io school of microelectronic engineering lecture ii basic semiconductor devices

io

School of Microelectronic Engineering

Lecture IIBasic Semiconductor

Devices

`


Objectives

`


Topics

`


Semiconductor Materials

`


What Is A Semiconductor?

Semiconductors are materials with electrical conductivity between conductors and insulators.

The most commonly used semiconductor materials are silicon and germanium.

Some compounds, such as GaAs, SiC and SiGe.

Most important property is its conductivity can be controlled by adding certain impurities in the process called doping.

`



Periodic Table of The Elements

`


`


Band Gap

Atom is basic building block of all materials

Classical mechanics – every atom has it own orbit structure.

Electron orbits are called shells.

The outermost shell is called valence shell.

When e leaves the valence shell, it becomes a free electron and can conduct electric current.

`


When 2 or more identical atoms bond together to form solid materials, their orbit overlap and form so called energy bands. Can be represented by the energy band diagram

The bottom of conduction band is called Ec, and the top of the valence band is called Ev.

Eg = Ec – Ev

Eg is defined as the energy required to break a bond in semiconductor to free an e to cond band and leave the hole in the valence band.

Electrons in conduction band are free to move and can conduct electric current

Electrons in the valence band are bonded with nuclei and connot move freely, therefore cannot conduct electric current

`


Resistivity

Resistivity is the capability of a material resisting electric current.

A good conductor has a very low resistivity and a good insulator has a very high reistivity.

Unit: Ohm.cm

`


Resistivity and Band Gap For most metals, conduction and valence bands almost overlap or very small band gap. Electron can easily jump from valence to conduction band. Therefore the conduction band has a lot of e.

For insulators, the band gap is so large that electrons cannot jump across it.

`


Semiconductor Materials and Its Applications

`


Crystal Properties of Semiconductors

`


Classification of Solids (Based on Atomic Arrangement) Amorphous

Single Crystal

Poly Crystal

`


Crystal Structures

`


Amorphous Structure

`


Polycrystalline Structure

`


Single Crystal Structure

`


Silicon Crystal Structure

Silicon has four electrons in the outermost shell.

In a single crystal structure, every atom is bonded with four atoms shares a pair of electrons with each of them.

`


Crystal Lattice

`


Unit Cell

`


Unit Cell of Single Crystal Structure


Crystal Plane and Miller Indices

`



Doping Semiconductors


Two types of Semiconductor Materials

Intrinsic Semiconductor

Extrinsic Semiconductor


Intrinsic Semiconductor

Pure semiconductor materials with no impurity atoms and no lattice defect.

At T=0 K, all energy states in valence band are filled with electrons, states in conduction band are empty.


Electrical Conduction in Intrinsic Semiconductor

Si Si Si Si

Si Si Si Si

Si Si Si Si

Si Si Si Si

e-

Silicon covalence bonding at T=0K


Si Si Si Si

Si Si Si Si

Si Si Si Si

Si Si Si Sie-

e

Valance band

Conduction band

Eg

+

e-

• As the temperature increase above 0K, a few valence bond electrons maygain enough thermal energy to break the bond and jump into the conduction band.

• As temperature increase further, more bonds broken, more electrons jumpto the conduction band and more “empty states or holes” created in the valenceband.


In intrinsic material, electrons and holes are created in pairs by thermal energy. So the number of electrons in conduction band is equal to the number of holes in the valence band

Electron concentration = hole concentration

ni = pi and

nipi = ni2 (MASS ACTION LAW) – the product of n p is

alwaysa constant for a given semiconductor material at given temperature


Extrinsic Semiconductor Extrinsic s/c is defined as a semiconductor in which controlled amounts of specific dopant or impurity atoms have been added so that the thermal equilibrium electron and hole concentration are different from the intrinsic carrier concentration.

Si Si Si Si

Si Si P Si

Si Si Si Si

Si Si Si Sie-

Si Si Si Si

Si Si Si Si

Si Si Si Si

Si Si Si Sie-

Intrinsic silicon lattice

e

Extrinsic silicon lattice


Doping of Semiconductors

The purpose of doping is to alter the conductivity of semiconductor materials.

Two types of dopant; p-type (B), n-type (P, As)

N-type dopants provide an electron in s/c materials, hence called donors.

P-type dopants provide a hole in s/c materials, hence called acceptor.


N-type Dopant P and As have 5 electron valens

When doped into Si, 4 electrons used to form the covalence bond with Si

1 extra electron is left in the outermost shell and will occupy a new energy level called Donor Energy.


Energy required to elevate donor electron is less than that for electron involved in covalence bonding. With small thermal energy, donor electron is elevated to the conduction band This process add electron to the conduction band without creating holes in the valence band. The resulting material is referred as n-type semiconductor.

valence band

conduction bandEc

Ev

EdEc

Ev

Ed+ ++

---


P-type Dopant B have 3 electron valens

When doped into Si, one empty state is created in the covalence bond

This empty state will occupy a new energy level called Acceptor Energy.


Si Si Si Si

Si Si B Si

Si Si Si Si

Si Si Si Sie-

Extrinsic silicon lattice dopedwith B

Si Si Si Si

Si Si B Si

Si Si Si Si

Si Si Si Sie-

+

Hopping of valence electron creatinghole movement

Some valence electron gain a small amount of energy to move around the crystal lattice. This electron would occupy the “empty” position associated with B atom. The vacated electron position is considered as holes.


valence band

conduction bandEc

EvEa

Ec

Ev

Ea

+ ++

---

This process generate holes in the valence band without creating electrons in the conduction band.

The resulting material is referred as p-type semiconductor


Si Si Si Si

Si Si Si Si

Si Si Si Si

Si Si Si Sie-

Si Si Si Si

Si Si Si Si

Si Si Si Si

Si Si Si Sie-

e

+ +

Si Si Si Si

Si Si Si Si

Si Si Si Si

Si Si Si Sie-

+

Si Si Si Si

Si Si Si Si

Si Si Si Si

Si Si Si Sie-

+


Dopant Concentration and Resistivity


Dopant Concentration and Resistivity

WHY?


Basic Semiconductor Devices


Resistor The simplest electronic device. In the IC fabrication, patterned doped silicon normally used to make resistors with resistance determined by the length, linewidth, junction depth and dopant concentration. Poly silicon also used a resistor.


Example 1 Many people use polysilicon to form gates and local interconnect. Resistivityof polysilicon is determined by dopant concentration, about 1022 cm-3, and ρ = 200 Ω.cm. Assume polysilicon gate and local interconnect line width, height, and legth are 1m, 1m and 100m respectively.

Calculate the resistance.

R = ρ l / wh

= 200 Ω.cm x (100 x 10-4) cm / [(1x10-4 cm) x 1X10-4cm)

= 2 x 108 Ω

= 200 Ω


Capacitor One of the most important IC components When two conducting materials are separated by a dielectric, a capacitor is formed.

C = 0h l

d

0- Absolute permittivity of vacuum (8.85 x 10-12 F/m

- Dielectric constant

Unwanted (parasitic) capacitor, as result of dielectric sandwiched between 2 metal layers. This will result in the RC delay of the IC circuit. Major limitation for current IC device speed. This application required low k dielectric and better conduction metal

Example 2 Calculate the capacitance for a capacitor shown with h = l = 10 m. Assumethe dielectric between the 2 conducting plates is silicon dioxide, with k=3.9and d=1000 Å.

C = 0h l

d

= 3.9 x 8.85x10-12 x10x10-6 x 10x10-6

1000x10-10

= 3.45x10-14 F

Example 3 Most IC chips use aluminum-copper alloy metal interconnection. The resistivityρ = 3.2 Ω.cm, metal line geometry width w, height h, length l, and line spacingd are 1 m, 1 m, 1 m (1 million transistors connected by one metal line at 1 m between each transistor), and 1 m respectively. CVD silicon oxide lies betweenthe metal line, with dielectric constant k = 4. Calculate the time delay.

C = 0h l

d

R = ρ l / wh

Answer = 1.13 x 10-8 sec

Diode P-N junction Allow electric conduction only in one way (positively biased)

When p-type and n-type semiconductors join together, they form a p-n junction diode. Holes in p-type region will diffuse to the n-type region, and electrons in n-type region will diffuse to the p-type region (at thermal equilibrium, without applied bias). The area dominated by minority carriers is called the transistion region. The voltage across the transistion region given by;

For Si at room temperature, V0 ~ 0.7 V

I-V curve for diode

MOSFET

NMOS Conducting gate (metal or polysilicon) Heavily doped sour ce and drain Ultra thin gate oxide P-type substrate

NMOS When no bias voltage is applied to the gate, no current flow. When gate is positively biased, positive charge will appear at the gate. Positive charge at the silicon surface will be expelled from the region. At certain voltage (Threshold Voltage), electron will be accumulated at silicon surface to form channel, and allow the electron flow from source to drain.

PMOS When no bias voltage is applied to the gate, no current flow. When gate is negatively biased, negative charge will appear at the gate. Negative charge at the silicon surface will be expelled from the region. At certain voltage (Threshold Voltage), holes will be accumulated at silicon surface to form channel, and allow the holes flow from drain to source.

Basic Circuits

Bipolar PMOS NMOS CMOS BiCMOS

Bipolar

BiCMOS

IC Device with different substrate

IC CHIPS

IC chips can be categorized into 3 main groups; Memory Microprocessor ASIC

Memory

DRAM

Stands for Dynamic Random Access Memory. Random access means each memory cell in the chip can be accessed to read or write in any order.

Memory Cell of DRAM Memory cell: location to store 1 bit of digital information (1 or 0) in a memory chip. Memory cell of DRAM consists of 1 MOS transistor and 1 capacitor.

MOS serves as a switch. It allows e to flow intoand store in the capacitor. The capacitor needs to be recharged periodicallyby the power supply Vdd to compensate the e loss. When power is removed from DRAM, the dataare lost.

SRAM

Stands for Static Random Access Memory.

EPROM, EEPROM

Stands for electric-erasable programmable read-only memory.

EPROM, EEPROM Memory Cell

TUNNEL OXIDE

EPROM, EEPROM Programming

SDA

GND A2 A0A1

SCL WP VCC

Charge Pump

16K-bits Memory Cell

Timer

Control Logic (Master)

EEPROM Design Layout

E/Wcircuit

E/Wcircuit

Decoder Xe

Decoder Xr

Dec Y

Dat

a ct

rlA

ddre

ss b

lock

Dec

oder

px

Microprocessor

Also called central processing unit (CPU) consists of 2 components; a controller arithmetic logic unit (ALU).

CPU is the brain of computers and other control system. 2 types of architecture;

complete instruction set computer (CISC) – IBM compatible reduced intruction set computer (RISC) – Apple

ASICS

Application specific integrated circuits

Majority of chips belong to this cathegory; DSP, power devices, IC for TV, radio, internet, telecommunication, automobiles, etc

BASIC MOS IC PROCESS

MOS Technology

PMOS Process – 1960s

NMOS Process – Mid 1970s

POLYSI GATE

CMOS Process – 1980s onwards

io school of microelectronic engineering lecture ii basic semiconductor devices

Documents

conduction band

cond band

small band gap

valence shell

energy band diagram

valence bands

electric current electrons

orbit structure