Semi-insulating polysilicon heterojunctions on silicon R.M. Ranade, S.S. Ang, W.D. Brown, H,A. Naseem and J.R. Yeargan Department of Electrical Engineering, University of Arkansas, Fayetteville, AR 72701, USA R.K. Ulrich Department of Chemical Engineering, University of Arkansas, Fayetteville, AR 72701, USA The electrical characteristics of undoped semi-insulating polysilicon (SlPOS)/p-type crystalline silicon (p-Si) and N+-SlPOS/p-Si hetero- junctions were investigated. The current-voltage characteristics of the undoped SIPOS/p-Si heterojunctions depart from a hyperbolic sine behavior as the refractive index of the SlPOS increases. Carrier conduction in the undoped heterojunctions is characterized by low- and high- temperature barrier heights, which also vary with refractive index. Current- voltage characteristics of the n+-SlPOS/p-Si structures were rectifying with a cut-in voltage of about 1 V, which decreased with increasing tem- perature. Increasing the SlPOS doping increased the current density for a given bias and reduced the cut-in voltage. The forward characteristics dis- played both a low and a high field activation energy with the difference attributable to the presence of interface states at the junction. Conse- quently, carrier conduction in these doped and undoped SlPOS/p-Si heterojunctions appears to be controlled by the SlPOS/p-Si interface. Howeve r, hyd roge n an neal i n g pass ivates the i nte rface states the reby alte r- ing the low field conduction region. 1. Introduction Semi-insulating polysilicon (SIPOS) films have been used primarily as a surface passivant for high-voltage semiconductor devices [1]. The finite conductivity of the undoped SIPOS film provides a field-shield effect for the passivated surfaces; yet the conductivity is low enough that leakage current through the film is reasonably low for many device applications. Other applications include use as the insulating layer in a power SIPOS MISS device [2] and as thin film high sheet resistance SIPOS resistors in CMOS SRAMs to replace the polysilicon decoupling resistors I31. Bolt and Simmons [4] investigated the electrical characteristics of undoped SIPOS/n-Si isotype heterojunctions. Assuming a "mosaic" model for the SIPOS as proposed by Tarng [5], they concluded that carrier transport through SIPOS was by thermionic emission (TE) of carriers over the grain boundaries at temperatures above room temperature, while carrier transport below room temperature was by thermionic field emission (TFE) in which significant tunneling through the barrier occurs. MICROELECTRONICS JOURNAL Vol. 22 Nos. 7-8 © 1991 Elsevier Science Publishers Ltd., England 47