Schottky-pn Diode Research Articles

Vertical Schottky diode on (113) oriented homoepitaxial diamond

Vertical Schottky diodes were fabricated on (113) oriented diamond substrates produced from thick heavily boron doped (NA ∼ 3.5 × 1020 cm−3) diamond layers grown on HPHT-grown synthetic Ib diamond crystals. The p+-substrates were overgrown by a low doped p-epilayer whose surface layer was compensated up to the depth of 0.1 to 0.3 μm. The surface was then oxygen terminated and covered by evaporated Mo/Au Schottky contacts. The bottom ohmic contact to p+-substrate was formed by Ti/Au bi-layer. Current- and capacitance-voltage characteristics of the realized diodes were measured at temperatures ranging from 25 to 180 °C. At room temperature, the realized diodes show high forward current densities JF = 100 A/cm2 and low differential ON-state specific resistance Ron_sp = 3.7 mΩ.cm2 at forward voltage VF = 4 V and a very low leakage (JR < 10−9 A/cm2) in the blocking regime. At 180 °C, forward characteristics significantly improve (JF = 5 kA/cm2, Ron_sp = 0.2 mΩ.cm2 at VF = 4 V) while the blocking characteristics remain almost unchanged (JR = 6 × 10−8 A/cm2 at VR = 4 V). The breakdown electric field is about 3 MV/cm and the rectification ratio keeps at the level of 1011. Characteristics of realized diodes, which are fully comparable to those of Schottky-pn diodes prepared on (111) oriented boron doped diamond, confirm the suitability of (113) orientation for realization of vertical diamond power devices.

Degradation of forward current density with increasing blocking voltage in diamond Schottky-pn diodes

Schottky-pn diodes (SPNDs) have been proposed as a novel solution allowing high current density in the lightly-doped drift regions of diamond and other semiconductors where shallow doping is difficult. Here we show, using simulations, that while SPNDs may have applications in low voltage diamond devices, development of high voltage devices is precluded by space-charge limited conduction physics inherent to the SPND structure at high current densities. For designed blocking voltages above 1000 V, simulations indicate that SPNDs are dramatically outperformed by properly designed conventional Schottky diodes.

(Invited) Development of SiC Schottky-PN Diode with Low on Resistance and High Blocking Voltage

4H-SiC is a promising material for low-loss, high frequency, and high temperature electronic devices. And nowadays, 4H-SiC Schottky diodes, MOSFETs, and its inverters were used in many applications. As well known, a conduction loss and a switching loss shows tread off relationship on these devices. Schottky diode shows high speed switching action and small recovery current based on its unipolar operation, but high series resistance of drift layer. On the other hand, PiN diode shows low series resistance of drift layer due to its conductivity modulation phenomenon based on the bipolar action. However, it causes an adverse reverse recovery current and limited switching speed. To break this tread off relationship, Schottky-PN diode structure so-called SPND or SNPD was proposed with a diamond semiconductor [1]. This diode has a structure with a low doped n- or p- drift layer where is sandwiched and completely depleted with Schottky junction and one side PN junction. Under forward bias condition, barrier height of PN junction is reduced and majority carrier was injected into the drift layer completely depleted with Schottky junction. Injected majority carrier drift to Schottky junction without recombining with minority carrier. It means minority carrier dose not contribute to current flow under forward bias. Then, this SPND structure is expected to show both very low on-resistance and high speed switching action. Under reverse bias condition, blocking voltage is shared throughout the drift layer. However, the diamond semiconductor material is under development and difficult to use production phase. In this study, we demonstrated the possibility of SPND by using 4H-SiC substrate under mass production. In this study, we grew low doped p-layer on n+ 4H-SiC C-face substrate by using hot-wall chemical vapor deposition (CVD) system with H2-SiH4-C3H8 gas system. Trimethylaluminum (TMA) was used as p-type dopant source. The thickness and doing concentration of grown p- layer is 1.1 μm and 5 x 1014 cm-3. Pt was evaporated in vacuum as Schottky electrode on as grown surface. The diameter of Schottky contact is 1mm. Al was used as ohmic back contact. The schematic cross-sectional structure of fabricated 4H-SiC SPND is shown inset of Figure 1. Figure 1 shows typical J-V characteristic of fabricated 4H-SiC SPND at room temperature. Good rectifying characteristic was observed. The blocking voltage of 300 V was obtained (Fig. 1(a)). From this result, we estimated dielectric breakdown field value of 2.5 MV/cm which is almost equal to ideal value of 4H-SiC one. On forward bias characteristic, the current raises at 2.2 V and the current density is reached at 1200 A/cm2 at 2.4 V (Fig. 1(b)). From this value, the differential on-resistance was estimated at 0.18 mΩcm2. In this study, we used 4H-SiC substrate with thickness of 200 μm and resistivity of 20 mΩcm. The calculated series on-resistance of 4H-SiC substrate part was estimated as 0.4 mΩcm2. This result means estimated differential on-resistance of 0.18 mΩcm2 is almost the resistance of substrate and the resistance of drift layer dose not contribute to the series resistance. These results mean 4H-SiC SPND structure was realized and its good performance of low on-resistance with high blocking voltage was confirmed. Details will be shown in this presentation.

Temperature dependence of electrical characteristics for diamond Schottky-pn diode in forward bias

A diamond Schottky-pn diode (SPND) was fabricated and its electrical properties were investigated by measuring the current-voltage characteristics. The diode showed high current density (>104 A/cm2 at 7 V) at room temperature with low differential on-resistance (~10−5 Ωcm2) under a wide temperature range (295–600 K). We constructed Richardson plots based on the characteristics and then estimated the potential barrier height on the hole current. From these results combined with the results of Poisson equation analysis, we viewed the change in hole barrier as a function of applied voltage and obtained the value of the hole activation energy as ~0.06 eV in the p+-type layer around the flat-band voltage. We clarified the transport mechanisms of the SPND and the method for estimating the activation energy in heavily boron-doped diamond exhibiting hopping conduction.

High Temperature Reliability Assessment and Degradation Analysis for Diamond Semiconductor Devices

The long-term reliability of Schottky pn diodes (SPNDs) on diamond having widely used Ti/Pt/Au electrodes was investigated at 500°C in order to identify degradation phenomena at higher temperatures. A vital degradation event was observed after the passage of about 100 hours in that both forward and reverse currents were progressively reduced. AES depth profiling and X-STEM-EELS analyses revealed that this occurred because the Ti contact material changed to insulating (or semiconductive) TiO2, causing large series resistance.

Diamond Schottky-pn diode using lightly nitrogen-doped layer

We successfully fabricated diamond Schottky-pn diodes (SPNDs) using a lightly nitrogen-doped (N-doped) layer. The diamond SPNDs showed a high forward current density of over 20,000A/cm2 at 7V, a high rectification ratio of over 1013, and a high breakdown electric field of 3.3MV/cm at room temperature. These properties mean that the N-doped layer acted as an n-type semiconductor. The N-doped n-type layer is effective in the SPNDs as same as a phosphorus-doped n-type layer.

Diamond electronic devices fabricated using heavily doped hopping p+ and n+ layers

Diamond diodes with extremely low on-resistance are introduced and discussed. Heavily boron-doped p+-type and phosphorus-doped n+-type diamond films with hopping conduction at room temperature are utilized. These diamond films have a unique property, that is, their resistivity decreases drastically even at a high impurity concentration of ∼1020 cm−3 without degrading the crystallinity. This unique property is applied to two kinds of diamond diodes. One is a p+–i–n+ junction diode. The other is a Schottky-pn diode consisting of the combination of the p+–n and n-type Schottky junctions. Current–voltage and capacitance–voltage characteristics are evaluated for both diodes. Switching characteristics are also evaluated for Schottky-pn diodes. The results show a low on-resistance on the order of mΩ cm2 and fast switching on the ns order. These results indicate that the low-loss diamond diodes can be realized by using the hopping p+- and n+-type layers.

Device Design of Diamond Schottky-pn Diode for Low-Loss Power Electronics

Device Design of Diamond Schottky-pn Diode for Low-Loss Power Electronics

Device Design of Diamond Schottky-pn Diode for Low-Loss Power Electronics

The device parameters of a novel diamond diode, namely, a Schottky-pn diode (SPND), are analyzed to realize a fast switching time, a low on-resistance, and a high blocking voltage simultaneously. The SPND is composed of an n-type active layer sandwiched between a highly doped p+-type layer and a Schottky metal. The key structure is the fully depleted n-type layer. From the simulations of the energy band diagram based on the key structure of the SPND using Poisson's equations, it is concluded that the low donor density in the n-type layer and the high acceptor density in the p+-type layer are key points for the high-performance SPND.

Diamond Schottky‐pn diode without trade‐off relationship between on‐resistance and blocking voltage

AbstractWe successfully improved a merged diamond diode, namely a Schottky‐pn diode (SPND), which is composed of a fully depleted n‐type active layer sandwiched between a highly doped p+‐type layer and a Schottky metal. According to the analysis of operation mechanisms based on band diagrams, we increased the acceptor concentration in the p+‐type layer in order to decrease the on‐resistance (RonS), and increased the n‐type layer width to reach higher blocking voltage (Vblock). Consequently, much lower RonS of 0.03 mΩ cm2 and higher Vblock than those of the previously reported diamond SPND were achieved simultaneously. Thus, we elucidated experimentally that the diamond SPND has no trade‐off relationship between RonS and Vblock, and is suitable for low‐loss high‐power switching devices.

Diamond Schottky p–n diode with high forward current density

AbstractWe successfully fabricated a diamond diode, namely a Schottky p–n diode (SPND), which is composed of a fully depleted n‐type active layer sandwiched between a highly doped p‐type layer and a Schottky metal. The diamond SPND showed a high forward current density (over 4000 A cm−2 at 6 V) with a low built‐in voltage (∼1.5 V) at room temperature while maintaining a high rectification ratio of ∼1010. Further improvement of the high forward current density characteristics was found at high temperature. The SPND can be realized with higher forward current density than that of conventional diamond Schottky barrier diode, p–n diode, and recently proposed merged diodes, while maintaining the high rectification ratio.

Diamond Schottky-pn diode with high forward current density and fast switching operation

We fabricated a diamond diode, namely a Schottky-pn diode (SPND), which is composed of a fully depleted n-type active layer sandwiched between a highly doped p-type layer and a Schottky metal layer. The SPND has superior characteristics that overcome the weak points of both a Schottky barrier diode and a pn diode. That is, the SPND showed high current density (over 4000 A/cm2) with low specific resistance (0.4 mΩ cm2) at a forward bias of 6 V while maintaining a high rectification ratio of ∼1010. Moreover, the SPND showed extremely fast turn-off speed of nanosecond order.