Semiconductor Threshold Voltage Control & Ion Implantation
This document details various parameters that can be adjusted to change the threshold voltage (VT) in semiconductor devices, along with an explanation of how ion implantation affects VT.
Threshold Voltage Adjustment Parameters
The threshold voltage (VT) of a semiconductor device, such as a MOSFET, can be precisely controlled by adjusting several key parameters:
Doping Concentration (Nsub): Doping concentration in the substrate affects the threshold voltage. Increasing the substrate’s doping concentration (Nsub) can decrease the threshold voltage (VT) because the gate’s electric field needs to counteract a stronger background charge.
Gate Material and Work Function (ϕ_m): The choice of gate material and its work function influences the threshold voltage. A gate with a higher work function can increase VT, while a lower work function can decrease it. This is due to a change in the energy barrier for charge carriers moving between the gate and the channel.
Oxide Thickness (Tox): The thickness of the oxide layer between the gate and the channel affects VT. Increasing the oxide thickness can increase VT as it increases capacitance, making it harder for the gate to influence the channel.
Channel Length (L): Reducing the channel length can increase VT, as shorter channels exhibit higher electric fields, requiring a stronger gate voltage to control the channel.
Channel Doping Concentration (Nch): Modifying the doping concentration in the channel region can impact VT. Higher doping concentrations tend to decrease VT, making the device easier to turn on.
Ion Implantation Dose and Energy: Ion implantation involves bombarding the substrate with ions to alter its properties. By adjusting the ion implantation dose (number of ions per unit area) and energy (kinetic energy of ions), the doping concentration and distribution in the substrate can be modified, thus affecting VT.
Ion Implantation & Threshold Voltage
Ion implantation involves introducing dopant ions into the substrate by accelerating them to high energies and embedding them into the crystal lattice. This process creates localized regions of altered doping concentration. When ion implantation is used to increase doping concentration in the channel region of a MOSFET, the following effects are observed:
Increased Doping Concentration: Ion implantation increases the doping concentration in the channel, reducing the channel region’s resistance.
Shift in Threshold Voltage: As doping concentration increases, the threshold voltage tends to decrease. This is because a stronger background charge in the channel makes it easier for the gate voltage to control the channel’s conductivity.
Change in Carrier Mobility: Ion implantation can also affect carrier mobility, impacting the rate at which charge carriers move in the channel. However, the effect on mobility might not be as significant as the change in doping concentration.
Subthreshold Slope: Ion implantation can affect the subthreshold slope, which is the rate of change of drain current with respect to gate voltage in the subthreshold region. A steeper subthreshold slope is desirable for better control over transistor behavior.
In conclusion, ion implantation is a powerful technique for modifying the threshold voltage of semiconductor devices by altering the doping concentration. By carefully adjusting ion implantation parameters such as dose and energy, the threshold voltage can be effectively tuned to achieve desired device characteristics.