![]() ![]() The threshold of both types of devices is slightly negative at low doping densities and differs by 4 times the absolute value of the bulk potential. Threshold voltage of n-type (upper curve) and p-type (lower curve) MOSFETs versus substrate doping density. ![]() The threshold voltage dependence on the doping density is illustrated with Figure 7.4.1 for both n-type and p-type MOSFETs with an aluminum gate metal. The threshold voltage of a p-type MOSFET with an n-type substrate is obtained using the following equations: Where the flatband voltage, V FB, is given by: The threshold voltage equals the sum of the flatband voltage, twice the bulk potential and the voltage across the oxide due to the depletion layer charge, or: In this section we summarize the calculation of the threshold voltage and discuss the dependence of the threshold voltage on the bias applied to the substrate, called the substrate bias effect. ÃŽ Si denotes the dielectric constant of Si.7.4 Threshold voltage 7.4.1. Threshold voltage calculation 7.4.2. The substrate bias effect The Equation and Q dep is the charge in the depletion region Imperfections in the silicon-dioxide interface and doping. ![]() Qf is the fixed charge due to surface states at the interface arise due to Polysilicon gate and the silicon substrate. Where f ms is the difference between the work functions of the Where V FB is the flat band voltage given by, Substrate is p-type and for PMOS transistor interface is p-type asįrom physics theory of NMOS transistor it is proved that the threshold for NMOS transistor interface is n-type as Transistor is defined as the gate voltage for which the oxide-substrate In semiconductor physics the threshold voltage (V TH) of a MOS The source and drain as shown in Figure below. If the gate-source voltageīecomes sufficiently negative an inversion layer consisting of holes isįormed at the oxide silicon interface providing a conduction path between Transistor but with all the polarities reversed. The turn ON phenomenon in a PMOS device is similar to that of NMOS If V GS rises further the charge in theĭepletion region remains relatively constant while the channel chargeĭensity continuously increases and provides greater current from source to Gate voltage at which this phenomenon occurs is called as the threshold Source and drain terminals and the transistor is turned 'ON'. Thus, a 'channel' of charge carriers is formed under the gate oxide between Hence a n-type channel is formedīetween the source and drain regions as shown in Figure below. The increased gate voltage results in additional electrons in the thin Where K is Boltzmann's constant, N A is substrate doping and n i is intrinsic carrier concentration of silicon. The gate potential is doubles the fermi potential (f F) strong The critical value at which n-type channel is created at the surface. Under this condition no current flowsīecause no charge carriers are available as shown in Figure below.īy increasing the gate potential surface inversion is starts and reaches Hence, the positive gate leads to positive charge to accumulate on gateĮlectrode and the negative charge to accumulate on the substrate whichįorms depletion region under gate. Holes from the p substrate starts repelling from the gate area under oxideĪnd leaves negative ions. When a positive voltage is applied to the gate with respect to source the Substrate forms the plates of a capacitor with the gate oxide as In very high resistance between drain and source. Therefore, both junctions have '0' V bias and considered OFF which results Substrate-Source and Substrate-Drain junctions. Consider a n-channel MOSFET connected to external voltages as shown inįigure below, Here, V GS = 0 and drain, source and bulk areĬonnected to ground, the drain and source are connected by back to back pn ![]()
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