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MOSFET Transistor

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Doping Silicon

doping silicon with phosphorus gas creates n-type silicon which has extra free valence electrons (shown in black).

doping silicon with boron gas creates p-type silicon which has a deficiency in valence electrons (shown in white) - i.e. a "hole" in the silicon matrix that readily accepts electrons.

ASK What happens when you put n-type next to p-type?

N-type silicon is silicon that has been chemically combined (doped) to make it conductive. A silicon atom has four electrons in its outer shell and bonds tightly with four surrounding silicon atoms creating a crystal matrix with eight electrons in the outer shells. However, phosphorus has five electrons, and when combined, the fifth electron becomes a "free" electron that moves easily within the crystal when a voltage is applied. Because the charge carriers are electrons, n-type refers to a negative charge.

In contrast, p-type silicon is silicon doped with boron gas that turns it into a conductive material that readily accepts electrons when voltage is applied. Boron has only three electrons in its outer shell and can bond with only three of the four surrounding silicon atoms. This leaves one silicon atom with a vacant location in its outer shell, called a "hole," that readily accepts an electron. Because the charge carriers are holes, p-type silicon is said to have a positive charge.

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Remember Silicon's properties?

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Transistor ON and OFF

OFF

ON

depletion region

electric

field

the goal is to turn the transistor ON and have current flow from Source to Drain - i.e. a low resistive path between Source and Drain.
RED is the depletion region
The source is so named because it is the source of the charge carriers (electrons for n-channel, holes for p-channel) that flow through the channel; similarly, the drain is where the charge carriers leave the channel.
With a large enough voltage at the gate (greater than the threshold Voltage V_th), the electric field is such that enough positively charged holes are forced away from the gate-insulator interface and the high concentration of negative charge carriers results in an inversion layer.
This "field effect" creates a conducting channel which extends between the source and the drain

current is only conducted through it when a voltage is applied between the source and the drain

Increasing the voltage on the gate leads to a higher electron density in the inversion layer and therefore increases the current flow between the source and drain.

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NMOS N-channel MOSFET

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NMOS

source (and body) are connected to ground

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NMOS

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NMOS Threshold Voltage (V_th)

The value of V_G(aka V_GS) at which there are sufficient electrons in the inversion layer to make a low resistance conductive path (i.e. a channel) between the source and drain.

if V_GS> V_th

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NMOS Threshold Voltage (V_th)

The threshold voltage (V_th) for an NMOS transistor is typically 0.7V - 1.0V and is the determined by the physical properties of the transistor

choice of oxide material
thickness of oxide
variation in implanted impurity concentrations (i.e. doping)

as well as temperature.

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NMOS Summary

To create a conductive channel in the p-type silicon between the source and the drain, we must attract electrons to the region beneath the gate.

To attract enough electrons to the region beneath the gate, the voltage V_GS must be sufficiently positive (i.e. V_GS > V_th)

To then get current to flow through this conductive channel from the source to the drain, V_DS (the voltage between the source and the drain) must be positive.

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NMOS

Gate vs Threshold Voltage

V_GS< V_th If the gate voltage (V_GS) is below the threshold voltage (V_th), the transistor is OFF since there is no channel and hence no current can flow from the drain to the source.
V_GS> V_thIf the gate voltage (V_GS) is above the threshold voltage (V_th), the transistor is ON since now there is a channel and hence current can flow from the drain to the source.

V_GS>> V_thThe larger V_GS, the larger the channel & the larger the amount of current that can flow.

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NMOS ON Two Modes

When the transistor is on (i.e. V_GS > V_th) it operates in two modes depending on V_DS

V_DS < V_GS - V_thLinear mode

� V_GS - V_DS> V_thor

V_GD< V_th

V_GD> V_th

V_DS > V_GS - V_th Saturation mode

V_GS - V_DS< V_thor

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NMOS ON Linear Mode V_GS > V_th& V_GD> V_th

Channel exists all the way between source and drain. A strong inversion region is present even near drain.

V_DS < V_GS - V_thLinear mode

� V_GS - V_DS> V_thor

V_GD> V_th

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NMOS ON Saturation Mode V_GS > V_th& V_GD< V_th

The conductive channel is pinched off near drain.

Although the channel does not extend the full length of the device, the electric field between the drain and the channel is still high enough for current conduction to continue.

V_GD< V_th

V_DS > V_GS - V_th Saturation mode

V_GS - V_DS< V_thor

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NMOS Off (cutoff) & ON

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I_D_rainvs V_D_rain_S_ource various V_G_ate_S_ourcevalues

linear mode�V_GS - V_DS> V_th

saturation mode�V_GS - V_DS< V_th

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NMOS in 3 Dimensions

What can be changed to change the current

A NMOS transistor on a chip is not a perfect switch - it is so small it can only conduct a limited amount of current between the source and the drain.

Why would we want more current

Why not

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Spice - MOSFET Transistor

Mname MOS transistor must start with a M, name must be unique for each transistor
ND, NG, NS, and NB are the node names¹ of the Drain, Gate, Source and Bulk terminals. In a CMOS circuit always connect PMOS Bulk to V_DD & NMOS Bulk to 0 (aka GND).
ModName is the name of the PMOS or NMOS transistor simulation model
# is the length and width of the gate (in m).

Example NMOS and PMOS transistor netlist

Mname ND NG NS NB ModName W=# L=#

Mn0 out inA net0 0 ibm0p13nmos W=2.0u L=0.12u

Mn1 net0 inB 0 0 ibm0p13pmos W=2.0u L=0.12u

¹the number 0 as a node name represents GROUND

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NMOS Simulation

Objective

Simulate the I_DRAIN vs V_DS of a NMOS transistor @ various V_GateSource values