Chapter 5

NMOS Transistor

NMOS & PMOS in Cutoff

Triode and Saturation

• N-channel enhancement-mode MOSFET (metal-oxide semiconductor field effect transistor)

Nmos regions

Region	Conditions	$i_{D}\,$
Cutoff	$v_{GS}<V_{to}\,$	0
Triode	$v_{DS}\leq v_{GS}-V_{to}\,$ and $v_{GS}\geq V_{to}$	$K[2(v_{GS}-V_{to})v_{DS}-v_{DS}^{2}]$
Saturation	$v_{DS}\geq v_{GS}-V_{to}\,$ and $v_{GS}\geq V_{to}$	$K(v_{GS}-V_{to})^{2}\,$
Boundry	$v_{GS}-v_{DS}=V_{to}\,$	$Kv_{DS}^{2}$

• Triode:

• The threshold voltage, $V_{to}$, is the minimum $V_{GS}$ needed to move the transistor from the Cutoff to Triode region. When is reached, a channel forms beneath the gate, allowing current to flow.

• $V_{to}$ is usually on the order of a couple of volts

• For small values of $V_{DS}$, $i_{D}$ is proportional to $V_{DS}$. The device behaves as a resistor whose value depends on $v_{GS}$

• Saturation:

• "Now consider what happens if we continue to increase $V_{DS}$. Because of the current flow, the voltages between points along the channel and the source become greater as we move toward the drain. Thus, the voltage between gate and channel becomes smaller as we move toward the rain, resulting in a tapering of the channel thickness as illustrated in Figure 5.5. Because of the tapering of the channel, its resistance becomes larger with increasing $v_{DS}$, resuling in a lower rate of increase of $i_{D}$." <ref>Electronics p. 291</ref>

MOSFET analysis

1. Analyze the DC circuit to find the Q-point (using nonlinear device equations or characteristic curves)
2. Use the small-signal equivalent circuit to find the impedance and gains

Small-signal equivalent circuits

• "Transconductance, g_m, is an important parameter in the design of amplifier circuits. In general, better performance is obtained with higher values of g_m."<ref>Electroincs p. 310</ref>
• Transconductance is defined as $g_{m}=2K(V_{GSQ}-V_{to})={\sqrt {2KP}}{\sqrt {W/L}}{\sqrt {I_{DQ}}}$.

• $i_{d}=g_{m}v_{gs}+{\frac {v_{ds}}{r_{d}}}$, where r_d is the drain resistance

caption

Type	Voltage Gain	Current Gain	Input Resistance	Output Resistance	Frequency Response
Common-Source	cell
Source Follower	cell

Questions

• Are FETs ever used in the triode region? Or is this just to have a "resistor" FET?
• What's the difference between the enhancement and depletion modes?
• NMOS and BJTs seem very similar. Why would you use one over the other?
• How do you find r_d?

References

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