ABSTRACT
Abstract. In this paper, I describe a state-of-the-art 250-W AlGaN/GaNHEMTs push-pull transmitter amplifier operated at a drain bias voltage of 50 V. The amplifier, combined with a digital pre-distortion (DPD) system, also achieved an adjacent channel leakage power ratio (ACLR) of less than -50 dBc for 4-carrier W-CDMA signals with a drain supply voltage of 50 V. I also demonstrate a stable operation under RE stress testing for 1000 h at a drain bias voltage of 60 V. Device fabrications on a 4-inch sapphire and 3-inch S.I.-SiC substrate were also addressed. These performances verify that an AlGaN/GaN HEMTs amplifier is suitable for 3G W-CDMA systems. 1
1. Introduction AlGaN/GaN-based HEMTs have potential for micro-wave power applications, including wireless base stations. There are several reports relating to their high output-power density characteristics [1-5]. However, only a few studies shown their high-efficiency using large gate-periphery devices with a drain bias voltage (Vds) of over 50 V due to instability related to current collapse [6-10]. In addition, higher gain over 12 dB with high total output power over 100 W is needed. In this paper, a state-of-the-art 250-W high-gain AlGaN/GaN-HEMTs push-pull transmitter amplifier operated at a drain bias voltage of 50 V will be addressed [10]. Base station systems also require highly efficient amplifiers [7,8,10,11]. We therefore need to ensure that the quiescent bias current is low [11]. Low distortion characteristics near class-B operation are required when using digital modulation schemes such as IMT2000 [8.10]. A digital pre-distortion (DPD) system, which was developed for practical W-CDMA base station amplifiers, controls a pre-distorted input signal into the power amplifier to cancel out the nonlinearity of the power amplifier. Thus, the AlGaN/GaN HEMT amplifier should be embedded in the DPD system. In this paper, the amplifier, combined with the DPD system, also achieved an adjacent channel leakage power ratio (ACLR) of less than -50 dBc for 4-carrier W-CDMA signals with a drain supply voltage of 50 V [8,10]. Moreover, reliability and cost must be discussed for mass-production. There are many reports concerning to the frequency dispersion related instabilities, such as large-signal current collapse and gm dispersion [12-14], which degrade the power performance of the
conventional Al-GaN/GaN HEMTs. To overcome these problems, we used an n-type doped GaN cap layer in the AlGaN/GaN HEMTs and controlled polarization-induced surface charges [3]. Device fabrication on a 4-inch sapphire [15] and a 3-inch S.I.-SiC will also be addressed to consider the cost issue.
