Role of Buffer Layers of High Power GaAs MESFETs for Higher Output Power

Junichiro Takeda, Yohei Otoki, Tadayoshi Tsuchiya*, Takeshi Meguro** and Yukio Sasaki**
Hitachi Cable America, Inc., 2665 North First Street, Suite 200, San Jose, CA 95134, USA
*Hitachi Cable Ltd., 3550 Kidamari, Tsuchiura, Ibaraki, 300-0026, JAPAN
**Hitachi Cable Ltd., 880 Isagozawa, Hitachi, Ibaraki, 319-1418, JAPAN
E-mail: takeda@hitachi-cable.com

Keywords: GaAs, Buffer, Power FET

Abstract
The GaAs MESFETs have been used for the high power amplifier for infrastructure use like a base station and a CATV system. These PAs are operated in class B or C, where depletion layer goes deep in the channel and across into the buffer layer. That means the buffer layer has important role for the performance of the PAs especially for output power. The buffer is usually consists of single or multiple AlGaAs layers. In this paper, the role of the AlGaAs buffer layers is investigated focused on the output power of the transistors and the optimized design is suggested.

The MESFETs with n-type GaAs channel of 1E17cm-3 with different buffer structures (Fig.1) were grown by MOVPE and DC performance was measured. The varied parameters of the buffer structures were Al composition (0.2 to 0.4), thickness (50nm to 300nm), and layer numbers (single and multi) as shown in Table1. We defined the “good buffer” for Power MESFET shown in Fig.2 that the current flowing near the buffer should be smooth and leakage through buffer should be minimum. Depended on this definition, we set the unique evaluation by DC measurements shown in Fig.2, where delta gm and delta Vd at high voltage of 11V show the smoothness of current and the leakage, respectively.

Using this method, we could evaluate the effect of the buffer parameters qualitatively as shown in Fig.3. It’s quite interesting that the high delta Gm well coincident with the mobility near the buffer layer as shown in Fig.4. That means high quality of the crystal gave us high Gm near the buffer. All the results lead us the optimized design of the buffer. We compared the power output of conventional one (300nm single layer, x=20%), and optimized one (50nm x 5 layers, 28%) and found this improvement gave us big improvement of 20% higher output as shown in Fig.5. We showed the buffer design is very important for the high power amplifier applications of GaAs MESFETs.

Paper 9b.3.pdf