Semiconductor Physics and DevicesChapter 11. MOSFET: Additional Concepts

& Thin Film Transistors (TFTs)

Seong Jun KangDepartment of Advanced Materials Engineering for Information and Electronics

Laboratory for Advanced Nano Technologies

Nonideal Effect

The experimental characteristics of MOSFETs deviate to some degree from the ideal relationsthat have been theoretically derived.

Nonideal Effect: Subthreshold Conduction

The experimental characteristics of MOSFETs deviate to some degree from the ideal relationsthat have been theoretically derived.

Subthreshold Conduction

The ideal current-voltage relationship predicts zero drain current when the gate-to-source voltageis less than or equal to the threshold voltage.

Figure shows a comparison between the idealcharacteristic that was derived, and theexperimental results.

The drain current, which exists for , isknown as the subthreshold current.

Nonideal Effect: Subthreshold Conduction

Subthreshold Conduction

Current-Voltage Relationship

Figure (a) shows an n-channel enhancement mode MOSFET with a gate-to-source voltage that isless than the threshold voltage and with only a very small drain-to-source voltage.

The source and substrate terminals are held at ground potential.

There is no electron inversion layer, the drain-to-substrate pn junction is reverse biased, and thedrain current is zero.

Current-Voltage Relationship

Figure (b) shows an n-channel enhancement mode MOSFET with an applied gate voltage higherthan threshold voltage.

An electron inversion layer has been created so that when a small drain voltage is applied, theelectrons in the inversion layer will flow from the source to the positive drain terminal.

The conventional current enters the drain terminal and leaves source terminal.

Nonideal Effect: Subthreshold Conduction

Subthreshold Conduction

Figure shows the energy-band diagram of an MOS structure with a p-type substrate biased sothat 2 .

At the same time, the Fermi level is closer to the conduction band than the valence band, so thesemiconductor surface develops the characteristics of a lightly doped n-type material.

Some conduction between the source and drain contacts would be observed through thisweakly inverted channel.

The condition for 2 is known asweak inversion.

Nonideal Effect: Subthreshold Conduction

In the case of 2 , the Fermi level at the surface is far above the intrinsic Fermi level.

The electron concentration at the surface is same as the hole concentration in the bulk.

This condition is the threshold inversion point, and the applied gate voltage creating thiscondition is the threshold voltage.

In the case of 2 , weak inversion would be observed.

The threshold voltage is defined as the applied gate voltage required to achieve the thresholdinversion point2 : Threshold inversion condition for p-type semiconductor

Nonideal Effect: Subthreshold Conduction

Figure shows the surface potential along the length of the channel at accumulation, weakinversion, and threshold for the case when a small drain voltage is applied.

There is a potential barrier between the source and channel region which the electrons mustovercome in order to generate a channel current. (figure (b) and (c)) Similar to the pn junction, the current is an exponential function of .

In the inversion mode (figure (d)), the barrier is so small that is more like an Ohmic contact. The current is proportional to the .

Nonideal Effect: Channel Length Modulation

We assumed the ideal current-voltage relationship with the channel length L was constant.

However, when the MOSFET is biased in the saturation region, the depletion region at the drainterminal extends laterally into the channel, reducing the effective channel length.

Since the drain current is inversely proportional to the channel length,

The I-V curves in the saturation region shows a positive slopes due to channel length modulation.

Nonideal Effect: Channel Length Modulation Nonideal Effect: Channel Length Modulation

Nonideal Effect: Ballistic Transport

Scattering events in a semiconductor limit the velocity of carriers to an average drift velocity.

The average drift velocity is a function of the mean time between collisions or the mean distancebetween scattering events.

As the MOSFET channel length is reduced, the mean distance between collisions may becomecomparable to the channel length.

If the channel length is further reduced (in submircron), then carriers could travel from the sourceto the drain without experiencing a scattering event.

This motion of carriers is called ballistic transport.

Ballistic transport means that carriers travel faster than the average drift velocity, and this effectcan lead to very fast devices.

Thin Film Transistors

Gate, Source and Drain Electrodes with an active channel materials and gate insulator.

No pn junction

Introduction

The first transistor was invented at Bell Laboratories on December 16,1947 by William Shockley (seated at Brattain's laboratory bench), JohnBardeen (left) and Walter Brattain (right).

This was perhaps the most important electronics event of the 20thcentury, as it later made possible the integrated circuit andmicroprocessor that are the basis of modern electronics. Prior to thetransistor the only alternative to its current regulation and switchingfunctions was the vacuum tube, which could only be miniaturized toa certain extent, and wasted a lot of energy in the form of heat.

Introduction

A transistor is a semiconductor device that controls current between two terminals based on the current or voltage at a third terminal.

Generally, a transistor is used for amplifying and switching of electrical signals.

Early electronics used vacuum tube amplifier for the electrical circuits. Then a transistor used a single crystal was developed.

Recently, scientist developed a nano size transistor using various semiconducting materials, such as silicon, organic materials, carbon nanotube.

Introduction

Vacuum tube was used as an amplifier or switching devices.

The electron from cathode to the plate can be controlled by the grid.

If we apply AC to the grid, the current between the cathode and the plate is changed according to the AC signal.

Using the combination of vacuum tube, the first computer was developed.

The size, energy loss, heating, etc were problem.

Thin Film Transistors

Why Thin Film Transistors (TFTs) is useful?TFT is a one of Field Effect Transistors fabricated on the glass substrates using thin film semiconductorsUsing those TFT arrays, we can operate the LCD (TFTLCD or AMLCD) and AMOLEDs

a-Si TFT Poly-Si TFT

TFTs are used in Active Matrix Displays as Pixel on/off switching devices

Thin Film Transistors: Materials

Substrate: GlassGate electrode: Metal film (Cr, Al, MoTa films)Source electrode: Metal film (Cr, Mo, Ta films)Drain electrode: Metal film (Cr, Mo, Ta films)Gate insulator: SiNx, SiONx, SiOx filmsActive layer (Semiconductor) : Crystalline SiAmorphous SiOxide semiconductor (InGaZnO)Organic semiconductor (Pentacene)

Passivation layer: SiNx

MOSFET vs. TFTs

Thin Film Transistors: use in Display

Where are the TFTs in the Pixel?

Gate Bus Line/Data Bus Line/Pixel electrode

Using On/OFF switching by TFT, we can makeImage and characters

Thin Film Transistors: several structures

Thin Film Transistors: Silicon active materials Organic Thin Film Transistors (OTFTs)

Organic Thin Film Transistors (OTFTs)

Superior flexibilityLow cost and simple processSolution based process

However, poor performance.

Organic Thin Film Transistors (OTFTs)

Organic Thin Film Transistors (OTFTs) Organic Thin Film Transistors (OTFTs)

(a) Linear Region :

(b) Saturation Region :

Top contact Pentacene OTFT

5000 SiO2 gate insulator

W/L = 1mm/15.4㎛ (L. Kosbar et.al. MRS Proc. 2001)

Output curve of Organic TFTs

Organic Thin Film Transistors (OTFTs)

Top contact Pentacene OTFT

5000 SiO2 gate insulator

W/L = 1mm/15.4㎛ (L. Kosbar et.al. MRS Proc. 2001)

Transfer curve of Organic TFTs

Threshold voltage

Organic Thin Film Transistors (OTFTs)

Solution based organic TFTs

Oxide Thin Film Transistors

Key issue : TFT turn-on-time

Future AMLCD TV

□ Large size □ High resolution (FHDUD) □ Motion picture (>120Hz)

□ TFT turn on time decrease : 5.2 us (FHD) 0.4 us (UD) Not enough charging

1. Increase of the number of gate line 2. RC delay due to increase of cell #

FHD UD~1/2 FHD

Line # :1080

Line # : 2160

5.2 us

UD

0.4 us2.6 us 3.4 us

@70inch, 120Hz

Oxide Thin Film Transistors

Oxide Thin Film Transistors

Why high mobility TFTs?Oxide Thin Film Transistors

Why high mobility TFTs?

• Silicon TFTs do not give thesolution !!! until now….a-Si : fundamental issue of weak

bondpoly-Si : manufacturing issues

(yield & investment)

• Reliable TFT under constantcurrent stress is needed

Oxide Thin Film Transistors

Oxide Thin Film Transistors

Natural 3D LCD

E- paper

LCD

OLED display

Flexible display

Low-T, Low-cost fabrication On-current (μ) is not critical (<<1cm2/Vs is OK) Off-current is not critical : E-Ink inself has a memory function

Low performance TFT (slow responses, Ion/off~103 are OK)

Moderately low-T (~300ºC), low-cost fabrication On-current (μ) is not critical : a-Si:H and organic TFTs (μFET<1cm2/Vs) are OK Low off-current is favorable : Ioff<10-12A is required.

High On-current (μFET>3cm2/Vs)

High On-current (μFE>4cm2/Vs) Very High stability (∆Vth<<1V)

Low-T process(<<200ºC) Flexibility (Brittleness)

Low temperatureHigh performance

Requirement of TFT performance for AMOLEDs

Oxide Thin Film Transistors

Prof. H. Hosono

Oxide Thin Film Transistors

SD HD FHD UD(4FHD)

Post UD(16FHD)

Several Times higher

Several ten Times higher

60H

z12

0Hz

240H

z48

0Hz

Limit of a-Si based TFTs

10.4 us

20.7 us 14.8 us

7.4 us

3.7 us

1.8 us 0.9 us

0.9 us

1.8 us

3.7 us

Ref

resh

Rat

e

Resolution

7.4 us

3.7 us

1.8 us

1 10 100 Mobility (cm2/Vs)

RT

150

300

450

A wide variety of applications flexible displays, OLED, …

Organic TFT

Furnace annealing at 300oC of higher is effective in obtaining good transfer characteristics

A-Si:HTFT

Poly-Si TFT

Poly-Si TFT a-Si TFT Oxide TFT

Semiconductor Polycrystalline Si Amorphous Si Amorphous IGZO

TFT uniformity Poor Good Good

Pixel circuit Complex (5T+2C) Complex (4T+2C) Simple (2T+1C)

Channel mobility ~100 cm2/Vs 1 cm2/Vs 10~40 cm2/Vs

Circuit integration YES No YES

TFT mask steps 5~11 4~5 4~5

Cost/Yield High/Low Low/High Low/High

Process Temperature 450~550 oC 150 ~ 350 oC ~350 oC

Device merits Temperature stabilityHigh reliability High uniformity No hot carrier effect

Excellent Ion/loff ratio

Why Transparent Oxide Semiconductors?Oxide Thin Film Transistors

Oxide Thin Film Transistors

Oxide TFT based UD 3D TV (LCD): Samsung

70 inch UD LCD TV (240 Hz 구동): Oxide TFT

The first atom-thick material

Graphite is a mineral of the carbon with a layered and planar structure.

The same structure of graphite was constructed artificially, called graphene, but in a monolayer structure.

Graphene has remarkable chemical, mechanical and physical properties.

New 2D materials MoS2 transistor technology

Atomic thick electronics

Ultimate heterostructure technology by layer-by-layer stacking of atomic films