A $\delta$-Doped InGaP/InGaAs pHEMT With Different Doping Profiles for Device-Linearity Improvement

In this paper, delta-doped InGaP/InGaAs pseudomorphic high-electron-mobility transistors (pHEMTs) with doping-profile modifications are investigated in order to improve the device linearity. The proposed modification was based on the third-order intermodulation distortion (IM3) and the third-order intercept point (IP3) analysis using a simple equivalent circuit of the devices. The correlations of the extrinsic transconductance (Gm) with IM3 and IP3 indicate that the flatness of Gm, as a function of gate-bias causes a lower IM3 level. On the other hand, a high Gm with a flatter Gm distribution results in higher IP3 value for the device. Therefore, doping modifications that improve the flatness of the Gm distribution will also improve the device linearity. Doping modifications in the Schottky layer (Schottky layer doped) and in the channel layer (channel doped) of the conventional delta-doped InGaP/InGaAs pHEMT were investigated. It was also found that extra doping, either in the channel region or in the Schottky layer, improved the flatness of the Gm distribution under different gate-bias conditions. This achieved a lower IM3 and a higher IP3 with a small sacrifice in the peak Gm value. The power performances of these devices were tested at different drain biases. Even though it had the lowest electron mobility among the three different types of devices studied, the channel-doped device demonstrated the best overall linearity performance, the highest IP3 value, the lowest IM3 level, and the best adjacent-channel power ratio under code-division multiple-access modulation.