Semiconductor Device Physics Lecture 4 Dr. Gaurav Trivedi, EEE Department, IIT Guwahati

Potential vs. Kinetic Energy Electron kinetic energy Increasing electron energy Ec Ev Hole kinetic energy Increasing hole energy Ec represents the electron potential energy:

Band Bending Since Ec, Ev, and Ei differ only by an additive constant The potential energy of a particle with charge –q is related to the electrostatic potential V(x): Since Ec, Ev, and Ei differ only by an additive constant

Band Bending Ec Variation of Ec with position is called “band bending” Until now, Ec and Ev have always been drawn to be independent of the position. When an electric field E exists inside a material, the band energies become a function of position. Ec Ev E x Variation of Ec with position is called “band bending”

Diffusion Particles diffuse from regions of higher concentration to regions of lower concentration region, due to random thermal motion (Brownian Motion).

1-D Diffusion Example Thermal motion causes particles to move into an adjacent compartment every τ seconds.

Diffusion Currents n x p x D is the diffusion coefficient [cm2/sec] Electron flow Hole flow Current flow D is the diffusion coefficient [cm2/sec]

Total Currents Drift current flows when an electric field is applied. Diffusion current flows when a gradient of carrier concentration exist.

Current Flow Under Equilibrium Conditions In equilibrium, there is no net flow of electrons or : The drift and diffusion current components must balance each other exactly. A built-in electric field of ionized atoms exists, such that the drift current exactly cancels out the diffusion current due to the concentration gradient.

Current Flow Under Equilibrium Conditions Consider a piece of non-uniformly doped semiconductor: n-type semiconductor Decreasing donor concentration Ev(x) Ec(x) EF Under equilibrium, EF inside a material or a group of materials in intimate contact is not a function of position

Einstein Relationship between D and m But, under equilibrium conditions, JN = 0 and JP = 0 Similarly, Einstein Relationship Further proof can show that the Einstein Relationship is valid for a non-degenerate semiconductor, both in equilibrium and non-equilibrium conditions.

Example: Diffusion Coefficient What is the hole diffusion coefficient in a sample of silicon at 300 K with p = 410 cm2 / V.s ? Remark: kT/q = 25.86 mV at room temperature

Recombination–Generation Recombination: a process by which conduction electrons and holes are annihilated in pairs. Generation: a process by which conduction electrons and holes are created in pairs. Generation and recombination processes act to change the carrier concentrations, and thereby indirectly affect current flow.

Generation Processes Band-to-Band R–G Center Impact Ionization Release of energy ET: trap energy level Due to lattice defects or unintentional impurities Also called indirect generation EG Only occurs in the presence of large E

Recombination Processes Band-to-Band R–G Center Auger Collision Rate is limited by minority carrier trapping Primary recombination way for Si Occurs in heavily doped material

(direct semiconductors) (indirect semiconductors) Direct and Indirect Semiconductors Ev Ec Ec Ev Phonon Photon Photon GaAs, GaN (direct semiconductors) Si, Ge (indirect semiconductors) Little change in momentum is required for recombination Momentum is conserved by photon (light) emission Large change in momentum is required for recombination Momentum is conserved by mainly phonon (vibration) emission + photon emission

Values under arbitrary conditions Excess Carrier Concentrations Values under arbitrary conditions Deviation from equilibrium values Equilibrium values Positive deviation corresponds to a carrier excess, while negative deviations corresponds to a carrier deficit. 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 For a p-type material Low-level injection condition However, the minority carrier concentration can be significantly affected.

Indirect Recombination Rate Suppose excess carriers are introduced into an n-type Si sample by shining light onto it. At time t = 0, the light is turned off. How does p vary with time t > 0? Consider the rate of hole recombination: NT : number of R–G centers/cm3 Cp : hole capture coefficient In the midst of relaxing back to the equilibrium condition, the hole generation rate is small and is taken to be approximately equal to its equilibrium value:

Indirect Recombination Rate The net rate of change in p is therefore: where For holes in n-type material Similarly, where For electrons in p-type material

Minority Carrier Lifetime The minority carrier lifetime τ is the average time for excess minority carriers to “survive” in a sea of majority carriers. The value of τ ranges from 1 ns to 1 ms in Si and depends on the density of metallic impurities and the density of crystalline defects. The deep traps originated from impurity and defects capture electrons or holes to facilitate recombination and are called recombination-generation centers.

Example: Photoconductor Consider a sample of Si at 300 K doped with 1016 cm–3 Boron, with recombination lifetime 1 μs. It is exposed continuously to light, such that electron-hole pairs are generated throughout the sample at the rate of 1020 per cm3 per second, i.e. the generation rate GL = 1020/cm3/s. c) What are p and n? d) What are np product? Note: The np product can be very different from ni2 in case of perturbed/agitated semiconductor

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 ET : energy level of R–G center