Chapter 5

NMOS Transistor

Circuit symbols for various FETs

N&P-channel enhancement MOSFETs in Cutoff

Triode (Linear) and Saturation (B.P.O = Beyond Pinch Off)
N-channel enhancement MOSFET

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

Conditions for various modes of operation

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

Alternate method

Region	$v_{GS}\,$	$v_{DS}\,$
Cutoff	$v_{GS}<V_{to}\,$
Triode	Failed to parse (SVG with PNG fallback (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle v_ |- | Saturation |} Determining the Operating Region *Cutoff: If V<sub>GS</sub> is less than V<sub>to</sub> *Triode: If not Cutoff and V<sub>DG</sub> is less than V<sub>to</sub> *Saturation: If not Cutoff or Triode Determining V<sub>to</sub> *Enhancement: Build the channel *Depletion: Pinch-off the channel *JFET: Pinch-off the channel *'''Triode:''' :*The threshold voltage, <math>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 Analyze the DC circuit to find the Q-point (using nonlinear device equations or characteristic curves) Use the small-signal equivalent circuit to find the impedance and gains Small-signal equivalent circuits "Transconductance, gm, is an important parameter in the design of amplifier circuits. In general, better performance is obtained with higher values of gm."<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 rd is the drain resistance Type Voltage Gain Current Gain Power Gain Input Impedance Output Impedance Frequency Response Common-Source $A_{v}<1$ $A_{i}>1$ $G>1$ High Low Source Follower Common-Gate Questions 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 rd? Roughly what are the breakdown voltages for JFETs? Learn how to check for operating regions via Vgs & Vds as compared to Vto. CMOS nand/nor gates JFET only goes to IDSS Small signal model of mosfets References <references/>