Lecture 1: Introduction, Properties and Structure

MSE 6001, Semiconductor Materials LecturesFall 2006

1 Semiconductor Materials and Devices

1.1 Introduction and overview

Semiconductor devices make modern information technology possible, and they are beginning toplay a much larger role in energy and lighting. The semiconductor industry is now larger thanthe steel industry, with sales of more than $250B this year. We are probably still in the earlystages of the microelectronics age. The control of semiconductor electrical and optical propertiesmake these materials useful for electronic and photonic devices. The properties include, for ex-ample, electrical resistivityρ and optical absorptionα, which are related to one another by thesemiconductor electronic structure. Properties depend on the molecular-scale structure, the atompositions and the electron states around these atoms. These electronic states and their occupationwith electrons is the electronic structure. The numerical values of the properties may be calculatedwith quantum mechanics, and in turn, semiconductor properties are increasingly engineered usingquantum mechanical design at the nanometer scale.

The most prominent device is the silicon metal-oxide-semiconductor field-effect transistor(MOSFET). The electron concentration at the silicon-silicon dioxide interface is controlled bythe gate voltage, which controls the MOSFET conductivity, allowing the MOSFET to be turned onand off. (Figs. 1 and 2)

MOSFET cross section

0 0.5 1

1e+09

1e+10

1e+11

1e+12

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surface potential (V)

n s, e

lect

ron

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entra

tion

(cm

–2)

MOSFET 2D electron concentration controlled

by surface potential

Na = 4x1015 cm–3

T = 300 K

n+ source

p-Si

gate oxide,

toxpoly-Si gate

L

depletion

n+ drain

• 2D electron sheet, ~ 2 nm

• surface potential (V)

• ns, 2DEG concentration (cm-2)

ns vs. surface potential

physical gate length

FIGURE 1: (left) Schematic cross sectional view of a MOSFET. (right) Calculated surface electron concen-tration in an MOS structure. The surface potential controls the surface electron concentrationns throughthe field effect in a MOSFET.

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Vds [V]

I ds [A

]

NMOSFETW/L = 30 µm/5 µmJ. Jackson and M. Sorenson, U Utah

Vg = 8 V

gate

source drain

5 µm

FIGURE 2: Drain-source characteristics of a MOSFET fabricated at the UU microfabrication lab. The gatevoltageVg controls the surface potential, which controls the drain-source currentIds

1.2 Semiconductor Materials (Inorganic)

Both organic and inorganic materials can be semiconductors; and both crystalline and amorphousmaterials can be semiconductors. Many oxides also have semiconducting properties. Most ofour physical models, understanding, and applications come from the study of crystalline inorganicsemiconductors.

Elemental semiconductors Elemental semiconductors include Si, Ge, and even C in the formof diamond and carbon nanowires. These column-IV materials are generally used for microelec-tronics, and Si and Ge are used in solar cells and photodetectors.

Compound semiconductors Prominent examples of compound semiconductors include mem-bers of the III-V semiconductor family, which includes GaAs and InP. The optical properties ofIII-V semiconductors makes them useful for optoelectronic devices, such as light emitting diodes(LEDs) and communication lasers for fiber optic networks, and these materials are also used forvery high frequency microelectronics, at 40 GHz and above. Other important families of compoundsemiconductors include the II-VI’s, such ad ZnSe and HgTe. The bonding in the column-IV, III-V,and II-VI materials is tetrahedralsp3, corresponding to the diamond lattice (Fig. 4).

An important feature of compound semiconductors is that the properties may be adjusted byusing alloys. For example, GaAs is alloyed with AlAs to form a semiconductor with intermediateproperties, used in TV remote controls and other applications.

Doping Semiconductors are useful in devices because the free carrier concentration, the con-centration of electrons and holes, may be in part controlled by doping with chemical impurities.Dopants in inorganic semiconductors are covalently-bonded, substitutional impurities having ei-

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B C N O

Al Si P S

Zn Ga Ge As Se

Cd In Sn Sb Te

Hg Tl Bi

IIB III IV V VI

FIGURE 3: Portion of the periodic table from which most of the major inorganic semiconductors are taken.

109°

FIGURE 4: Bond angles in the diamond lattice.

ther one extra valence electron, which can be donated to the crystal, or one fewer valence electron,which can accept an electron from the crystal to leave behind a hole.

1.3 Semiconductor Devices

Perhaps over one hundred semiconductor devices have been invented, with applications rangingfrom megawatt switches in the power distribution grid to communication lasers to nano-scale mem-ory elements. The devices may be broadly categorized as electronic or photonic, with photonicdevices also often referred to a optoelectronic devices.

Electronic devices Transistors are three-terminal devices used for switching and amplification.The MOSFET is an example of a field effect transistor, with other examples including junctionfield effect transistors (JFETs) and metal-semiconductor field effect transistors (MESFETs). Fieldeffect transistors are unipolar devices, conducting electrical current with a single carrier type, eitherelectrons or holes, and the current is a drift current driven by an electric field. MOSFETs areused for the largest and integrated circuits, such CPUs (central processing units) in computers andmemories, which can have over a billion transistors on a chip.

Bipolar junction transistors (BJTs) operate using both carrier types, and the currents are diffu-

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Transmission electron micrograph of MOSFET,

Intel 65 nm process

A 70 Mbit SRAM test vehicle with >0.5 billion transistors and incorporating all of the features described in this paper has been fabricated on this technology. The aggressive de-sign rules allow for a small 0.57!m2 6-T SRAM cell that is also compatible with high performance logic processing. A top view of the cell after poly patterning is shown in Figure 12. In addition to small size, this cell has a robust static noise margin down to 0.7V VDD to allow low voltage opera-tion (Fig. 13). Figure 14 is a Shmoo plot for the 70 Mb SRAM operating frequency vs voltage, showing the SRAM operates at 3.43 GHz at 1.2V. A die photo is shown in Fig-ure 15.

VII. Conclusion We have developed an industry leading 65nm CMOS tech-nology for high performance microprocessors with excel-lent transistor and interconnect performance, along with aggressive dimensional scaling. A high performance, high density 70 Mbit SRAM test vehicle has been successfully fabricated utilizing all of the 65nm process features. This 65nm technology is on track for high volume manufactur-ing in 2005.

References [1] K. Mistry, et al., Symp. VLSI Tech. Dig., 2004. [2] T. Ghani, et al., IEDM Tech. Dig., pp. 197-200, 2003. Layer Pitch

(nm) Thick (nm)

AspectRatio

Isolation 220 320 - Polysilicon 220 90 - Contacted gate pitch 220 - - Metal 1 210 170 1.6 Metal 2 210 190 1.8 Metal 3 220 200 1.8 Metal 4 280 250 1.8 Metal 5 330 300 1.8 Metal 6 480 430 1.8 Metal 7 720 650 1.8 Metal 8 1080 975 1.8 Table 1: Layer pitch, thickness and aspect ratio

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AM

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l Are

a (u

m2)SRAM Cell Area

0.5x every 2 years

65nmContacted Gate Pitch

0.7x every 2 years

250nm

180nm

130nm

90nm

Figure 1: Intel contacted gate pitch and SRAM area trends

Figure 2: Transistor size trend for technology nodes.

Figure 3: TEM cross section of 35nm NMOS

PMOSPMOS

Figure 4: TEM cross section of 35nm PMOS

L = 35 nm

FIGURE 5: P. Bai, et al., 65 nm Logic Technology Featuring 35 nm Gate Lengths, Enhanced ChannelStrain, 8 Cu Interconnect Layers, Low-k ILD, and 0.57µm2 SRAM Cell (IEDM Proceedings, 2004)

sion currents driven by carrier concentration gradients. BJTs are used for higher power and higherfrequency circuits, such as for the radio transmitters in cell phones.

Photonic devices Photonic devices emit or detect light. Solar cells are simple semiconductordiodes that convert optical power to electrical power. LEDs convert electrical power into opticalpower. In the coming decade or so, most lighting will be done by LEDs rather than incandescent orflorescent bulbs because of the much greater efficiency of LEDs. Communication lasers are basedon LED-like structures with mirrors and optical waveguides added to provide a cavity to producecoherent light.

FIGURE 6: Micrograph of 256 Mb DRAM die.

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