Question 10.SG.Q.30: The source-body junction capacitance is approximately (a) 2 ......
Common Data for Questions 30 and 31: In the three-dimensional view of a silicon N channel MOS transistor shown in the figure, δ = 20 nm. The transistor is of width 1 mm. The depletion width formed at every P—N junction is 10 nm. The relative permittivities of Si and SiO_2, respectively, are 11.7 and 3.9, and \varepsilon_0=8.9 \times 10^{-12} F / m.
The source-body junction capacitance is approximately
(a) 2 fF (b) 7 fF
(c) 2 pF (d) 7 pF
Source-body junction capacitance
C_{ j }=\frac{\varepsilon A}{d}=\frac{\varepsilon_0 \varepsilon_{ r } A}{d}
Here, \varepsilon_{ r }=11.7 as the channel is of Si.
A=1 \mu m \times 0.2 \mu m =0.2 \times 10^{-12} m ^2d = Depletion width of P—N junction = 10 nm = 10^{-8} mn
Therefore,
C_{ j }=\frac{8.9 \times 10^{-12} \times 11.7 \times 0.2 \times 10^{-12}}{10^{-8}}=2 pF
Ans. (a)