tailieunhanh - Micowave and Millimeter Wave Technologies Modern UWB antennas and equipment Part 16

Tham khảo tài liệu 'micowave and millimeter wave technologies modern uwb antennas and equipment part 16', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 442 Microwave and Millimeter Wave Technologies Modern UWB antennas and equipment Wide band gap semiconductors such as GaN and SiC are very promising technologies for microwave high power devices. The advantages of these materials over conventional semiconductors GaAs and Si include high breakdown field Eg high saturation electron velocity vsat and high thermal conductivity. GaN AlGaN high electron mobility transistors HEMTs offer even higher power performance due to the higher carrier sheet density and the higher saturation velocity of the bidimensional electron gas channel 2DEG compared to SiC metal semiconductor field effect transistors MESFETs . The diagram in Fig. 1 summarizes GaN-HEMT properties and its benefits. This technology has demonstrated a power density of 30 W mm using devices with dimensions of pm2 at 4 GHz when biased at 120 V Wu et al. 2004 . Fig. 1. GaN HEMT properties and benefits. In a HEMT the conduction channel is confined to the interface between two materials with different band gap. This region known as 2DEG has very few ionized impurities to scatter the electrons resulting in a very high mobility device. AlGaN GaN heterostructures have a high sheet carrier density in the 2DEG interface without intentional doping of the structure. The spontaneous and piezoelectric polarization effects are the key factors for the charge distribution in the AlGaN GaN HEMT Ambacher et al. 2000 . Fig. 2. AlGaN GaN HEMT model. Broadband GaN MMIC Power Amplifiers design 443 The model of a HEMT that shows the small-signal parameters and the 2DEG channel is depicted in Fig. 2. Source and drain ohmic contact as well as Schottky gate can be observed. The gate voltage Vgs controls the current Ids that flows between the source and the drain. When Vgs reaches pinch-off voltage the electrons below the gate are depleted and no current can flow from drain to source. Since AlGaN GaN HEMTs for HPA applications work under high power conditions nonlinear models have .