Chapter 10. Junctions Junctions • Surface ~ simplest one; junction between vacuum/surface • Metal/Metal qq potential is set up MM12 • Metal/Semiconductor Blocking contact ~ Schottky barrier Ohmic contact ~ low resistance • Semiconductor/Semiconductor a. Homojunction ~ both side of junctions are same b. Heterojunction ~ each side is different c. Iso-type ~ types are same to both sides < n types or p types > d. Anisotype < p + n > • M/I/M, MIS, SIS, MOS Surfaces Termination of the periodic potential Chemisorption of oxygen on n-type → localized states at the surface Semiconductor surface Surface state bending of band gives Ɛ positively charged O - depletion region Ec 2 chemisorption + + separation of charge EF - d - negatively charged surface Ev state - • As more O2 are formed, the energy bands bend at the surface of the semiconductor because of the local field that is built up. Metal/Metal qcp Ɛ q qB q A A contact qB potential B B A A • Transfer of electron until EF are same for both of metals • A potential difference cp known as the contact potential is set up between two metals. • If qqqqqABcpAB, • EF has to be same in thermal equilibrium for whole system Although an internal field exists, no potential can be measured in an external circuit connecting the two metals together. Local electric field is measured by the Kelvin Probe method – vibrating method Metal/Semiconductor – Schottky Barrier q D qS S ≡ electron qM qM affinity S q E - S b - ++ Wd qEE () SS cF Blocking contact • Blocking contact if Φ Φ for n-type and Φ Φ for p-type • A flow of electrons from the semiconductor to the metal in order to equalize the Fermi energies in the two materials. • An internal field Φ is developed in the semiconductor. • Φ Φ Φ : diffusion potential, built-in potential • Φ : Energy barrier Metal/Semiconductor – Schottky Barrier • The width of the depletion region + Ionized donor density = ND E r 0 dE Nq D dx r 0 d E dx d 2 Nq D Boundary conditions 2 : Poisson's equation dx r 0 Nq 2 0 at qW D 2 at DD 0 at 0 2r 0 Metal/Semiconductor – Schottky Barrier • In the depletion region Wd, potential change Φ ,ΦΦ / 2Φ Φ → The more donors, the smaller . • If we include the effects of an applied voltage Φapp, / 2Φ Φ Φ The more donors or the larger applied voltage can recued the contact resistance. → The larger applied voltage, the smaller . Metal/Semiconductor – Schottky Barrier metal semiconductor depletion region Wd conductor dielectric nearly conductor 1 / 2Φ Φ / 1 M SD slope ∝ 1 2 Φ Φapp app / Method to measure and Φ Metal/Semiconductor – Schottky Barrier • JV characteristics of Schottky barrier (n-type semiconductor) J qq () + DMS + qMS V EE q Metal - Metal + cF SS • Thermionic emission → Φ / → Φ Φapp/ (only change by Φapp) Metal/Semiconductor – Schottky Barrier • net → → Φ / Φapp/ 1 ≡ J0 , not depend on the Φapp • Basic equation for current in a M/S junction J net Φapp/ 1 For forward bias with positive Φapp Φapp/ forward J0 For reverse bias with negative Φapp Φapp reverse Metal – Metal + reverse forward Metal/Semiconductor – Schottky Barrier J J0 Φapp reverse forward • In a forward bias, contact potential is reduced from Φ to Φ Φ) → forward current (metal to semiconductor) • In a reverse bias, contact barrier increases to Φ Φ) → negligible electron flow from semiconductor to metal • In both cases, metal-to-semiconductor electron flow is determined by Φ • Diode equation: Φapp/ 1 Ohmic contact qqDSM() q qM S S e- SM q metal n-semiconductor Accumulation region (reservoir of e-) n-type: Φ Φ • Electrons flow without any barrier contact does not make any change in J. p-type: Φ Φ (opposite of Schottky barrier) Ohmic contact • The accumulation layer in the semiconductor serves as a ready reservoir of electrons for conduction in the material available at the contact, and thus application of an electric field measures only the conductivity (R) of the semiconductor. J R V p-n junction - homojunctions q n e- n Φ Φ Φ qp p p h+ p-type n-type Depeletion region W = Wp+ Wn Similar to two Schottky barriers p-type n-type Wp Wn p-n junction - homojunctions When no electric field is applied (Φapp = 0) When an electric field (Φapp) is applied Minority carrier n qkTapp / p nep y Majority y carrier qqDapp qapp Majority x carrier x peqkTapp / Minority n pn carrier Majority carriers ~ e- in n-type, h+ in p-type Minority carriers ~ h+ in n-type, e- in p-type p-n junction - homojunctions J • app/ 1 J0 Φ app p - p + • The p-n junction shows the similar form with a Schottky barrier (Φapp dependence of J ) Note: The pre-exponential reverse saturation current (J0) has a different definition with Schottky barrier. Applications of p-n junctions • Rectifier • Amplifier • p-n-p type transistor • Field-effect transistor • Photodetector • Phototransistor • Solar cell • Light emitting diode • Tunnel diode Applications of p-n junctions Photodetector: operating under a reverse bias J e-h pair - light - + + Φapp ΔI • Increase of current by light irradiation number of collected electrons Photoconductivity gain ≡ 1 number of absorbed photons Applications of p-n junctions Solar cell: no applied bias e-h pair - light - + + Open circuit voltage O Short circuit current The band gap of about 1.4 eV proves to be optimal for solar energy conversion application. Applications of p-n junctions Light emitter: Forward biased p-n junction e-h pair - - + - LED, Laser + + • Direct band gap materials are favored for light emitting application. • GaAsP (R), InGaN (B), GaN (B) • By coating a blue LED with phosphor materials, a portion of the blue light can be converted to red and green (or yellow) lights for white light. Semiconductor-semiconductor junctions: Heterojunctions An isotope heterojunction between two n-type materials with the same electron density, but showing a discontinuity at the interface because of a difference in electron affinities. Ec 12 EEEvGG21 2 1 qEEEEDcFcF()()11 1 2 2 2 DD211122// NN D D A positive value of Δ or a negative value of Δ implies a spike impeding the transport of electrons or holes, respectively. Semiconductor-semiconductor junctions: Heterojunctions Energy band diagrams for p-n heterojunctions. The materials in (a) and (b) have the same band gaps, but in (a) the p-type material has a smaller electron affinity than the n-type material, whereas in (b) the situation is reversed. van der Waals Heterojunctions Multiple quantum well Epitaxially grown MQW Lattice mismatch (MQW)laser 2D semiconductors van der Waals heterostructure WSe2 MoS2 MoS2 WSe2 MoS2 WSe2 EF • Stacking of 2D semiconductors for heterostructure • Less interaction between layers (almost flat band) Appl. Phys. Lett. 102, 012111 (2013) • No issue of lattice mismatch.