Lecture #5

1. Lecture #5 OUTLINE Intrinsic Fermi level Determination of EF Degenerately doped semiconductor Carrier properties Carrier drift Read: Sections 2.5, 3.1

2. Intrinsic Fermi Level, Ei • To find EF for an intrinsic semiconductor, use the fact that n = p: EE130 Lecture 5, Slide 2

3. n(ni, Ei) and p(ni, Ei) • In an intrinsic semiconductor, n = p = ni and EF = Ei: EE130 Lecture 5, Slide 3

4. Example: Energy-band diagram Question: Where is EF for n = 1017 cm-3 ? EE130 Lecture 5, Slide 4

5. Dopant Ionization Consider a phosphorus-doped Si sample at 300K with ND = 1017 cm-3. What fraction of the donors are not ionized? Answer: Suppose all of the donor atoms are ionized. Then Probability of non-ionization  EE130 Lecture 5, Slide 5

6. Nondegenerately Doped Semiconductor • Recall that the expressions for n and p were derived using the Boltzmann approximation, i.e. we assumed The semiconductor is said to be nondegenerately doped in this case. Ec 3kT EF in this range 3kT Ev EE130 Lecture 5, Slide 6

7. Degenerately Doped Semiconductor • If a semiconductor is very heavily doped, the Boltzmann approximation is not valid. In Si at T=300K: Ec-EF < 3kT if ND > 1.6x1018 cm-3 EF-Ev < 3kT if NA > 9.1x1017 cm-3 The semiconductor is said to be degenerately doped in this case. • Terminology: “n+”  degenerately n-type doped. EFEc “p+”  degenerately p-type doped. EFEv EE130 Lecture 5, Slide 7

8. Band Gap Narrowing • If the dopant concentration is a significant fraction of the silicon atomic density, the energy-band structure is perturbed  the band gap is reduced by DEG : N = 1018 cm-3: DEG = 35 meV N = 1019 cm-3: DEG = 75 meV EE130 Lecture 5, Slide 8

9. Mobile Charge Carriers in Semiconductors • Three primary types of carrier action occur inside a semiconductor: • Drift: charged particle motion under the influence of an electric field. • Diffusion: particle motion due to concentration gradient or temperature gradient. • Recombination-generation (R-G) EE130 Lecture 5, Slide 9

10. Electrons as Moving Particles In vacuum In semiconductor F = (-q)E = mn*a where mn* is the electron effective mass F = (-q)E = moa EE130 Lecture 5, Slide 10

11. In an electric field, E, an electron or a hole accelerates: Electron and hole conductivity effective masses: Carrier Effective Mass electrons holes * * EE130 Lecture 5, Slide 11

12. Thermal Velocity Average electron kinetic energy EE130 Lecture 5, Slide 12

13. 2 3 1 electron 4 5 Carrier Scattering • Mobile electrons and atoms in the Si lattice are always in random thermal motion. • Electrons make frequent collisions with the vibrating atoms • “lattice scattering” or “phonon scattering” • increases with increasing temperature • Average velocity of thermal motion for electrons: ~107 cm/s @ 300K • Other scattering mechanisms: • deflection by ionized impurity atoms • deflection due to Coulombic force between carriers • “carrier-carrier scattering” • only significant at high carrier concentrations • The net current in any direction is zero, if no electric field is applied. EE130 Lecture 5, Slide 13

14. 2 3 1 electron 4 5 E Carrier Drift • When an electric field (e.g. due to an externally applied voltage) is applied to a semiconductor, mobile charge-carriers will be accelerated by the electrostatic force. This force superimposes on the random motion of electrons: • Electrons drift in the direction opposite to the electric field  current flows • Because of scattering, electrons in a semiconductor do not achieve constant acceleration. However, they can be viewed as quasi-classical particles moving at a constant average drift velocityvd EE130 Lecture 5, Slide 14

15. Electron Momentum • With every collision, the electron loses momentum • Between collisions, the electron gains momentum • (-q)Etmn • tmn is the average time between electron scattering events EE130 Lecture 5, Slide 15

16. Carrier Mobility mn*vd = (-q)Etmn |vd|= qEtmn / mn* =mn E • n [qtmn / mn*]is the electron mobility |vd|= qEtmp / mp* mp E Similarly, for holes: • p [qtmp / mp*]is the hole mobility EE130 Lecture 5, Slide 16

17. Electron and Hole Mobilities  has the dimensions of v/E : Electron and hole mobilities of selected intrinsic semiconductors (T=300K) EE130 Lecture 5, Slide 17

18. Example: Drift Velocity Calculation a) Find the hole drift velocity in an intrinsic Si sample forE= 103 V/cm. b) What is the average hole scattering time? Solution: a) b) vd = mn E EE130 Lecture 5, Slide 18

19. Mean Free Path • Average distance traveled between collisions EE130 Lecture 5, Slide 19

20. Summary • The intrinsic Fermi level, Ei, is located near midgap • Carrier concentrations can be expressed as functions of Ei and intrinsic carrier concentration, ni : • In a degenerately doped semiconductor, EF is located very near to the band edge • Electrons and holes can be considered as quasi-classical particles with effective mass m* • In the presence of an electric field e, carriers move with average drift velocity , where m is the carrier mobility EE130 Lecture 5, Slide 20