2013.01.22 SD Lab. SOGANG Univ. Doohyung Cho Power MOSFETs(5) 2013.01.22 SD Lab. SOGANG Univ. Doohyung Cho

Contents 6.18 Integral Body Diode 6.17.1 Bipolar Second Breakdown 6.17.2 MOS Second Breakdown 6.18 Integral Body Diode 6.18.1 Reverse Recovery Enhancement 6.18.2 Impact of Parasitic Bipolar Transistor 6.19 High-Temperature Characteristics 6.19.1 Threshold Voltage 6.19.2 On-Resistance 6.19.3 Saturation Transconductance 6.20 Complementary Devices 6.20.1 The p-Channel Structure 6.20.2 On-Resistance 6.20.3 Deep-Trench Structure 6.21 Silicon Power MOSFET Process Technology 6.21.1 Planar VD-MOSFET Process 6.21.2 Trench U-MOSFET Process 6.22 Silicon Carbide Devices 6.22.1 The Baliga-Pair Configuration 6.22.2 Planar Power Structure 6.22.3 Shielded Planar Power MOSFET Structure 6.22.4 Shielded Trench-Gate Power MOSFET Structure 6.23 Summary

6.17.1 Bipolar Second Breakdown The lateral current flow path through the P-base region produces a forward bias across the emitter–base junction of the parasitic bipolar transistor at location A. The drain bias at which the emitter current becomes very large can be obtained. The bipolar second breakdown mode can be suppressed by reduction of the P-base resistance with the addition of a P+ region

6.17.2 MOS Second Breakdown The voltage drop across the P-base resistance due to the avalanche breakdown induced current produces a body bias effect The MOS second breakdown mode can also be suppressed by reduction of the Pbase resistance with the addition of a P+ region

6.18 Integral Body Diode The primary difficult with utilization of the integral diode in the power MOSFET structure is its poor reverse recovery characteristic due to the high lifetime in the N-drift region. In addition, the presence of the N+ source region can lead to the turn-on of the parasitic N–P–N bipolar transistor during the reverse recovery transient leading to destructive failure

6.18.1 Reverse Recovery Enhancement Methods for reducing the lifetime consist of the addition of deep level centers such as gold or platinum or by the use of high energy particle bombardment The application of these techniques to the power MOSFET structure is hindered by the degradation of the gate oxide and its interface properties with the silicon. it was discovered that this oxide charge can be removed by annealing the devices at relatively low temperatures Electron irradiation provides a clean and precise process for improving the reverse recovery behavior of the integral diode without compromising any of the power MOSFET device characteristics.

6.18.2 Impact of Parasitic Bipolar Transistor The drain blocking voltage during the reverse recovery transient can exceed the open-base breakdown voltage of the parasitic N–P–N transistor resulting in destructive failure

6.19 High-Temperature Characteristics Power MOSFET structures have superior high-temperature operating capability when compared with bipolar devices with the same voltage ratings. Commercial devices are available with a maximum junction temperature capability of up to 200°C.

6.19.1 Threshold Voltage The threshold voltage for power MOSFET structures decreases with increasing temperature due to the increase in the intrinsic carrier concentration.

6.19.2 On-Resistance The on-resistance of power MOSFET structures is observed to increase with increasing temperature. This is attributed to the reduction of the mobility in the drift region and the inversion layer The increase in the on resistance due to the increased temperature favors a redistribution of the current to other locations within the device. This prevents the formation of “hot spots”

6.19.3 Saturation Transconductance The temperature-dependent terms in this expression are the channel mobility and the threshold voltage. The net result is a reduction of the transconductance with increasing temperature

6.20 Complementary Devices&6.20.1 The p-Channel Structure all the bias voltages must be changed from positive to negative values for the p-channel structure

6.20.2 On-Resistance The components of the on-resistance for the p-channel structure are identical to those for the corresponding n-channel structure. However, the magnitude for the specific on-resistance for the various components is larger in the p-channel structure due to the lower mobility for holes than electrons in silicon.

6.20.3 Deep-Trench Structure deep-trench architecture, the trench is extended from the upper surface into the N+ or P+ substrate. When the gate bias is applied to turn on the device, an inversion layer is formed along the trench sidewalls within the base region extending all the way from the source to the drain region allowing a reduction of the specific on resistance due to elimination of the drift region component

6.21 Silicon Power MOSFET Process Technology V-groove process (1970s) - with the sharp point of the V-groove gate structure and instabilities associated with the potassium hydroxide-based etching solutions. VD-MOS process(1980s) - utilization of improved design rules to reduce the size of the diffusion window leading to a lower specific on-resistance.(BUT presence of JFET) U-MOS process(1990s) - availability of reactive ion etching (RIE), achievement of lower specific on-resistance

6.21.1 Planar VD-MOSFET Process

6.21.2 Trench U-MOSFET Process

6.22 Silicon Carbide Devices The Baliga’s figure of merit for 4H-SiC is more than 2,000 times superior to that for silicon. First, the quality of the interface between the thermally grown oxide and the silicon carbide surface has been poor resulting in low inversion layer mobility. Second, the high electric field generated within the silicon carbide produces very high electric field within the gate oxide leading to its rupture during operation.

6.22.1 The Baliga-Pair Configuration In the Baliga-Pair configuration, a low breakdown voltage, normally-off, silicon MOSFET and a high-voltage, normally-on, silicon carbide JFET/MESFET are connected together It is important that the silicon carbide FET structure be designed for normally-on operation with a low specific on-resistance. It is also necessary for the silicon carbide FET to be able to block the drain bias voltage with a gate potential less than the breakdown voltage of the silicon power MOSFET.

6.22.1 The Baliga-Pair Configuration Voltage Blocking Mode as the voltage at the drain (DM) of the silicon MOSFET increases, an equal positive voltage develops at the source (SSiC) of the silicon carbide FET. Since the gate (GSiC) of the silicon carbide FET is connected to the ground terminal, this produces a reverse bias across the gate–source junction of the silicon carbide FET When the depletion region pinches off the channel at location A, further increase in the bias applied to the drain (DB) of the composite switch is supported across the silicon carbide FET. value close to the pinch-off voltage of the silicon carbide FET.

6.22.1 The Baliga-Pair Configuration Forward Conduction Mode Due to the low specific on-resistance of both structures, the total on resistance of the Baliga-Pair configuration is also very small. Current Saturation Mode Due to the low specific on-resistance of both structures, the total on resistance of the Baliga-Pair configuration is also very small. Baliga-Pair behaves like a silicon power MOSFET from the point of view of the external circuit on both the input and output side(safety device)

6.22.1 The Baliga-Pair Configuration Switching Characteristics The switching speed of the Baliga-Pair is consequently very high because of the availability of silicon power MOSFETs designed for high frequency applications. Fly-back Diode The Baliga-Pair contains an inherent high-quality fly-back rectifier. When the drain bias is reversed to a negative value, the gate–drain contact/junction of the silicon carbide FET becomes forward biased.

6.22.2 Planar Power Structure The diffusion rate for dopants is extremely small in silicon carbide even at very high temperatures together with the location of the ion implantation edges. These devices have been called DIMOSFETs because of the double-implant process used for their fabrication

6.22.2 Planar Power Structure Blocking Characteristics large leakage current can occur when the depletion region in the P-base region reaches through to the N+ source region. The doping concentration and thickness of the P-base region must be designed to prevent the reach-through phenomenon from limiting the breakdown voltage. At any given P-base doping concentration, the thickness for 4H-SiC is about six times larger than for silicon. This implies that the minimum channel length required for silicon carbide devices is much larger than for silicon devices, resulting in a big increase in the on-resistance.

6.22.2 Planar Power Structure Threshold Voltage The much larger threshold voltage for silicon carbide is physically related to its larger band gap (which results in an extremely small value for the intrinsic concentration in (6.282)) as well as the higher P-base doping concentration required to suppress reach-through breakdown.

6.22.2 Planar Power Structure Oxide Field maximum electric field in the silicon drift region remains below 3 × 105 V cm−1, the electric field in the oxide does not exceed its reliability limit of about 3 × 106 V cm−1. However, for 4H-SiC, the electric field in the oxide reaches a value of 9 × 106 V cm−1 important to monitor the electric field in the gate oxide when designing and modeling the silicon carbide MOSFET structures.

6.22.2 Planar Power Structure Oxide Field This leads to hot electron injection-induced instability of the threshold voltage. The trapping of these electrons within the gate oxide can cause shifts in the threshold voltage of the MOSFET leading to reliability problems.

6.22.3 Shielded Planar Power MOSFET Structure Blocking Mode fully utilize the high breakdown electric field strength available in silicon carbide, it is important to screen the gate oxide from the high field within the semiconductor this is achieved by the formation of a potential barrier at location A by the depletion of the JFET region at a low drain bias voltage.

6.22.3 Shielded Planar Power MOSFET Structure Threshold Voltage declining influence of the P+/N junction at the gate oxide interface when the doping concentration of the N-base region is increased

6.22.4 Shielded Trench-Gate Power MOSFET Structure Blocking Characteristics high electric field can develop in the N-drift region below the P+ shielding region while the electric field at the Pbase region remains low. sheet resistance of the ion-implanted P+ region can be quite high

6.22.4 Shielded Trench-Gate Power MOSFET Structure Forward Conduction first one, labeled RJFET1 in the figure, is formed between the P-base region and the P+ shielding region with the current constricted by their zero-bias depletion boundaries. second JFET region, labeled RJFET2 in the figure, is formed between the P+ shielding regions. Its resistance is determined by the thickness of the P+ shielding region

6.22.4 Shielded Trench-Gate Power MOSFET Structure Forward Conduction It can be seen that the drift region resistance is dominant here while the JFET resistances are very small.

6.23 Summary especially for devices designed to support low blocking voltages. Since the specific on-resistance now approaches the ideal value, further improvements in the performance of power MOSFET devices have focused on reduction of the input gate capacitance and the gate charge. 4H-polytype. With 4H-SiC, it is possible to reduce the specific on-resistance by over a factor of 1,000 times in switches designed for operation at above 1,000 V. The commercialization of this technology is expected to result in improvements in the size, weight, and efficiency of high power electronic systems in the future.