Calculate the cutoff frequency of an ideal MOSFET with a constant mobility.

Assume that the electron mobility in an n-channel device is $\mu_{n}=400 \mathrm{~cm}^{2} / \mathrm{V}-\mathrm{s}$ and that the channel length is $L=4 \mu \mathrm{m}$ . Also assume that $V_{T}=1 \mathrm{~V}$ and $V_{G S}=3 \mathrm{~V}$ .

Question

Calculate the cutoff frequency of an ideal MOSFET with a constant mobility.

Assume that the electron mobility in an n-channel device is [latex]\mu_{n}=400 \mathrm{~cm}^{2} / \mathrm{V}-\mathrm{s}[/latex] and that the channel length is [latex]L=4 \mu \mathrm{m}[/latex]. Also assume that [latex]V_{T}=1 \mathrm{~V}[/latex] and [latex]V_{G S}=3 \mathrm{~V}[/latex].

Accepted Answer

From Equation (10.96), the cutoff frequency is

[latex]\begin{array}{c}f_{T}=\frac{g_{m}}{2 \pi C_{G}}=\frac{\frac{W \mu_{n} C_{\mathrm{ox}}}{L}\left(V_{G S}-V_{T} ight)}{2 \pi\left(C_{\mathrm{ox}} W L ight)}=\frac{\mu_{n}\left(V_{G S}-V_{T} ight)}{2 \pi L^{2}} \ \end{array}[/latex] (10.96)

[latex]\begin{array}{c} f_{T}=\frac{\mu_{n}\left(V_{G S}-V_{T} ight)}{2 \pi L^{2}}=\frac{400(3-1)}{2 \pi\left(4 imes 10^{-4} ight)^{2}}=796 \mathrm{MHz}\end{array}[/latex]

Comment

In an actual MOSFET, the effect of the parasitic capacitance will substantially reduce the cutoff frequency from that calculated in this example.

Question 10.10: Calculate the cutoff frequency of an ideal MOSFET with a con...

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