Gate leakage current induced trapping in AlGaN/GaN Schottky-gate HFETs and MISHFETs
© Liao et al.; licensee Springer. 2014
Received: 30 June 2014
Accepted: 26 August 2014
Published: 8 September 2014
This study examined the correlation between the off-state leakage current and dynamic on-resistance (RON) transients in AlGaN/GaN heterostructure field-effect transistors (HFETs) with and without a gate insulator under various stress conditions. The RON transients in a Schottky-gate HFET (SGHFET) and metal-insulator-semiconductor HFET (MISHFET) were observed after applying various amounts of drain-source bias stress. The gate insulator in the MISHFET effectively reduced the electron injection from the gate, thereby mitigating the degradation in dynamic switching performance. However, at relaxation times exceeding 10 ms, additional detrapping occurred in both the SGHFET and MISHFET when the applied stress exceeded a critical voltage level, 50 V for the SGHFET and 60 V for MISHFET, resulting in resistive leakage current build-up and the formation of hot carriers. These high-energy carriers acted as ionized traps in the channel or buffer layers, which subsequently caused additional trapping and detrapping to occur in both HFETs during the dynamic switching test conducted.
Recently, AlGaN/GaN heterostructure field-effect transistors (HFETs) have been considered as a disruptive technology for high-power switching . However, the degradation in dynamic switching performance is a crucial problem limiting the application of GaN-based HFETs [2, 3]. To clarify the physical mechanisms, several studies have attributed this degradation in performance to two main sources. One source is the surface states associated with electrons injected from the gate. Injected electrons that are trapped in surface states form a negative potential that reduces the electrons in two-dimensional electron gas (2DEG) channels and acts as a ‘virtual gate’ in HFETs [4, 5]. This degradation can be mitigated by using surface passivation techniques. The other source is the trapping of hot electrons in defective epitaxial layers,  which implies that the electrons in 2DEG channels can be driven by high electric field and trapped at barrier or buffer layers. Recent studies have indicated that a relationship exists between gate leakage-induced electron injection and defective epitaxial layers [7, 8]. However, no study has clarified this leakage behavior and the involved trapping mechanism. Therefore, the behavior of dynamic on-resistance (RON) transients in relation to VDS-dependent off-state leakage currents in HFETs under various stress conditions is discussed in this paper. Furthermore, the behavior of RON transients in HFETs with and without a gate insulator was compared, and the results revealed that a severe degradation in dynamic switching performance is due to a resistive leakage current formed by high electric field but not high electron injection.
To explain the leakage mechanism, a technology computer-aided design simulation was performed using Atlas (Silvaco, Santa Clara, CA, USA) to examine the electric field. The epitaxial layers and layout of the simulation device were identical to those of the SGHFET (Figure 1). The carbon doping was modeled according to a compensation mechanism proposed by Armstrong et al. . The C N -C Ga states were autocompensated with E CGa = 0.11 eV (donors) and E CN = 3.28 eV (acceptors), and the concentrations of both E CGa and E CN were set at 1 × 1018 cm−3. Previously, Verzellesi et al. employed the CDS-VDS measurement to verify the carbon-doping model used in this study .
The PF effect implies that the injected carriers underwent a series of capture and emission processes. These processes prevent the applied electric field from effectively accelerating the injected carriers; consequently, an increased number of carriers are trapped near the surface. The high density of trapped carriers caused an electric field gradient to limit the current density. The current resulting from the presence of a space-charge effect is called space-charge-limited conduction (SCLC) . However, when the applied VDS exceeded Vc 2, the leakage current increased considerably, indicating that part of the carriers moved freely through the barrier layer. This characteristic curve was observed in both the SGHFET and MISHFET. However, the critical voltage Vc 2 of the SGHFET was approximately 50 V, as shown in Figure 2c, and a higher value of 60 V was observed in the MISHFET. These values are similar to those shown in Figure 2b.
In this study, the degradation in dynamic switching performance was determined by calculating the ratio of dynamic RDS,on to RDC. The value of dynamic RDS,on was obtained under test conditions in which VDS,test and VGS,test were respectively set to 1 and 0 V after applying the off-state stress. HFETs were stressed in high VDS off-state (VDS,stress) for 1 s then synchronous switching VGS and VDS to the test condition by Agilent B1505 power device analyzer (Agilent Technologies, Santa Clara, CA, USA). The value of R DC was obtained under test conditions in which VDS and VGS were respectively set at 1 and 0 V without applying the off-state stress. After each dynamic RDS,on measurement, the initial condition of these devices can be fully recovered by shining microscope light for 10 min.
When the applied VDS,stress exceeded the value of Vc 2 (Figure 5c), the amount of injected carriers was not limited by the localized trapping or SCLC effect. The root cause to overcoming this limitation can be attributed to either Fowler-Nordheim tunneling or deeper acceptor-like traps and emission mechanisms invoked by the PF effect. However, the characteristic curve in this VDS,stress region was difficult to analyze because the electric field was not distributed in the AlGaN barrier layer alone; the depletion region in the 2DEG channel, which was extended under high VDS conditions, should also be considered. Under high VDS,stress conditions, the high electric field may have caused resistive leakage current, thereby causing part of the carriers to move freely through the AlGaN barrier layer. These free carriers can be driven by high electric fields that subsequently form hot carriers. These high-energy carriers could be trapped in the barrier, channel, or buffer layers; thus, a ‘global trapping’ effect occurred. Because the gate insulator mitigated the effect of the electric field on the barrier layer, the critical voltage of the MISHFET was higher than that of the SGHFET. However, the global trapping effect continued because high VDS,stress applied to the MISHFET controlled the electron injection, which explains why a similar but less pronounced behavior was observed in the MISHFET (Figure 4b) as a result of the detrapping behavior.
This study compared the off-state leakage current and characteristic curves of RON transients in AlGaN/GaN SGHFETs and MISHFETs to explain how the behavior of gate-injected electrons causes trapping and detrapping. The off-state leakage current follows PF effect for low-bias VDS. The gate insulator in the MISHFET effectively reduced the electron injection from the gate, thereby mitigating the degradation in dynamic switching performance. When the applied VDS,stress exceeded the critical voltage, 50 V for the SGHFET and 60 V for MISHFET, resistive leakage current build-up caused part of the injected carriers to move freely through the barrier layer. These carriers can be accelerated by applying a high electric field to form hot carriers that act as ionized traps in the channel or buffer layers, thereby enhancing the trapping/detrapping effect in both SGHFETs and MISHFETs.
This work was partially supported by the National Science Council of Republic of China under contract No. NSC 102-2221-E-008-082-MY2 and 102-2622-E-008-012-CC1 and by the Joint Research Center of National Central University and Delta Group under Contract No. 102G908-10.
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