1	Chapter 26 Introduction to Semiconductors
2	Semiconductor Basics Atoms Protons Neutrons Electrons
3	Semiconductor Basics Electron shells: K, L, M, N, etc. Conductor 1 electron in outer shell (valence shell) Insulator 8 in valence shell (outer shell full) Semiconductor 4 in valence shell
4	Semiconductor Basics Most common semiconductors Silicon (Si) Germanium (Ge)
5	Semiconductor Basics Valence electrons have greatest energy Electrons have discrete energy levels that correspond to orbits
6	Semiconductor Basics Valence electrons have two energy levels Valence Band Lower energy level Conduction Band Higher energy level
7	Semiconductor Basics Differences in energy levels provide Insulators Semiconductors Conductors
8	Semiconductor Basics Energy gap between Valence and Conduction Bands
9	Semiconductor Basics Conductor has many “free” electrons These are called “conduction” electrons Energy Gap is between valence and conduction band
10	Semiconductor Basics Atomic Physics Energy expressed in electron volts (eV) 1 eV = 1.602 ´ 10^–¹⁹ joules Energy gap Small for conductors Large for insulators
11	Semiconductor Basics Silicon has 4 electrons in its valence shell 8 electrons fill the valence shell Silicon forms a lattice structure and adjacent atoms “share” valence electrons
12	Semiconductor Basics Electrons are shared so each valence shell is filled (8 electrons) Valence shells full No “free” electrons at 0 K
13	Conduction in Semiconductors At temperatures > °K Some electrons move into conduction band Electron-Hole pairs are formed Hole is vacancy left in lattice by an electron that moves into conduction band Continuous recombination occurs
14	Conduction in Semiconductors Electrons available for conduction Copper ≈ 10²³ Silicon ≈ 10¹⁰(poor conductor) Germanium ≈ 10¹² (poor conductor)
15	Conduction in Semiconductors Hole: absence of an electron in the lattice structure Electrons move from – to + Holes (absence of electrons) move from + to – Recombination When an electron fills a hole
16	Conduction in Semiconductors
17	Conduction in Semiconductors As electrons move toward + terminal Recombine with holes from other electrons Electron current is mass movement of electrons Hole current is mass movement of holes created by displaced electrons
18	Conduction in Semiconductors Effect of temperature Higher energy to electrons in valence band Creates more electrons in conduction band Increases conductivity and reduces resistance Semiconductors have a negative temperature coefficient (NTC)
19	Doping Adding impurities to semiconductor Creates more free electron/hole pairs Greatly increased conductivity Known as “doping”
20	Doping Terminology Pure semiconductor known as intrinsic Doped semiconductor known as extrinsic
21	Doping Creates n-type or p-type semiconductors Add a few ppm (parts per million) of doping material n-type More free electrons than holes p-type More holes than free electrons
22	Doping Creating n-type semiconductors Add (dope with) atoms with 5 valence electrons Pentavalent atoms Phosphorous (P) Arsenic (As) Antimony (Sb) – Group V on periodic table
23	Doping Creating n-type semiconductors New, donor atoms become part of lattice structure Extra electron available for conduction
24	Doping Intrinsic semiconductors Equal number of holes and electrons Conduction equally by holes and electrons Very poor conductors (insulators)
25	Doping n-type extrinsic semiconductor Free electrons greatly outnumber free holes Conduction primarily by electrons Electrons are the “majority” carriers
26	Doping Conduction in an n-type semiconductor
27	Doping Creating p-type semiconductors Add (dope with) atoms with 3 valence electrons Trivalent atoms Boron (B) Aluminum (Al) Gallium (Ga) – Group III on periodic table
28	Doping Creating p-type semiconductors New, acceptor atoms become part of lattice structure Extra hole available for conduction
29	Doping p-type extrinsic semiconductor Free holes greatly outnumber free electrons Conduction primarily by holes Holes are the “majority” carriers Electrons are the “minority” carriers
30	The p-n Junction Abrupt transition from p-type to n-type material Creation Must maintain lattice structure Use molten or diffusion process
31	The p-n Junction Example Heat n-type material to high temperature Boron gas diffuses into material Only upper layer becomes p-type p-n junction created without disturbing lattice structure
32	The p-n Junction Joined p-type and n-type semiconductors +++++++ +++++++ ------------ ------------ Diffusion across junction creates barrier potential ++-+++-++ -++-++-++- +--+--+--+ ---+----+---
33	The p-n Junction Joined p-type and n-type semiconductors +++++++ +++++++ ------------ ------------ Diffusion across junction creates barrier potential ++-+++-++ -++-++-++- +--+--+--+ ---+----+---
34	The p-n Junction Depletion region Barrier voltage, V_B Silicon V_B ≈ 0.7 volts at 25°C
35	The p-n Junction Germanium V_B ≈ 0.3 volts at 25°C V_B must be overcome for conduction External source must be used
36	The Biased p-n Junction Basis of semiconductor devices Diode Unidirectional current Forward bias (overcome V_B) – conducts easily Reverse bias – virtually no current p-type end is anode (A)
37	The Biased p-n Junction Diode n-type end is cathode (K) Anode and cathode are from vacuum tube terminology
38	The Biased p-n Junction Diode symbol Arrow indicates direction of conventional current for condition of forward bias (A +, K -) External voltage source required External resistance required to limit current
39	The Biased p-n Junction Holes are majority carriers in p-type Electrons are majority carriers in n-type
40	The Biased p-n Junction Reverse biased junction Positive (+) terminal draws n-type majority carriers away from junction Negative (–) terminal draws p-type majority carriers away from junction No majority carriers attracted toward junction Depletion region widens
41	The Biased p-n Junction Electrons are minority carriers in p-type Holes are minority carriers in n-type Reverse biased junction Minority carriers drawn across junction Very few minority carriers
42	The Biased p-n Junction Reverse biased current Saturation current, I_S Nanoamp-to-microamp range for signal diodes
43	The Biased p-n Junction Reverse biased junction Positive terminal of source connected to cathode (n-type material)
44	The Biased p-n Junction
45	The Biased p-n Junction p-type Holes are majority carriers n-type Electrons are majority carriers
46	The Biased p-n Junction Forward biased junction + terminal draws n-type majority carriers toward junction – terminal draws p-type majority carriers toward junction Minority carriers attracted away from junction Depletion region narrows
47	The Biased p-n Junction Forward biased junction Majority carriers drawn across junction Current in n-type material is electron current Current in p-type material is hole current Current is referred to as I_majorityor I_F (for forward current)
48	The Biased p-n Junction Voltage across Forward biased diode ≈ V_B Often referred to as V_F (for forward voltage) V_B ≈ 0.7 for Silicon and 0.3 for Germanium Forward biased current Majority and Minority current Minority current negligible
49	The Biased p-n Junction Forward biased junction Positive terminal of source connected to Anode (p-type material)
50	The Biased p-n Junction Forward biased junction Conducts when E exceeds V_B For E < V_B very little current flows Total current = majority + minority current Diode current, I_F ≈ majority current V_F ≈ 0.7 volts for a silicon diode
51	Other Considerations Junction Breakdown Caused by large reverse voltage Result is high reverse current Possible damage to diode Two mechanisms Avalanche Breakdown Zener Breakdown
52	Other Considerations Avalanche Breakdown Minority carriers reach high velocity Knock electrons free Create additional electron-hole pairs Created pairs accelerated Creates more electrons “Avalanche” effect can damage diode
53	Other Considerations Peak Inverse Voltage (PIV) or Peak Reverse Voltage (PRV) rating of diode
54	Other Considerations Zener Breakdown Heavily doped n-type and p-type materials in diode Narrows depletion region Increases electric field at junction Electrons torn from orbit Occurs at the Zener Voltage, V_Z
55	Other Considerations Zener Diodes Designed to use this effect An important type of diode
56	Other Considerations Diode junction +++++++ +++++++ ------------ ------------
57	Other Considerations Like a capacitor Thickness of depletion region changes with applied voltage Capacitance dependent on distance between plates