The main factor affecting the output resistance in longer MOSFETs is channel length modulation as just described.
The effect of channel-length modulation upon the MOSFET output resistance varies both with the device, particularly its channel length, and with the applied bias. However, no simple formula used for λ to date provides accurate length or voltage dependence of r O for modern devices, forcing use of computer models, as discussed briefly next. (A more elaborate approach is used in the EKV model. For a 65 nm process, roughly V E ≈ 4 V/μm. Where V E is a fitting parameter, although it is similar in concept to the Early Voltage for BJTs. In textbooks, channel length modulation in active mode usually is described using the Shichman–Hodges model, accurate only for old technology: The similarity in effect upon the current has led to use of the term "Early effect" for MOSFETs as well, as an alternative name for "channel-length modulation". In bipolar devices, a similar increase in current is seen with increased collector voltage due to base-narrowing, known as the Early effect. In the weak inversion region, the influence of the drain analogous to channel-length modulation leads to poorer device turn off behavior known as drain-induced barrier lowering, a drain induced lowering of threshold voltage. The effect is more pronounced the shorter the source-to-drain separation, the deeper the drain junction, and the thicker the oxide insulator. Because resistance is proportional to length, shortening the channel decreases its resistance, causing an increase in current with increase in drain bias for a MOSFET operating in saturation. In the figure at the right, the channel is indicated by a dashed line and becomes weaker as the drain is approached, leaving a gap of uninverted silicon between the end of the formed inversion layer and the drain (the pinch-off region).Īs the drain voltage increases, its control over the current extends further toward the source, so the uninverted region expands toward the source, shortening the length of the channel region, the effect called channel-length modulation. Instead of flowing in a channel, beyond the pinch-off point the carriers flow in a subsurface pattern made possible because the drain and the gate both control the current. However, near the drain, the gate and drain jointly determine the electric field pattern. The channel is formed by attraction of carriers to the gate, and the current drawn through the channel is nearly a constant independent of drain voltage in saturation mode. To understand the effect, first the notion of pinch-off of the channel is introduced.
It also causes distortion in JFET amplifiers. It is one of several short-channel effects in MOSFET scaling. The result of CLM is an increase in current with drain bias and a reduction of output resistance. Effect in field effect transistors Cross section of a MOSFET operating in the saturation regionĬhannel length modulation ( CLM) is an effect in field effect transistors, a shortening of the length of the inverted channel region with increase in drain bias for large drain biases.
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