA is the doping concentration in the substrate
- ni is the intrinsic carrier concentration of the semiconductor
The zero-bias threshold voltage V_T0 is typically given by the manufacturer of the MOSFET, but can be influenced by device fabrication and other factors.
These equations combine the fundamental properties of the MOSFET and its materials, allowing us to calculate the threshold voltage as a function of various parameters. Please note that these are basic equations and many modern MOSFETs have more complicated behaviors due to short-channel effects, high doping levels, non-uniform substrate doping, and other factors.
It is believed that these calculations are accurate, but not guaranteed. Use at your own risk!