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Channel length modulation (CLM) is an effect in
field effect transistors The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs control ...
, a shortening of the length of the inverted channel region with increase in drain bias for large drain biases. The result of CLM is an increase in current with drain bias and a reduction of output resistance. It is one of several
short-channel effect In electronics, short-channel effects occur in MOSFETs in which the channel length is comparable to the depletion layer widths of the source and drain junctions. These effects include, in particular, drain-induced barrier lowering, velocity satur ...
s in
MOSFET scaling The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
. It also causes distortion in
JFET The junction-gate field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or resistors, or to build amplifier ...
amplifiers. To understand the effect, first the notion of pinch-off of the channel is introduced. 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. However, near the drain, the gate ''and drain'' jointly determine the electric field pattern. 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. 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). As 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''. 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 The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
operating in saturation. The effect is more pronounced the shorter the source-to-drain separation, the deeper the drain junction, and the thicker the oxide insulator. 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 Drain-induced barrier lowering (DIBL) is a short-channel effect in MOSFETs referring originally to a reduction of threshold voltage of the transistor at higher drain voltages. In a classic planar field-effect transistor with a long channel, the bo ...
, a drain induced lowering of threshold voltage. In bipolar devices, a similar increase in current is seen with increased collector voltage due to base-narrowing, known as the
Early effect The Early effect, named after its discoverer James M. Early, is the variation in the effective width of the base in a bipolar junction transistor (BJT) due to a variation in the applied base-to-collector voltage. A greater reverse bias across ...
. 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".


Shichman–Hodges model

In textbooks, channel length modulation in active mode usually is described using the Shichman–Hodges model, accurate only for old technology: where I_\text = drain current, K'_n = technology parameter sometimes called the transconductance coefficient, ''W, L'' = MOSFET width and length, V_\text = gate-to-source voltage, V_\text =
threshold voltage The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important s ...
, V_\text = drain-to-source voltage, V_\text = V_\text - V_\text, and λ = channel-length modulation parameter. In the classic Shichman–Hodges model, V_\text is a device constant, which reflects the reality of transistors with long channels.


Output resistance

Channel-length modulation is important because it decides the MOSFET
output resistance Output may refer to: * The information produced by a computer, see Input/output * An output state of a system, see state (computer science) * Output (economics), the amount of goods and services produced ** Gross output in economics, the value o ...
, an important parameter in circuit design of
current mirror A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being "copied" can be, and ...
s and
amplifiers An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
. In the Shichman–Hodges model used above, output resistance is given as: ::\begin r_\text &= \frac \\ &= \frac\left(\frac + V_\text\right) \\ &= \frac \end where V_\text = drain-to-source voltage, I_\text = drain current and \lambda = channel-length modulation parameter. Without channel-length modulation (for λ = 0), the output resistance is infinite. The channel-length modulation parameter usually is taken to be inversely proportional to MOSFET channel length ''L'', as shown in the last form above for ''r''O: ::\lambda \approx \frac, where ''V''E is a fitting parameter, although it is similar in concept to the
Early Voltage The Early effect, named after its discoverer James M. Early, is the variation in the effective width of the base in a bipolar junction transistor (BJT) due to a variation in the applied base-to-collector voltage. A greater reverse bias across ...
for BJTs. For a
65 nm process The 65  nm process is an advanced lithographic node used in volume CMOS ( MOSFET) semiconductor fabrication. Printed linewidths (i.e. transistor gate lengths) can reach as low as 25 nm on a nominally 65 nm process, while the pitc ...
, roughly ''V''E ≈ 4 V/μm. (A more elaborate approach is used in the EKV model. ). However, no simple formula used for λ to date provides accurate length or voltage dependence of ''rO'' for modern devices, forcing use of computer models, as discussed briefly next. 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. The main factor affecting the output resistance in longer MOSFETs is channel length modulation as just described. In shorter MOSFETs additional factors arise such as:
drain-induced barrier lowering Drain-induced barrier lowering (DIBL) is a short-channel effect in MOSFETs referring originally to a reduction of threshold voltage of the transistor at higher drain voltages. In a classic planar field-effect transistor with a long channel, the bo ...
(which lowers the threshold voltage, increasing the current and decreasing the output resistance),
velocity saturation Saturation velocity is the maximum velocity a charge carrier in a semiconductor, generally an electron, attains in the presence of very high electric fields. When this happens, the semiconductor is said to be in a state of velocity saturation. C ...
(which tends to limit the increase in channel current with drain voltage, thereby increasing the output resistance) and
ballistic transport In mesoscopic physics, ballistic conduction (ballistic transport) is the unimpeded flow (or transport) of charge carriers (usually electrons), or energy-carrying particles, over relatively long distances in a material. In general, the resistivity ...
(which modifies the collection of current by the drain, and modifies
drain-induced barrier lowering Drain-induced barrier lowering (DIBL) is a short-channel effect in MOSFETs referring originally to a reduction of threshold voltage of the transistor at higher drain voltages. In a classic planar field-effect transistor with a long channel, the bo ...
so as to increase supply of carriers to the pinch-off region, increasing the current and decreasing the output resistance). Again, accurate results require
computer models Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be deter ...
.


References and notes


External links


What is channel length modulation?
- OnMyPhD
MOSFET Channel-Length Modulation
- Tech brief


See also

*
Threshold voltage The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important s ...
*
Short channel effect In electronics, short-channel effects occur in MOSFETs in which the channel length is comparable to the depletion layer widths of the source and drain junctions. These effects include, in particular, drain-induced barrier lowering, velocity satu ...
*
Drain-induced barrier lowering Drain-induced barrier lowering (DIBL) is a short-channel effect in MOSFETs referring originally to a reduction of threshold voltage of the transistor at higher drain voltages. In a classic planar field-effect transistor with a long channel, the bo ...
* MOSFET operation *
Hybrid-pi model The hybrid-pi model is a popular circuit model used for analyzing the small signal behavior of bipolar junction and field effect transistors. Sometimes it is also called Giacoletto model because it was introduced by L.J. Giacoletto in 1969. The ...
*
Transistor models Transistors are simple devices with complicated behavior. In order to ensure the reliable operation of circuits employing transistors, it is necessary to scientifically model the physical phenomena observed in their operation using transistor mod ...
{{DEFAULTSORT:Channel Length Modulation Electronic design MOSFETs