Negative Differential Conductivity in AlGaN/GaN HEMTs: Real Space Charge Transfer from 2D to 3D GaN States?
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As shown below, piezoelectric and pyroelectric effects in AlGaN/GaN heterostructures hinder the real space transfer into the AlGaN layer. However, as we discuss below, real space transfer into the 3D states in GaN is a possible mechanism explaining the observed NDC. EXPERIMENTS
Ipeak
Vg = +3.0 V
8
Isat
6 4 2
Vg = 0 V
0 0
1
2
3
4
5
1.5 3 1.4 1.3
Vvalley
2
1.2
Vpeak 1
1
Source-Drain Voltage (V)
2
1.1
3
1.0
Peak-Valley Current Ratio
Drain Current (mA)
10
Drain Source Voltage ( V )
The Al0.2Ga0.8N/GaN HEMTs were grown on sapphire substrates by low-pressure metal organic chemical vapor deposition (MOCVD). The epilayer design of the samples was described in [5]. The measured 2DEG sheet density was on the order of 1012cm-2 and the Hall mobility was 1,000 cm2/V.s. Figure 1(a) shows the current-voltage characteristics of the 45 µm wide AlGaN-GaN HEMT. The source-drain spacing Lds was 5 µm and the gate length Lg was 1.7µm. The applied gate bias was as high as 3.0 V. The negative differential conductivity (NDC) was observed at the gate biases Vg > 1.5 V. The drain current increased with Vds to the peak value, Ipeak, determined by the gate bias, and then decreased until saturation current (Isat) has been reached. For HEMTs with the drainto-gate spacing of 1 µm and 2 µm the peak drain current was achieved at the source-drain voltages of approximately Vpeak = 1.5 V and Vpeak = 2.5 V, respectively. The NDC was observed at the source-drain voltages Vpeak < Vds < Vvalley, where Vvalley was the drain bias when the NDC effect stopped. The drain current reached its saturation beyond Vvalley. In Figure 1(b), we show the gate bias dependence of Vpeak, Vvalley and the peak-valley current ration R=Ipeak/Ivalley. This figure demonstrates that Vvalley increased with the gate bias with a slope of about 1, while the increase rate of Vpeak was much smaller. On the other hand, R also increased almost linearly with Vg.
Gate Source Voltage ( V )
(a) (b) Figure 1. Drain current-voltage characteristics of the 45 µm wide AlGaN-GaN HEMT. (a) Drain current dependence on Vd, Vg varies from 0 to 3V with the step of 0.5V. (b) The Vpeak, Vvalley and Ipeak/Isat as functions of the gate bias. We measured NDC in the HEMTs with the source-drain spacing from 5µm to 15µm and the offset gate length from 2µm to 12µm. The source-to-gate and gate-to-drain distances were approximately the same and equal to 2µm and 1µm, respectively. Figure 2 demonstrates the normalized current drop as a function of the gate length for 95 µm wide HEMTs (∆Ineg = Ipeak - Isat). The obtained results clearly showed strong dependence of NDC on Lg. At the gate bias of +3.0 V, the increase in the gate length for about 6 times (source-drain spacing increases 3 times) reduces NDC by a factor of larger than 30. The saturation current for the same gate lengths decreases
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linearly only by one third, from 12 mA to 8 mA. The ∆Ineg dependence on the gate length can be described by expression ∆Ineg ∼ Lg-α, where α is from 1.5 to 2. 0.6 0.5
Vg = 3V
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