Question

+ LTE . 11:07 multiple_choice Leliji > Compared to long channel devices, in short channel MOS transistors a. The saturation current is i. Smaller ii. Equal iii. Larger iv. Doubled b. The Threshold voltage is i. Larger ii. Smaller iii. Equal iv. equal to VDD c. The carrier mobility in the channel is i. Larger ii. Smaller iii. Equal iv. equal for electrons and holes d. The MOSFET current saturates at a smaller drain voltage due to i. Pinch off ii. Velocity saturation iii. High channel resistance iv. Very low channel resistance f. The drain and source series resistance are i. More important 11. less important iii. equally important iv. None of the above e. The electron velocity in the channel is i. more dependent on the electric field ii. less dependent on the electric field iii. directly proportional to the electric field iv. equal to the hole velocity

Answer 1

a). iii. Larger

In long channel MOS transistors, the drain current becomes constant in saturation while in short channel MOS transistors, on increasing drain to source voltage the drain current increases continuously. Hence saturation current is larger. This is evident from following characteristics.

n-channel V = 1, 2, 3, 4.5 V L = 0.5, 0.75 m W = 20 am le (MA) III 2 3 4 Vos (V)

Here Solid line represents drain current for long channel devices and dotted line represent drain current for short channel devices which is clearly larger.

b) ii. The Threshold voltage decreases in short channel length MOSFET

THRESHOLD VOLTAGE (V) O ON EXP - CALCULATED V10V 2 8 9 10 3 4 5 6 7 CHANNEL LENGTH (um)

In long channel MOSFETs, gate has control over the channel and supports most of the charge. As we go to short channel lengths as seen in the graph above, the threshold voltage begins to decrease as the charge in the depletion region is now supported by the drain and the source also. Thus the gate needs to support less charge in this region and as a result, VT falls down. This phenomenon is known as charge sharing effect. Now since IDS is propotional to (VGS - VT), therefore as VT begins to fall in case of short channels, IDS starts increasing resulting in larger drain currents. Also when VGS is zero and the MOSFET is in the cut off mode, since VT is small, (VGS - VT) will be a small negative value and will result in leakage current which further multiplied by the drain voltage will result in leakage power. In case of long channel MOSFETs, VT is large enough and (VGS - VT) is a comparatively larger negative value, in cut off mode leakage power is very small.

c) ii. The mobility becomes smaller

NAV- <100 O, - 1. Noi - 1.25 x 10cm Na - 1.33 x 10 (V- us . (cm?NV-sec) 2 x 10 4 6 8 109 Eelf (V/cm)

Mobility degradation graph

We can describe this mobility degradation by two effects:

i. Lateral Field Effect: In case of short channels, as the lateral field is increased, the channel mobility becomes field-dependent and eventually velocity saturation occurs . This results in current saturation.

ii. Vertical Field Effect: As the vertical electric field also increases on shrinking the channel lengths, it results in scattering of carriers near the surface. Hence the surface mobility reduces. Thus for short channels, we can see the mobility degradation which occurs due to velocity saturation and scattering of carriers.

Pso Hs = where, E 1+1 Eo Er is the effective vetical electric field and Ec is the critical electric field.

d) ii The MOSFET drain current saturates due to velocity saturation at smaller drain voltage

In case of short channels, as the lateral field is increased, the channel mobility becomes field-dependent and eventually velocity saturation occurs. This results in current saturation

Velocity ration

Figure : IDS Vs VGS for short channel

f) i More Important

As MOSFET channel length decreases, the influence of parasitic source-drain series resistance on the current characteristics becomes more and more important and can overtake the pure channel resistance

e) i It becomes more field dependent.

In short channel MOSFET, lateral field effect occurs due to which electron mobility decreases and hence velocity decreases.