Recent Progress in GaN Based Field Effect Transistors
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semiinsulating substrate (and a relatively low dielectric constant), makes GaN-based materials extremely appealing for microwave and millimeter wave applications.
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Figure 1: Electron drift velocity at 300 K in GaN, SiC, and GaAs (a)and electron drift velocity in GaN at 300 K, 500 K, and 750 K (b). 12 The calculations of the low field mobility generally agree with the results of the Monte Carlo simulations and experimental data. 13-16. Shur et al. 16 compared the temperature dependencies of the electron mobility in the two dimensional electron gas (2DEG) in A1GaN/GaN heterostructures and in doped bulk GaN. Based on their experimental data and calculations, they demonstrated a large enhancement of the electron mobility in the 2DEG compared to bulk GaN (see Fig. 2).
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Khan et al. 17 reported on the measurements of the Hall mobility in doped channel GaN heterostructures. Their results (in agreement with the computed dependencies shown in Fig. 2) confirm that the electron mobility at room temperature decreases relatively little with doping. This important result led to the development of the AlGaN/GaN Doped Channel Heterostructure Filed Effect Transistors (DC-HFETs) as discussed below.
Temperature (K) Figure 2:
Measured and calculated Hall electron
mobility in bulk GaN and in the 2DEG. 16
IV. GaN-BASED MESFETs, MISFETs, and HFETs. All basic types of field effect transistors - GaN MESFETs, GaN MISFETs, Inverted Channel A1GaN/GaN HFETs, AlGaN/GaN DC-HFETs - have been developed (see Fig. 3). Khan et al. reported on the microwave performance of short channel (0.2 gtm gate length) AlGaN/GaN Heterostructure Field Effect Transistors (HFLTL) at room " and elevated temperatures up to 110 oC with the cutoff frequency, fT, up to 20 GHz and the maximum frequency of oscillations, fnax, up to 77 GHz. '9 Binari et al. 2,21 reported on the microwave performance of the AlGaN/GaN Inverted HFETs, GaN MESFETs, and Si3 N4/GaN MISFETs 22 with the gate length L = 0.8 .tmat room temperature. Binari et al. 21, 22 and Khan et al. also presented the results of the dc and microwave measurements showing that these devices can operate at least up to 360 OC (see Fig. 4). 18
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Figure 3: Schematic structure of different GaN and AIGaN/GaN FETs. GaN MESFET (a), GaN MISFET (b), AlGaN/GaN HFET (c), and AIGaN/GaN DC-HFET (d). In certain cases A GaN buffer layer may be used.
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Figure 4: I-V characteristics of AIGaN/GaN HFETs at different temperatures.
18
As was mentioned above, the FET epilayer
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