Thermionic Injection and Space-Charge-Limited Current in Reach-Through p+np+ Structures

Abstract

The current transport mechanisms of reach-through p+np+ and its related structures have been studied.It has been established that when the applied voltages is slightly greater than the reach-through voltage, at which the n layer is completely depleted, the current increases exponentially with voltage by thermionic injection mechanism. The current-voltage relationship is given by J= A*T^2 exp〔-q(VFB -V)^2/4kTVFB〕where A* is the effective Richardson constant, T the temperature, V the applied voltage, and VFB the flat- band voltage defined as qNDL^2/2εS where ND and L are the ionized impurity density and the length of the n layer respectively. When the injected carrier density rises to a value comparable to the impurity density the space- charge-limited (SCL) effect causes the current to vary less rapidly with the applied voltage . The SCL effect is derived based on an accurate expression of the velocity-field relation, i.e., vs/(1+Es/E),where vs is the scattering-limited velocity and Es is the critical field given by the ratio of vs to the low-field mobility. In the hogh current limit we obtain the linear current-voltage expression J≈qNDvs(V/VFB). Experimental structures are made from epitaxial n on p+ silicon substrate with an epitaxial layer thickness of 8.5 μm and doping concentration of 5*10^14cm^-3. The second p+ layer of about 1μm is formed by diffusion. Good agreement have been obtained between the experimental results and theoretical predictions.