The threshold voltage of an n-channel MOSFET can be increased by
increasing the channel doping concentration
Read the threshold expression and every option answers itself :
\(V_{T}=V_{FB}+2\phi_{F}+\dfrac{\sqrt{2q\varepsilon_{s}N_{A}\left(2\phi_{F}\right)}}{C_{ox}}\qquad C_{ox}=\dfrac{\varepsilon_{ox}}{t_{ox}}\)
which reduces, for the purposes of this question, to
\(V_{T}\propto t_{ox}\sqrt{N_{A}}\)
| Change | Effect on VT | Why |
|---|---|---|
| NA increased | Rises | More depletion charge to balance before inversion |
| NA reduced | Falls | Less depletion charge |
| tox reduced | Falls | Cox rises, so the last term shrinks |
| Channel length reduced | Falls | Short-channel effect — VT roll-off |
So only option 1 raises the threshold. Physically, heavier substrate doping means a wider band of fixed acceptor charge must be depleted before the surface can invert, and the gate must supply the field to balance it — so more gate voltage is required.
Option 4 deserves a note because its answer is not obvious from the simple formula. As the channel shortens, the source and drain depletion regions occupy an appreciable fraction of it, and part of the depletion charge is then controlled by those junctions rather than by the gate. The gate has less work to do, so \(V_{T}\) falls — the threshold roll-off that makes short-channel devices hard to control and drives the need for halo implants and ever-thinner oxides.
The two other handles used in practice :
Body effect. Reverse-biasing the source-substrate junction widens the depletion region and raises the threshold:
\(V_{T}=V_{T0}+\gamma\left(\sqrt{2\phi_{F}+V_{SB}}-\sqrt{2\phi_{F}}\right)\)
Threshold-adjust implantation. A shallow ion implant into the channel shifts \(V_{FB}\), and it is how a CMOS process sets its n- and p-channel thresholds independently and symmetrically — a far more controllable method than adjusting the bulk doping, which would also change the body effect and the junction capacitances.
Hence, the threshold voltage is increased by increasing the channel doping concentration.
Similar Questions
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
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:
Approximate oxide capacitance value (Cgd) for saturation operating mode of MOS transistor 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 a MOSFET the drain saturation current is
The threshold voltage of a MOSFET can be lowered by
1. using a thinner gate oxide
2. reducing the carrier concentration in the substrate
3. increasing the carrier concentration in the substrateOf these statements :
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 :
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 :
Important Questions from MOSFET
Which industry does aluminium smelting belong to?
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
A switched mode power supply operating at 20 kHz to 100 kHz range uses as the main switching element is __________.
Which semiconductor power device out of the following, is not a current triggering device?
A CMOS amplifier when compared to an N–channel MOSFET, has the advantage of
