Thermionic injection and space-charge-limited current in reach-through p+np+ structures

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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 voltage 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 or diffusion over the potential barrier. The current‐voltage relationship is given by J ≅ A*T 2 exp[−q(V FB − V)2/4kTV FB], where A* is the effective Richardson constant, T the temperature, V the applied voltage, and V FB the flat‐band voltage defined as qNDL 2/2ε s , 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 high‐current limit, we obtain the linear current‐voltage expression J ≃ qNDvs (V/V FB). Experimental structures are made from epitaxialn on p +silicon substrate with an epitaxial layer thickness of 8.5 μm and doping concentration of 5 × 1014 cm−3. The second p + layer of about 1 μm is formed by diffusion. Good agreement has been obtained between the experimental results and theoretical predictions.