The anomalous behavior of Schottky barrier diodes made on lightly doped GaAs

Abstract

When Schottky barrier diodes are used to evaluate lightly dopedn type (n∼10¹⁵ per cu cm) epitaxial GaAs onn ⁺ substrates, the diodes do not always have the expected varactor characteristics. The capacitance variation with bias is sometimes too small, and the voltage intercept on the 1/C ² vsV plot too high. This behavior has been attributed either to a thin intrinsic semiinsulating layer between substrate and epi layer,¹ or to deep trap impurities in the epi layer.² The effects seen in this work are shown to be due to traps by the dependence of diode characteristics on heat, light, and frequency. These diodes also have conductances which decrease with reverse bias. The model of Sah and Reddi³ leads to a reduction in high frequency junction capacity and capacitance-bias variation range. The large, bias dependence conductance observed in our diodes must also be explained, however. A schematic model is proposed in which a series resistancer, due to the resistance of the undepleted epi layer at high frequency, is shunted by a capacitanceC _u, also due to the undepleted layer. A junction conductance 1/R is in parallel with the junction capacitanceC. The equivalent circuit seen by the capacitance bridge, used for obtaining data, is a measured capacitanceC _m in parallel with the measured shunt resistanceR _m. Much of the observed diminution of capacity and varactor action can be accounted for in terms of this model. The series resistancer seems to be large only at high frequencies, and is associated with the traps in the epi layer.