Contacts Introduction MetalMetal Contacts Seebeck Effect and Thermocouples
Contacts • Introduction • Metal-Metal Contacts • Seebeck Effect and Thermocouples • Metal-Semiconductor Contacts • Thermoelectric Cooler EE 3406/ECE/NUS 40
Introduction - Different kinds of contacts/Junctions (i) Metal-Metal (ii) Semiconductor-semiconductor (pn junction, discussed in EE 2004) Meaning of contact, good & bad contact (iii) Metal-Semiconductor (iv) Metal-Insulator-Semiconductor (MIS capacitor, discussed in EE 2004) EE 3406/ECE/NUS 41
Metal-Metal Contacts (I) Consider the contact of Platinum (Pt: work function ФM 1=5. 36 e. V) and Molybdenum (Mo: work function ФM 2=4. 2 e. V): - Before contact, two systems are isolated and neutral. - After contact, electrons in Mo which are at higher energy move from Mo to Pt to fill up the available lower energy states. This reduces the total energy of the electrons in the Pt-Mo system. - Pt surface becomes negative charged and a contact potential develops at the junction. This potential difference favors the electron motion in the opposite direction (Pt Mo) - Electron transfer (Mo Pt) continues until the contact potential is large enough to prevent further transfer equilibrium achieved. (This is equivalent to alignment of Fermi levels) EE 3406/ECE/NUS 42
Metal-Metal Contacts (II) Principle to draw band diagram at thermal equilibrium: - Fermi level has to be aligned Why? - Band-bending occurs and the vacuum level has a potential drop work must be done to get an electron from Mo to Pt (in this case, the potential drop=1. 16 V) Looks familiar? Recall pn junction band diagram in EE 2004… EE 3406/ECE/NUS - M-M contacts are found applications in thermo-couples or in multi-layered bondpads in ICs 43
Seebeck Effect: Principle Consider a conductor eg. an aluminum rod heated at one end and cooled at the other: - Electrons in hot region more energetic velocity greater than those in cold region diffuse from hot end toward cold end positive metal ions left in hot region and accumulated electrons in cold region. - Diffusion continues until electric field developed between hot and cold regions prevent further electron motion voltage develops between hot and cold end. - The creation of a positive difference ΔV across a piece of metal due to a temperature difference ΔT is called Seebeck effect. EE 3406/ECE/NUS 44
Seebeck Effect: Seebeck Coefficient - The Seebeck coefficient (or thermoelectric power) is defined as the potential difference developed per unit temperature difference S=d. V/d. T where the sign of S represents potential of cold side wrt to hot side - Some discussion on S: (i) In most cases of metal, S is negative. (ii) in some special cases (such as in Cu), S is positive. The reason is that net diffusion is more complicated. For details, see textbook. EE 3406/ECE/NUS 45
Thermocouples: Application of Seebeck Effect - A thermocouple uses two different metals with different Seebeck coefficients: one junction maintained as reference temperature T 0 and other used to sense T - Potential difference between two wires is given by where, SAB=SA-SB is defined as thermoelectric power for thermocouple pair A-B. - In general, Food for thought: what will happen if Au is replaced by Ni? EE 3406/ECE/NUS is known as thermocouple equation where a and b are thermocouple coefficients and ΔT=T-T 0 is temperature wrt reference temperature T 0. 46
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