Chapter 2 Semiconductor Materials and Diodes

1. Chapter 2Semiconductor Materials and Diodes

2. Classification of Materials • Classification according to the way materials react to the current when a voltage is applied across them: • Insulators • Materials with very high resistance - current can’t flow • mica, rubber • Conductors • Materials with very low resistance – current can flow easily • copper, aluminum • Semiconductors • Neither good conductors nor insulators (silicon, germanium) • Can be controlled to either insulators by increasing their resistance or conductors by decreasing their resistance

3. Semiconductor Materials and Properties • An atom is composed of a nucleus, which contains positively charged protons and neutral neutrons, and negatively charged electrons that orbit the nucleus. • Electrons in the outermost shell are called valence electrons.

4. A portion of the periodic table in which the more common semiconductors are found • Elemental Semiconductors Silicon (Si) and germanium (Ge) are in group IV. Hence, they have 4 electrons in their outer shells Do you still remember? A stable atoms need ? electrons at its outermost shell 8

5. Si Si Si Si Si • Si have 4 electrons in their outer shells • needs another 4 to become stable • So, when there are 4 other Si nearby = 4 electrons: Sharing of electrons occurred; and this bond is known as the covalent bond • Atoms come into close proximity to each other and so the valence electrons interact to form a crystal.

6. BANDGAP ENERGY, Eg • Now, in order to break the covalent bond, a valence electron must gain enough energy to become free electrons. • The minimum energy required is known as the bandgap energy, Eg

7. ILLUSTRATION WHEN A VALENCE ELECTRON IS FREE 1. Becomes free electron 3. Electron moves to fill space 5. Electron moves to fill space 2. Becomes empty 4. Becomes empty

8. Intrinsic Semiconductor • Intrinsic Semiconductor • A single-crystal semiconductor material with no other types of atoms within the crystal. • The densities of electrons and holes are equal. • The notation niis used as intrinsic carrier concentration for the concentration of the free electrons as well as that of the hole: B = a coefficient related to the specific semiconductor material Eg = the bandgap energy (eV) T = the temperature (Kelvin) remember that K = °C + 273.15 k = Boltzmann’s constant (86 x 10-6 eV/K)

9. Intrinsic Semiconductor • The values of B and Eg for several semiconductor materials: Example: Calculate the intrinsic carrier concentration in silicon at T = 300 K.

10. Example 2 Find the intrinsic carrier concentration of Gallium Arsenide at temperature = 300K k = Boltzmann’s constant (86 x 10-6 eV/K) Answer: 1.8 x 106 cm-3

11. Example 3 Answer: 1.4 eV

12. Extrinsic Semiconductor • Since intrinsic concentration, ni is very small, so, very small current is possible • So, to increase the number of carriers, impurities are added to the Silicon/Germanium. • The impurities will be from Group V and Group III

13. Extra electron • Group V – 5 electrons in the outer shell; Example, Phosphorus, Arsenic • The 5th electron are loosely bound to the Phosphorus atom • Hence, even at room temperature, the electron has enough energy to break away and becomes free electron. • Atoms from Group V are known as donor impurity (because it donates electrons) Group V + Si = n-type semiconductor

14. Extra hole • Group III – 3 electrons in the outer shell; Example, Boron • The valence electron from outer shells are attracted to fill the holes added by the insertion of Boron • Hence, we have movement of holes • Atoms from Group III are known as acceptor impurity (because it accept electrons) Group III + Si = p-type semiconductor

15. The materials containing impurity atoms are called extrinsic semiconductors, or doped semiconductors. • Effects of doping process • controls the concentrations of free electrons and holes • determines the conductivity and currents in the materials. • The relation between the electron and hole concentrations in thermal equilibrium: no = the thermal equilibrium concentration of free electrons • po= the thermal equilibrium concentration of holes • ni= the intrinsic carrier concentration

16. For N-type – electrons are the majority carriers • At room temperature (T = 300 K), each donor atom donates a free electron to the semiconductor. • If the donor concentration Nd is much larger than the intrinsic concentration, approximately: • Then, the hole concentration:

17. For P-type – holes are the majority carriers • Similarly, at room temperature, each acceptor atom accepts a valence electron, creating a hole. • If the acceptor concentration Na is much larger than the intrinsic concentration, approximately: • Then, the electron concentration:

18. Example 1 Calculate the thermal equilibrium electron and hole concentrations. Consider silicon at T = 300 K doped with phosphorous at a concentration of Nd = 1016 cm-3 and ni= 1.5 x 1010 cm-3.

19. Example 2 Calculate the majority and minority carrier concentrations in silicon at T = 300K if • Na = 1017cm-3 • Nd = 5 x 1015cm-3 • Calculate ni • For part (a) – it is p-type • For part (b) – it is n-type Answer: a) majority = 1017cm-3 minority 2.25x 103 cm-3 b) Majority 5 x 1015cm-3, minority 4.5 x 104 cm-3

20. k = Boltzmann’s constant (86 x 10-6 eV/K) • EXAMPLE 1 Calculate the intrinsic carrier concentration of Silicon at T = 250K • EXAMPLE 2 A silicon is doped with 5 x 1016 arsenic atoms • Is the material n-type or p-type? • Calculate the electrons and holes concentration of the doped silicon at T=300K Answer: ni = 1.6 x 108 cm-3 • Answer: • n-type • no = 5 x 1016 cm-3 and po = 4.5 x 103 cm-3