Impact ionization in GaAs metal–semiconductor field-effect transistors with a lightly doped drain structure and an Al0.2Ga0.8As/GaAs heterobuffer layer

Abstract

Kink effects (abrupt increases in drain current) were observed with an abrupt increase of gate current (IG), substrate current (Isub), substrate potential (Vsub), and photoemission intensity (Iphoto) in GaAs metal–semiconductor field-effect transistors (MESFETs) with a lightly doped drain (LDD) structure and an Al0.2Ga0.8As/GaAs heterobuffer layer. The kink drain voltage (VKD) increases as substrate temperature increases. Since impact ionization has a negative coefficient for increase of temperature, the increase of VKD indicates that impact ionization occurs at VKD. VKD also increases as VG becomes more negative and as the ratio of the dose of the silicon ions is reduced in the LDD region. Thus it is confirmed that impact ionization occurs at the drain side along the channel current path because those changes of VG and dose ratio reduce the electric field at the drain-side channel. In addition, impact ionization coefficients were calculated from IG and Isub based on the method proposed by Hui et al. Both the coefficients were exponentially proportional to the negative inverse of the electric field, although the magnitude of Isub was approximately 250× greater than that of IG. On the other hand, it was found that Iphoto was linearly proportional to Vsub. It suggests that Iphoto depends on the number of holes which overflow the n-GaAs channel/i-GaAs buffer interface potential barrier into the channel layer. These behaviors of IG, Isub, Iphoto, and Vsub are attributed to this FET structure which has a thin channel layer with a high donor density and a heterobuffer layer.