2.3 Basic MOSFET Operation

2.3 Basic MOSFET Operation

In the MOSFET, an inversion layer at the semiconductoroxide interface acts as a

conducting channel. For example, in an nchannel MOSFET, the substrate is ptype

silicon and the inversion charge consists of electrons that form a conducting channel

between the n + ohmic source and the drain contacts. At DC conditions, the depletion

regions and the neutral substrate provide isolation between devices fabricated on the

same substrate. A schematic view of the nchannel MOSFET is shown in Figure 2.10.

As described above for the MOS capacitor, inversion charge can be induced in the

channel by applying a suitable gate voltage relative to other terminals. The onset of

strong inversion is defined in terms of a threshold voltage VT being applied to the gate

electrode relative to the other terminals. In order to assure that the induced inversion

channel extends all the way from source to drain, it is essential that the MOSFET gate

structure either overlaps slightly or aligns with the edges of these contacts (the latter is

achieved by a selfaligned process). Selfalignment is preferable since it minimizes the

parasitic gatesource and gatedrain capacitance.

When a drainsource bias VDS is applied to an nchannel MOSFET in the abovethreshold

conducting state, electrons move in the channel inversion layer from source to drain. A

change in the gatesource voltage VGS alters the electron sheet density in the channel,

modulating the channel conductance and the device current. For VGS > VT in an nchannel

device, an application of a positive VDS gives a steady voltage increase from

source to drain along the channel that causes a corresponding reduction in the local gatechannel

bias VGX (here X signifies a position x within the channel). This reduction is

greatest near drain where VGX equals the gatedrain bias VGD.

Somewhat simplistically, we may say that when VGD = VT, the channel reaches threshold

at the drain and the density of inversion charge vanishes at this point. This is the socalled

pinchoff condition, which leads to a saturation of the drain current Ids. The

corresponding drainsourcevoltage, VDS = VSAT, is called the saturation voltage. Since

VGD = VGS – VDS, we find that VSAT = VGS – VT.

When VDS > VSAT, the pinchedoff region near drain expands only slightly in the

direction of the source, leaving the remaining inversion channel intact. The point of

transition between the two regions, x = xp, is characterized by XS ( p ) SAT V x » V , where

( ) XS p V x is the channel voltage relative to source at the transition point. Hence, the drain

current in saturation remains approximately constant, given by the voltage drop VSAT

across the part of the channel that remains in inversion. The voltage VDS – VSAT across

the pinchedoff region creates a strong electric field, which efficiently transports the

electrons from the strongly inverted region to the drain.

Typical currentvoltage characteristics of a longchannel MOSFET, where pinchoff is

the predominant saturation mechanism, are shown in the following figure.

However, with shorter MOSFET gate lengths, typically n the submicrometer range,

velocity saturation will occur in the channel near drain at lower VDS than that causing

pinchoff. This leads to more evenly spaced saturation characteristics than those shown in

this figure, more in agreement with those observed for modern devices. Also, phenomena

such as a finite channel conductance in saturation, a drain biasinduced shift in the

threshold voltage, and an increased subthreshold current are important consequences of

shorter gate lengths.