AlGaN-Based Microwave Transmit and Receive Modules
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AlGaN-Based Microwave Transmit and Receive Modules John C. Zolper Office of Naval Research, code 312 Arlington, VA 22217 [email protected]
ABSTRACT AlGaN High Electron Mobility Transistors (HEMTs) have made great progress for solid state power amplifiers with the demonstration of an X-band power density up to 9.8 W/mm. This high power density is the result of the high current and voltage capability of this material system. Recently, it has also been shown that these devices can achieve low microwave added noise figures (NF = 0.6 dB at 10 GHz) while maintaining a large breakdown voltage (>60 V) and hence a large dynamic range. These results imply that AlGaN HEMTs can be used to perform the active transmit and receive functions in more robust, higher dynamic range modules. In this paper, the progress in AlGaN microwave HEMTs is reviewed and the issues related to AlGaN transmit and receive (T/R) modules are described.
INTRODUCTION
normalized figure of merit
300 250 200 JFOM BHFFOM
150 100 50 0 Si
GaAs
InP
JFOM ∝ (vsat Ec)2
SiC
GaN
BHFFOM ∝ µEc2
Figure 1: Johnson’s and Baliga’s High Frequency Figures of Merit normalized to silicon. T2.4.1
AlGaN transistors have a unique combination of high current density, high breakdown electric field, and good thermal conductivity that enable previously unrealizable microwave power performance for solid state transistors. For microwave transistor performance, two figures of merit (FOMs) have been developed for comparing the inherit semiconductor material capabilities. These FOMs are Johnson’s FOM (vsatEc)2 and the Baliga’s High Frequency FOM (µeEc2) where Ec is the critical breakdown, vsat is the electron saturation velocity, and µe is the low field electron mobility.1,2 Figure 1 shows these figures of merit normalized to silicon for all the potential microwave semiconductor materials. Diamond is not included in Figure 1 since it has not yet been shown to be controllably doped n-type as required for a practical microwave device. The FOM comparison clearly shows the advantage of the GaN material system. The high current density of the AlGaN/GaN heterostructure (up to 1.6 A/mm has been reported3) is a result of the large polarization induced field and large conduction band offset in the AlGaN/GaN system. This polarization field has both a piezoelectric, strain induced, as well as a spontaneous polarization component.4 For the AlGaN HEMTs most widely studied to date, the spontaneous polarization field dominates. This built-in field induces a two dimensional electron gas (2DEG) that is linearly proportional to the Al-mole fraction x (0≤x≤1) as ns= x(5x1013 cm-2).5 For the 30% Al-composition most widely studied, a channel sheet electron density of ~1.5x1013 cm-2 can be realized that is a factor of 5-10 higher than typical GaAs or InP PHEMTs. The associated mobility at this high current density is in the range of 1000 to 1500 cm2/Vs which is well below that achieved in the conventional III-V material, however, the combined µens product is competitive. It is the µens product
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