Conservation of Lateral Momentum in Heterostructure Integrated Thermionic Coolers
- PDF / 403,014 Bytes
- 15 Pages / 612 x 792 pts (letter) Page_size
- 19 Downloads / 183 Views
Conservation of Lateral Momentum in Heterostructure Integrated Thermionic Coolers Daryoosh Vashaee and Ali Shakouri* Jack Baskin School of Engineering University of California Santa Cruz, CA 95064
ABSTRACT Thin film thermionic coolers use selective emission of hot electrons over a heterostructure barrier layer from emitter to collector resulting in evaporative cooling. In this paper a detailed theory of electron transport perpendicular to the multilayer superlattice structures is presented. Using Fermi-Dirac statistics, density-of-states for a finite quantum well and the quantum mechanical reflection coefficient, the currentvoltage characteristics and the cooling power density are calculated. The resulting equations are valid in a wide range of temperatures and electric fields. It is shown that conservation of lateral momentum plays an important role in the device characteristics. If the lateral momentum of the hot electrons is conserved in the thermionic emission process, only carriers with sufficiently large kinetic energy perpendicular to the barrier can pass over it and cool the emitter junction. However, if there is no conservation of lateral momentum, the number of electrons participating in thermionic emission will dramatically increase. The theoretical calculations are compared with the experimental dark current characteristics of quantum well infrared photodetectors and good agreement over a wide temperature range is obtained. Calculations for InGaAs/InGaAsP superlattice structures show that the effective thermoelectric power factor (electrical conductivity times the square of the effective Seebeck coefficient) can be improved comparing to that of bulk material. We will also discuss methods by which the conservation of lateral momentum in thermionic emission process can be altered such as by creating a controlled roughness at the interface of the superlattice barriers. The improvement in the effective power factor through thermionic emission can be combined with the other methods to reduce the phonon thermal conductivity in superlattices and thus obtain higher thermoelectric figure-of-merit ZT. I- INTRODUCTION Heterostructure Integrated Thermionic (HIT) coolers have been recently made and characterized for applications to integrated cooling (See Fig. 1) [1-9]. In these structures a potential barrier is used for the selective emission of hot electrons and evaporative cooling of the electron gas. The HIT cooler could operate in two modes. For a single barrier structure in the strong nonlinear regime, electron transport is dominated by the supply of electrons in the cathode layer [1]. Since only hot electrons (with energy bigger greater than Ef) are emitted above the barrier, electron-electron and electron-phonon interactions try to restore the quasi Fermi distributions in the cathode layer by absorbing *
[email protected]
G6.5.1
heat from the lattice, thus cooling the layer. This heat is deposited on the anode side. Theoretical estimates [1,2,16] show that there is an optimal barrier width of the order of
Data Loading...
