# Session 5: Solid State Physics. Charge Mobility Drift Diffusion Recombination-Generation

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1 Session 5: Solid State Physics Charge Mobility Drift Diffusion Recombination-Generation 1

2 Outline A B C D E F G H I J 2

3 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) 3

4 Electrons as Moving Particles In vacuum In semiconductor where is the electron effective mass 4

5 Thermal Velocity We saw that: In an electric field,, an electron or a hole accelerates electron and hole effective masses Si Ge GaAs electrons: holes: Average electron kinetic energy= /

6 Carrier Scattering Mobile electrons and atoms in the Si lattice are always in random thermal motion. Electrons make frequent collisions with the vibrating atoms called lattice scattering or phonon scattering (increases with increasing temperature) Average velocity of thermal motion for electrons: ~ K electron 5 Other scattering mechanisms: deflection by ionized impurity atoms deflection due to Columbic 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. 6

7 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: electron electron 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 velocity 7

8 Carrier Mobility With every collision, the electron loses momentum = Between collisions, the electron gains momentum = is the average time between electron scattering events (mean time between collisions) In steady state is the electron mobility is the electron mobility Electron and hole mobilities of selected intrinsic semiconductors (T=300K) Si Ge GaAs InAs [ /] [ /]

9 Mean Free Path Average distance traveled between collisions is called mean free path This is an important length, structures at the order or smaller that m.f.p. show different performance. 9

10 Mechanisms of Carrier Scattering Dominant scattering mechanisms: Phonon scattering (lattice scattering) Impurity (dopant) ion scattering Phonon scattering mobility decreases when T increases: 1 phonon density carrier thermal velocity 1 / - There is less change in the electron s direction of travel if the electron zips by the ion at a higher speed. Boron Ion - - Arsenic Ion + / 10

11 Matthiessen s Rule The probability that a carrier will be scattered by mechanism within a time period is /, where is the mean time between scattering events due to mechanism. Hence, The probability that a carrier will be scattered within a time period is / Temperature Effect on Mobility: 11

12 Mobility Dependence on Doping (Si) 12

13 Velocity Saturation 10 8 Carrier velocity vs. electric field / 10 7 GaAs Si 10 6 electrons holes / 13

14 Drift Velocity = volume from which all holes cross plane in time = # of holes crossing plane in time = charge crossing plane in time = charge crossing plane per unit time = hole current Hole current per unit area = Conductivity of a semiconductor is Resistivity 1 (Unit: ohm-cm) 14

15 Resistivity vs. Doping For n-type material: 1 Z X5 For p-type material: 1 Z T5Note: This plot does not apply to the compensated material! 15

16 Electrical Resistance Resistance (Ohms) 1 Q: Consider a Si sample doped with 10 / Boron. What is its resistivity? A: 10 /, 0( hence p-type) 10 / and 10 / 1 ~ Ω homogeneously doped sample Q: Consider the same Si sample doped with 10 / Arsenic. What is its resistivity? A: 10 /, 10 / ( hence n-type) 910 / and / 1 ~ Ω 16

17 Potential vs. Kinetic Energy Q: Consider a Si sample doped with 10 As. How will its resistivity change when the temperature is increased from T=300K to T=400K? A: The temperature dependent factor in (and therefore ) is. From the mobility vs. temperature curve for 10, we find that decreases from 770 / at 300K to 400 / at 400K. As a result, increases by

18 Potential vs. Kinetic Energy Increasing electron energy electron K.E. Conduction band Valance band hole K.E. Increasing hole energy represents the electron potential energy:.. reference 18

19 Band Bending The potential energy of a particle with charge is related to the electrostatic potential :.. 1 ref. 1 Variation of Ec with position is called band bending. 19

20 Diffusion Particles diffuse from regions of higher concentration to regions of lower concentration region, due to random thermal motion Higher particle concentration Lower particle concentration Direction of Diffusion 20

21 1-D Diffusion Example 1024 Thermal motion causes particles to move into an adjacent compartment every t seconds. Each particle has an equal probability of jumping to the left and to the right

22 Diffusion Current _ is the diffusion constant, or diffusivity. _ electron flow electron current hole flow hole current 22

23 Einstein Relationship Consider a piece of a non-uniformly doped semiconductor: In equilibrium, is constant; therefore, the band energies vary with position: In equilibrium, there is no net flow of electrons or holes 0 0 The drift and diffusion current components must balance each other exactly. (A built-in electric field exists, such that the drift current exactly cancels out the diffusion current due to the concentration gradient.) 23

24 Non Uniformly Doped Semiconductor The position of EF relative to the band edges is determined by the carrier concentrations, which is determined by the dopant concentrations. n-type semiconductor Decreasing donor concentration Under equilibrium conditions: Similarly: Note: The Einstein relationship is valid for a non-degenerate semiconductor, even under non-equilibrium conditions 24

25 Potential Difference due to n(x), p(x) Consider a piece of a non-uniformly doped semiconductor: n-type semiconductor Decreasing donor concentration Similarly: ln ln Therefore: ln 1 ln 25

26 Fermi Energy p-type n-type p-type n-type Density of state: then 1 Fermi dist.:

27 27

28 Non-Equilibrium Process Whenever the thermal-equilibrium condition of a semiconductor system is disturbed processes exist to restore the system to equilibrium Generation and recombination processes act to change the carrier concentrations, and thereby indirectly affect current flow Recombination mechanisms: Generation mechanisms: Direct R-G Center Band-to-band R-G Center Photon (light) Thermal energy Photon Thermal energy Auger Impact Ionization Recombination in Si is primarily via R-G centers 28

29 Recombination Mechanisms Photon (light) Direct or Band to Band: Basis for light emission devices Photon (single particle of light) or multiple phonons (single quantum of lattice vibration equivalent to saying thermal energy) Thermal energy R-G Center: Also known as Schockley-Read-Hall (SRH) recombination Photon (single particle of light) or multiple phonons (single quantum of lattice vibration equivalent to saying thermal energy) Note: Trap level, Two steps: 1st Carrier is trapped at a defect/impurity, 2nd Carrier (opposite type) is attracted to the RG center and annihilates the 1st carrier Auger: Requires 3 particles, Two steps: 1st carrier and 2nd carrier of the same type collide instantly annihilating the electron hole pair (1st and 3rd carrier). The energy lost in he annihilation process is given o the 2nd carrier. 2nd carrier gives off a series of phonons until it s energy returns to equilibrium energy (E~Ec) 29

30 Generation Mechanisms Photon Direct or Band to Band: Does not have to be a direct bandgapmaterial Mechanism that results in Basis for light absorption devices such as semiconductor photodetectors, solar cells, etc. Thermal energy R-G Center: Two steps: A bonding electron is trapped at an unintentional defect/impurity generating a hole in the valence band This trapped electron is then promoted to the conduction band resulting ina new electron-hole pair Almost always detrimental to electronic devices Impact Ionization: Requires 3 particles and typically high electric fields 1st carrier is accelerated by high electric fields Collides with a lattice atom Knocks out a bonding electron Creates an electron hole pair What is it called when this process repeats and what device is it useful for? 30

31 Low-Level Injection Excess Carrier Concentrations: equilibrium values Charge neutrality condition: Low-Level Injection: Often the disturbance from equilibrium is small, such that the majority-carrier concentration is not affected significantly: For an n-type material so For a p-type material so However, the minority carrier concentration can be significantly affected 31

32 Indirect Recombination Rate Suppose excess carriers are introduced into an n-type Si sample (e.g. by temporarily shining light onto it) at time t = 0. How does p vary with time t > 0? 1. Consider the rate of hole recombination via traps: Capture coefficient # or traps 2. Under low-level injection conditions, the hole generation rate is not significantly affected:.. 3. The net rate of change in is therefore where 1 32

33 Relaxation to Equilibrium State Consider a semiconductor with no current flow in which thermal equilibrium is disturbed by the sudden creation of excess holes and electrons. The system will relax back to the equilibrium state via the R-G mechanism: for electrons in p-type material: for holes in n-type material: The minority carrier lifetime is the average time an excess minority carrier survives in a sea of majority carriers. ranges from 1ns to 1ms in Si and depends on the density of metallic impurities (contaminants) such as Au and Pt, and the density of crystalline defects. These deep traps capture electrons or holes to facilitate recombination and are called recombination-generation centers. 33

34 Example: Photoconductor Consider a sample of Si doped with 10 boron, with recombination lifetime 1μs. It is exposed continuously to light, such that electron-hole pairs are generated throughout the sample at the rate of 10 per per second, i.e. the generation rate 10 / / 1. What are and? 10 cm 10 cm 2. What are and? What are and? What is the product? 10 34

35 Net Recombination Rate (General Case) For arbitrary injection levels and both carrier types in a non-degenerate semiconductor, the net rate of carrier recombination is: where and For low level injection: for electrons in p-type material: for holes in n-type material: 35

36 Derivation of Continuity Equation Consider carrier-flux into/out-of an infinitesimal volume: Area,Volume 1 1 Continuity Equation:

37 Minority Carrier Diffusion Equation The minority carrier diffusion equations are derived from the general continuity equations, and are applicable only for minority carriers. Simplifying assumptions 1. The electric field is small, such that ~ ~ 2. and are independent of (uniform doping) 3. low-level injection conditions prevail Starting with the continuity equation for electrons 1 1 in p-type material in n-type material 37

38 Minority Carrier Diffusion Equation The subscript or is used to explicitly denote n-type or p-type material, e.g. is the hole (minority-carrier) concentration in n-type material is the electron (minority-carrier) concentration in p-type material Thus the minority carrier diffusion equations are in p-type material Simplifications (Special Cases): Steady state: 0, 0 in n-type material No diffusion current: 0, 0 No R-G: 0, 0 No light: 0 38

39 Example Consider the special case: 1. constant minority-carrier (hole) injection at 0 2. steady state; no light absorption for 0: 0 0 light Light absorbed at x=0 0 Silicon bar 0 is the hole diffusion length: The general solution to the equation is B.C Hence solution is: where, are constants determined by boundary conditions:

40 Example Minority Carrier Diffusion length Physically, and represent the average distance that minority carriers can diffuse into a sea of majority carriers before being annihilated. Q: Find if 10 ; / 10 / 30 40

41 Quasi-Fermi Levels Whenever 0,. However, we would like to preserve and use the relations: These equations imply, however. The solution is to introduce two quasi-fermi levels and such that ln ln 41

42 Example: Quasi-Fermi Levels Consider a Si sample with 10 and 10. What are p and n? 10, What is the npproduct? 10 Find and : ln ln ln ln

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