Gate voltage swing enhancement of InGaP/InGaAs pseudomorphic HFET with low-to-high double doping channels

Abstract

Direct current characteristics of an InGaP/InGaAs pseudomorphic HFET employing low-to-high double doping channels are first demonstrated and compared with the conventional single doping-channel device. Because the lower InGaAs channel is heavily doped and two-dimensional electron gas is formed in this channel, the threshold voltage is extended to 23.7 V. Experimentally, the transconductance within 50% of its maximum value for gate voltage swing up to 4.5 V is achieved in the studied device, which is greater than that of 3.85 V in the single doping-channel device.Introduction: Recently, heterostructure field effect transistors (HFETs) have been widely applied in signal amplifiers and digital integrated circuits [1, 2]. Among the HFETs, the transconductance of high electron mobility transistors (HEMTs) may be relatively high resulting from the formed two-dimensional electron gas (2DEG) in the modulated channel; however, the magnitude of forward gate bias is severely limited owing to the so-called parallel conduction problem [3]. This will degrade the current driving capability and device linearity. But, transistor performance with high output current and device linearity is essential for linear amplification in circuit applications. A variable HFET, i.e. doping-channel FET (DCFET), has been demonstrated to exhibit broad gate voltage swing for improving device linearity and reducing the higher order harmonic terms in linear amplifier applications [4,5]. The basic structure of DCFETs consists of a narrow bandgap doped material as the channel layer to enhance carrier conduction and a wide bandgap undoped (or low-doping) barrier layer as an 'insulator' to improve the gate Schottky characteristic.In this Letter, owing to the lower channel with heavy doping concentration and the formation of 2DEG in this channel, the magnitude of the threshold voltage is extended and the gate voltage swing is effectively improved by the employment of a low-to-high doping channel profile in the InGaP/InGaAs pseudomorphic HFET.