As is known, pseudomorphic high electron mobility transistors (PHEMTs) are extensively used in wireless communication systems for switching, power and low noise amplifier applications. These transistors find wide market acceptance because of their high RF gain and power added efficiency (PAE) and low noise figure (NF). The excellent properties of these transistors also make them attractive for use in satellite communication systems including direct broadcast satellite television (DBS-TV) and global satellite communication systems. PHEMT technology is also used in high-speed analog and digital IC's such as 2.5-10 Gb/s lightwave communication systems. The higher frequency response of PHEMTs are currently finding use in millimeter wave communications (40 Gb/s) and radar systems.
The increasing market demand for RF power devices with higher and higher performance for wireless communication systems, radar detection, satellite and electronic warfare systems has led the electronics industry to extend the operating frequencies of available power devices and technologies. To increase operating frequencies of power devices up to the millimeter wave range, several key technological features have been proposed including an optimized PHEMT epitaxial layer structure, an optimized device layout of the unit cell for specific applications, and thin-film technology development adequate for high frequency functions. In particular, the most significant requirement set for the epitaxial layer structure is associated with the condition of ensuring that free electrons in the conductive channel are physically separated from ionized donors. This solution allows for a significant reduction in ionized impurity scattering, enhancing electron mobility and therefore marked performance improvements over conventional metal-semiconductor field effect transistors (MESFET).
Additionally, pseudomorphic high electron mobility transistors PHEMTs usually require a negative gate voltage bias to operate (see for example U.S. 2002/024057 A1). Therefore, as compared to heterojunction bipolar transistors (HBTs) that use a positive gate voltage, the introduction of such a negative voltage disadvantageously requires a dedicated voltage supply circuit that increases chip complexity and manufacturing costs.
A PHEMT power device capable of operating with a single voltage supply is for example disclosed in U.S. Pat. No. 6,593,603. The PHEMT power device includes an epitaxial substrate including a GaAs buffer layer, an AlGaAs/GaAs superlattice layer, an undoped AlGaAs layer, a first doped silicon layer, a first spacer, an InGaAs electron transit layer, a second spacer, a second doped silicon layer having a different doping concentration from the first doped silicon layer, a lightly doped AlGaAs layer, and an undoped GaAs cap layer stacked sequentially on a semi-insulating GaAs substrate; a source electrode and a drain electrode formed on and in ohmic contact with the undoped GaAs cap layer; and a gate electrode formed on the lightly doped AlGaAs layer to extend through the undoped GaAs cap layer.