Chaos, Communication and ConsciousnessModule PH19510

Lecture 8

Entering the Transistor Age

Overview

Metals, Insulators & Semiconductors The junction diode The bipolar transistor Planar Processing

Metals, Insulators and Semiconductors Metals

Overlap of valence & conduction bands empty states for electrons to move into conduction

Insulators Large gap between valence & conduction bands No free states No conduction

Semiconductors (Silicon, Germanium, GaAs etc) Small gap between valence & conduction bands Some free states some conduction – depends on impurities, temperature etc.

Column IIIB, IVB & VB

Column IIIB B, Al … 3 electrons in outer shell

Column IVB C, Si, Ge … 4 electrons in outer shell Diamond like crystal

Column VB N, P, As … 5 electrons in outer shell

Crystal structure of Silicon

Diamond like Tetrahedral bonds Hard Brittle

Intrinsic Semiconductor

2D representation of crystal lattice

Semiconductor All electrons paired Valence band filled No-conduction

n-Type Semiconductor

Electron donor impurity

Column V (As, P) Low concentration ≈1:108 Add ‘free’ electron Electron Movement conduction

As

p-Type Semiconductor

Electron acceptor impurity

Column III (Boron) Low concentration ≈1:108 Add ‘hole’ Hole Movement conduction

B

pn Junction

n-type & p-type junction Spare electrons from n-

type fill holes in p-type depletion region n-type –ve

depletion region shrinks forward bias (conduction)

n-type +ve depletion region grows reverse bias (no conduction)

n p

n p-ve +ve

n p -ve+ve

Point contact PNP junction

First Transistor 1947 Brattain, Bardeen & Shockley

Ge

p pn

NPN Bipolar Junction Transistor

Emitter at ground +ve voltage on collector Collector-Base reverse

biased no current Apply +ve voltage on

base Electrons pulled from

emitter into base Collector base depletion

region shrinks many electrons flow

from Emitter to Collector Amplification

n

n

p

Collector

Base

Emitter

Shockley and Silicon Valley

1955 Shockely leaves Bell Forms Shockley Semiconductor in Palo Alto, CA 12 keen young engineers ‘Difficult’ management style

1957 Staff leave (The Traitorous 8) & form Fairchild

Semiconductor

From Germanium to Silicon

Germanium rare but easier to refine Silicon Common

25.7% of earth crust Electric Arc refining

Silicon forms stable oxide Better electrical and mechanical properties

21900

2 COSiCSiO C

Refining

Pure material crystallises first from melt

Impurities left behind

Printing Transistors on Silicon

Planar process developed at Fairchild Possible to dope n-type so it becomes p-

type and vice versa Use of Silicon Dioxide (SiO2) as mask for

processing

Planar Transistors

n-Substrate Grow Oxide Pattern & etch oxide Diffuse p-dopant Grow Oxide Pattern & etch oxide Diffuse n-dopant Grow Oxide Pattern & etch oxide Metal Contactsn

CollectorBaseEmitter

Vertical Transistor

Review of Lecture 8

Metals, Insulators & Semiconductors The junction diode The bipolar transistor Planar Processing

ReferencesWhere to go for more Information inventors.about.com/library/weekly/aa061698.htm US Patent 2524035

www.eepatents.com/feature/2524035.html Intel Education programme

www.intel.com/education/transworks/ Principles of semiconductor devices

ece-www.colorado.edu/~bart/book/book/ Webelements.com The Britney Spears guide to Semiconductor Physics

britneyspears.ac/physics/basics/basics.htm - www.icknowledge.com