If Z × L is the total channel area and C'par is the total input parasitic capacitance, then for microwave performance the cut off frequency can be defined as : (A) \(\dfrac{g_{m}}{2\pi\left(C'_{ca}+C'_{par}\right)}\) Choose the most appropriate answer from the options given below :
(B) \(\dfrac{g_{m}}{2\pi C_{gs}}\)
(C) \(\dfrac{g_{m}}{2\pi C_{gd}}\)
(D) \(\dfrac{g_{m}}{2\pi\left(ZLC_{ox}+C'_{par}\right)}\)
(A) and (D) Only
Statements (A) and (D) are the same expression written two ways — option 4 — and recognising that is the whole point of the question.
The cut-off frequency is the frequency at which the device's current gain falls to unity:
\(f_{T}=\dfrac{g_{m}}{2\pi C_{in}}\)
where \(C_{in}\) is the total capacitance the gate must drive.
Why (A) and (D) are identical. The channel capacitance is the oxide capacitance per unit area multiplied by the channel area:
\(C'_{ca}=Z\times L\times C_{ox}\)
Substituting that into (A) gives (D) exactly. The question defines \(Z\times L\) as the channel area in its own preamble, which is the hint that the two are meant to be recognised as one.
| Statement | Capacitance used | Verdict |
|---|---|---|
| (A) | Channel + parasitic | ✓ Complete |
| (B) | Cgs alone | ✗ Ignores the parasitics |
| (C) | Cgd alone | ✗ The wrong capacitance entirely |
| (D) | ZLCox + parasitic | ✓ The same as (A) |
Why the parasitic term cannot be dropped, which rules out (B). The intrinsic channel capacitance is only part of what the gate drives. Overlap capacitance where the gate extends over the source and drain diffusions, fringing capacitance from the gate edges, and the capacitance of the interconnect all add to it — and as devices are scaled down the intrinsic part shrinks while the overlap and fringing parts do not shrink as fast, so the parasitics come to dominate. An expression that omits them overestimates \(f_{T}\) badly for a real microwave device.
Why (C) is wrong in principle. \(C_{gd}\) is the feedback capacitance from output back to input. It matters greatly — through the Miller effect it is multiplied by the gain when referred to the input — but it is not the input capacitance, and it appears in the maximum oscillation frequency \(f_{max}\) rather than in \(f_{T}\).
How to raise fT : shorten the channel, which raises \(g_{m}\) and lowers \(C_{ca}\) together, giving roughly \(f_{T}\propto1/L^{2}\) in the long-channel limit — which is why the drive to shorter gates has been as much about speed as about density.
Hence, the correct statements are (A) and (D).
Similar Questions
The threshold voltage of an n-channel MOSFET can be increased by
Assertion (A) : MOS ICs based on MOSFET structure find wide applications in digital field.
Reason (R) : MOS ICs have small size and are easy to fabricate.
In MOSFET, the carrier velocity between constant mobility regime and the saturation velocity can be described as :
Consider the following statements :
(A) The origin of the punch through phenomena is the lowering of the barrier near the source.
(B) For a long channel device, a drain bias can change the effective channel length, but the barrier at the source end remains constant.
(C) For a short channel device, this barrier is no longer fixed.
(D) The lowering of the source barrier do not cause any injection of extra carriers.
(E) The punch through condition normally occurs inside the bulk region of the semiconductor.Choose the most appropriate answer from the options given below :
In MOSFET, the linear region current is :
(A) \(\dfrac{\mu_{n}C_{ox}w}{L}\left(V_{gs}-V_{th}-\dfrac{V_{ds}}{2}\right)V_{ds}\)
(B) \(\dfrac{\mu_{n}C_{ox}w}{L}\left(V_{gs}-V_{th}\right)V_{ds}\)
(C) \(\dfrac{\mu_{n}C_{ox}w}{2L}\left(V_{gs}-V_{th}\right)V_{ds}^{2}\)
(D) \(\dfrac{\mu_{n}C_{ox}w}{L}\left(V_{gs}-V_{th}\right)^{2}\)Choose the most appropriate answer from the options given below :
Approximate oxide capacitance value (Cgd) for saturation operating mode of MOS transistor is:
For an n-channel MOS transistor with $\mu_n$ = 600 cm2/Vs, Cox = 7 x 10-8 F/cm2, W = 40 $\mu_m$, L = 4$\mu_m$ and VTO=1.0 V, the value of K parameter is:
In MOS
A. The substrate fermi potential ϕF is negative in NMOS
B. The substrate fermi potential ϕF is positive in NMOS
C. The substrate bias voltage VSB is positive in NMOS, negative in PMOS
D. The substrate bias voltage VSB is negative in NMOS, positive in PMOS.
Choose the correct answer from the options given below:
In enhancement mode MOSFET the saturation (drain) current is given by
(a) \(K\dfrac{W}{L}(V_{gs}-V_{th})^{2}\)
(b) \(K\dfrac{W}{L}(V_{gs}-V_{th})(1+\lambda V_{ds})\)
(c) \(K\dfrac{W}{L}(V_{gs}-V_{th})^{2}(1+\lambda V_{ds})\)
(d) \(K\dfrac{W}{L}(V_{gs}-V_{th})^{2}(1-\lambda V_{ds})\)
Out of these
In a MOSFET the drain saturation current is
Important Questions from MOSFET
The O in a MOSFET stands for _______ layer which provides _______ to the device.
Which industry does aluminium smelting belong to?
In an N-channel MOSFET, the drain current ID increases as _______.
Consider an ideal long channel nMOSFET (enhancement-mode) with gate length 10 µm and width 100 µm. The product of electron mobility (µn) and oxide capacitance per unit area (COX) is µn COX = 1 mA/V2 . The threshold voltage of the transistor is 1 V. For a gate-to-source voltage VGS = [2 − sin (2t)] V and drain-to-source voltage VDS = 1 V (substrate connected to the source), the maximum value of the drain-to-source current is ________.
Given, Vgs is the gate-source voltage, Vds is the drain source voltage, and Vth is the threshold voltage of an enhancement type NMOS transistor, the conditions for transistor to be biased in saturation are
