A theoretical explanation for the enhanced operation of the thermal diode
P.L. Hagelstein1, Y. Kucherov2
1Massachusetts Institute of Technology, Cambridge, MA 02139, USA
2Eneco, Inc., 391-B Chipeta Way, Salt Lake City, UT 84108, USA
The figure of merit of a near-intrinsic thermoelectric semiconductor has been observed to increase by as much as a factor of 8 in thermal diode devices. We conjectured initially that this was due to a current injection effect from the emitter, a conjecture that has so far not been well received by the community since any discontinuity in the fermi level would be shorted out in an n*n structure. At ICT02, we proposed that this could be prevented by inserting a p-type layer in between the n* and n layers. Experiments on devices with p-type separating layers show that this proposal is correct. Calculations of the open-circuit voltage for n*pn devices based on the numerical evaluation of a formal solution to the transport equation in a constant relaxation time approximation lead to a jump in the fermi level in the region of the gradient in doping profile when the gradient occurs over a few scattering lengths. In this case, the forward current injection is balanced by an ohmic return current as usual, except here the injected current is very large and the conductivity is very low.