Bologna / 06_TCAD_laboratory_MOSFET_GBB_20150223H1655

Technology Computer Aided

Design (TCAD) Laboratory

Lecture 6, Metal-Oxide-

Semiconductor Field-Effect-

Transistor (MOSFET)

[Source: Synopsys]

Giovanni Betti Beneventi

E-mail: [e-mail] ; [e-mail]

Office: Engineering faculty, ARCES lab. (Ex. 3.2 room), viale del Risorgimento 2, Bologna

Phone: [phone]

Advanced Research Center on Electronic Systems (ARCES)

University of Bologna, Italy

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Outline

•Review of basic properties of the MOSFET

•Sentaurus Workbench setup (SWB)

•Implementation of Input files

–Sentaurus Structure Editor (SDE) command file

–Sentaurus Device (SDevice)

•command file

•parameter file

•Run the simulation

•Post-processing of results

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The MOSFET: a qualitative introduction (1)

•MOSFET= Metal-Oxide-Semiconductor Field-Effect Transistor

•Workhorse of all digital systems since ‘70s

–Good electrical switch (little parasitic effects)

–High integration density and relatively simple manufacturing process

•Structure

–4 terminals device: source (S), drain (D), gate (G), bulk (B)

–The voltage applied to the gate determines if and how much current flows between the source and the drain port. The body serves to modulate device characteristics and parameters.

–The device can be considered as a voltage-controlled switch. When the voltage applied to the gate is

larger than a given value called the threshold voltage , a conducting channel is formed between drain and source. Then, in the presence of a voltage difference between drain and source, current flows. Conversely, when the gate voltage is lower than no channel exists and the switch is open.

NMOS

(bulk contact not shown)

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The MOSFET: a qualitative introduction (2)

•Two types of MOSFET:

–NMOS: n+ drain and source regions embedded in a p-type substrate. Above , the current is carried out by electrons (drift) moving through an n-type channel between source and drain.

–PMOS p+ drain and source regions embedded in a n-type substrate. Above , the current is carried out by holes (drift) moving through a p-type channel between source and drain.

–Since (above ) either electrons or holes contribute to current flow, MOSFET is said to be a unipolar device (in pn-junction diodes both electrons and holes contribute to the current, so the diode is a bipolar device).

–In the most used silicon technology, the CMOS (Complementary MOS) technology, both NMOS and PMOS are present.

NMOS

PMOS

n-substrate

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The MOSFET: a qualitative introduction (3)

Consider an NMOS

•= , drain, source and bulk connected to ground. The substratesource and substrate-drain junctions are reverse-biased pn-junctions

high-resistance between source and drain

•0 < < , gate and substrate form the plates of a capacitor with the gate oxide as the dielectric. The positive gate voltage causes positive charge to accumulate on the gate electrode and negative charge on the substrate side. The latter manifests itself initially by repelling mobile holes. Hence, a depletion region is formed below the gate. This depletion region is similar to the one occurring in a pnjunction diode.

•= : as the gate voltage increases, the potential at the silicon surface at some point reaches a critical value, where the semiconductor surface inverts to n-type material. Further increases in the gate voltage produce no further changes in the depletion-layer width, but results in additional electrons at the thin inversion layer directly under the oxide. These are drawn in the inversion layer from the heavily doped n+ source region, the conductivity of which is

modulated by the gate-source voltage. The value of where strong inversion occurs is called the threshold voltage . is a function of several quantities, most of which are material constants, such as the difference in the work-function between gate and substrate material, the oxide thickness, the impurity charge trapped at the surface between channel and gate oxide and the doping.

= 0

0 < <

>

The MOSFET: basic operation regimes (1)

•Resistive operation

•> , small voltage applied between drain and source.

•	=	−	>		channel

•Current flow is due to drift of electrons. It can be shown that

												2
		= ′					−			−
												2

where	′	=		=

effective transistor width*

(* extending perpendicular to the plane of the slide)effective channel length

electron mobility

silicon oxide thicknessdielectric permittivity

•For small the quadratic term can be dropped and the − characteristics is linear (i.e. the transistor behaves like a resistor), therefore the MOSFET is said to

be working in the linear or ohmic region.

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The MOSFET: basic operation regimes (2)

•Saturation region

•> , high voltage applied between drain and source.

•As the value of is further increased, the assumption that the channel voltage ( ) is larger

than		all along the channel ceases to hold. At	where	<	. .	> the induced
				,
charge is zero, and the conducting channel disappears or is pinched-off.

•It can be shown that, under these conditions, the conductive channel thickness is gradually reduced from source to drain until pinch-off occurs. Under these conditions (at least one pinch-

off point for which < at the drain region), the transistor is said to be working in the saturation region, and the − equation reads

= ′ ( − )2 2

• In saturation, ideally, does not depend on and has a squared dependence on

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Channel length modulation

•According to the equation in the saturation region, it seems that current

between source and drain contact does not depend on V . Actually, this is only a first order approximation. The effective length of the conductive channel is indeed modulated by : increasing V causes the depletion region at the drain junction to grow due to the fact that at the transistor channel is decreased. This means that the actual is reduced, hence increases.

•A more accurate description of current in MOS under saturation condition is

′

=

−

2(1 + )

V

dependence through

2

where is an empirical parameter called channel-length modulation.

•In shorter transistor, the depletion region at the drain junction is larger (not to be used as current sources).

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Velocity saturation

•The behavior of transistors with very short channel lengths (short-channel devices) deviates considerably from the resistive and saturated models presented so far. The main cause of this discrepancy is the velocity saturation effect. In fact, while for simplicity it is usually indicated that the carrier velocity = , actually = ( ), and in particular, at high fields, is reduced due to scattering effects (collisions suffered by the carriers).

•When the electric field along the channel (i.e. longitudinal component of the

electric field) reaches a critical value , the velocity of the carriers tends to saturate to (i.e. increase counterbalanced by decrease).

p-type Silicon channel

= 105 m/s

constant velocity constant mobility = slope

Models of velocity saturation can be introduced in the Sdevice command file in the Mobility physics section by specifying a Field-dependent model

~ 1.5 x 104 V/cm (higher for n-type Silicon channels)	G. Betti Beneventi 9

Drain current vs. Voltage chart (1): output characteristics

Considering NMOS devices.

Output characteristics= vs. at fixed

resistive: voltage-controlled resistor	saturation at small due to
saturation: voltage-controlled current-source	velocity saturation effects

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