Channel length modulation in a MOSFET

[jaus tail]

Hi,
I'm struggling to understand how channel length modulation takes place in mosfet. The books n internet said that as drain side is positive, there is less potential difference between positive gate and positive drain, while there is more potential difference between positive gate and zero voltage source. thus the channel is less dense at the drain side.

but i don't understand this. shouldn't the positive drain also repel holes and help the positive gate in creating the channel? won't the two (gate and drain) act like voltage sources in parallel and help each other in creating the channel?

can anyone help me understand this?

 

[davenn]

Hi,
I'm struggling to understand how channel length modulation takes place in mosfet. The books n internet said that as drain side is positive, there is less potential difference between positive gate and positive drain, while there is more potential difference between positive gate and zero voltage source. thus the channel is less dense at the drain side.

hi there
this explanation may be easier to understand ...
https://en.wikipedia.org/wiki/Channel_length_modulation

One of several short-channel effects in MOSFET scaling, channel length modulation (CLM) is 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. Channel length modulation occurs in all field effect transistors, not just MOSFETs.

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 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, 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 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".

Dave

 

[trurle]

Hi,
I'm struggling to understand how channel length modulation takes place in mosfet. The books n internet said that as drain side is positive, there is less potential difference between positive gate and positive drain, while there is more potential difference between positive gate and zero voltage source. thus the channel is less dense at the drain side.

but i don't understand this. shouldn't the positive drain also repel holes and help the positive gate in creating the channel? won't the two (gate and drain) act like voltage sources in parallel and help each other in creating the channel?

can anyone help me understand this?

You are mixing in question onset of pinch-off and DIBL (drain-induced barrier lowering) effects - at onset of pinch-off 2nd derivative I_d(V_ds) is negative, but DIBL produce positive derivative of I_d(V_ds).

 

[jaus tail]

Sorry for the late reply. My exams are going on, so I just read the reply and didn't get back to you guys. What I'm struggling to understand is that if there is no channel, how do electrons go from channel-pinch off to drain? Is it like a capacitor where the electric field helps the electrons fly the space that doesn't have the channel?

Ids = K/2 (V_gs-V_t)² In Saturation Region.

 

[trurle]

Sorry for the late reply. My exams are going on, so I just read the reply and didn't get back to you guys. What I'm struggling to understand is that if there is no channel, how do electrons go from channel-pinch off to drain? Is it like a capacitor where the electric field helps the electrons fly the space that doesn't have the channel?

Ids = K/2 (V_gs-V_t)² In Saturation Region.

Very similar discussion with answer is on researchgate:
https://www.researchgate.net/post/Why_actually_the_pinch_off_happens_at_the_MOSFET_drain_region_when_drain_to_source_voltage_is_increasing_for_constant_VGS