P-type silicon carbide substrate is generally used to make power devices, such as insulated gate bipolar transistors (IGBT, Insulate-Gate Bipolar Transistor).
IGBT= MOSFET+BJT, it is a non-on or off switch. MOSFET=IGFET (Metal Oxide Semiconductor Field Effect Transistor, or Insulated Gate Field Effect Transistor). BJT (Bipolar Junction Transistor, bipolar junction transistor, also known as triode), bipolar means that there are both electrons and holes participating in the conduction process during operation. Generally, there is a PN junction participating in conduction.
A simple example is given: look at the upper part, the gate+emitter+N is completely a vertical MOSFET structure, but a P-type semiconductor is added at the bottom to form a PNP-type BJT.
The principle of energization is divided into two parts:
- Voltage is applied to the gate, and the upper part is turned on, then the entire device is transformed into an NP structure from the top to the bottom.
- Voltage is applied to the collector, so the lower half is turned on, and the PN junction is turned on normally.
Finally, the current flows from the collector to the emitter. Because of the existence of the PN junction, there are electrons and holes that conduct electricity.
Give a few examples.
Connecting wires to the above structure, it can be used as an IGBT single tube. The appearance is similar to the MSOFET; after all, it is also used as a switch.
If several IGBTs are put together, or put together with other devices, they can be used as IGBT modules.
IGBT merges the advantages of BJT and MOSFET, such as low driving power and reduced saturation voltage.
The development of silicon carbide IGBT is shown below:
1996: first 6H-SiC Trench gate IGBT
1999: first 4H-SiC Trench gate pIGBT
2005: first 4H-SiC Planar gate nIGBT
2007: charge storage layer introduced to JFET region
2010: free-standing technology proposed
2012: first 12kV/10A SiC IGBT module
2015: * 15kV/40A IGBT module
* first 7kV/8kW SST
* first 32kV Marx Generator
* 27kV SiC nIGBT
Structurally, the IGBT mainly has three development directions:
1) Longitudinal structure: non-transparent collector area NPT type, buffer layer PT type, transparent collector area NPT type, FS electric field cut-off type;
2) Gate structure: planar gate structure, Trench-trench structure;
3) Substrate: epitaxial growth technology and type.
In order to inherit the structure of the MOSFET with an N-type substrate, it is necessary to grow an IGBT on a P-type silicon carbide substrate. The P-type silicon carbide substrate generally refers to an Al-doped silicon carbide substrate. Al is +3 valence, replacing part of the +4 valence Si in SiC, forming Al and a +1 valence hole. Holes are P-type semiconductors. In addition to Al, other trivalent elements will also be used as P-type dopants, including B, Ga, In and so on. Doping silicon carbide with nitrogen and phosphorus can form n-type semiconductors, while doping with aluminum, boron, gallium, and beryllium will form p-type semiconductors. Aluminum-doped silicon carbide is a type II semiconductor, but boron-doped silicon carbide is a type I semiconductor. The heterojunction types are as follows: Type I heterojunction: a small forbidden band is enclosed in a large forbidden band; Type Ⅰ’heterojunction: two forbidden bands intersect; Type Ⅱ heterojunction: Two forbidden bands are staggered.
However, the P-type silicon carbide substrate is not practical at this stage because the resistivity is very large, at 100mΩ•cm. The common resistivity of N-type SiC substrate is 15-30mΩ•cm, or less than 10mΩ•cm.
The high resistivity is caused by the difficulty of Al doping.
- Selection of Al source
Al is directly mixed in silicon carbide, and it will be depleted in the early growth stage, forming defects. However, if Al3C4 is heated by an external heating system, and then transported by He gas, the Al doping concentration is difficult to control. By this method, a 100mΩ•cm P-type silicon carbide substrate with a concentration of 10^20cm-3 can be obtained. Al (CH3)3(trimethylaluminum, TMA for short) will react with graphite, and then forming aggregates to block the pipeline. However, the epitaxial growth of P-type silicon carbide layer will still be prepared with H2 as a carrier gas.
Conventional N-doped 3×10^19cm-3 generally produces stacking faults. N/Al co-doping can increase the critical value of N to produce stacking faults to 8.8×10^19cm-3. By this method, an N-type silicon carbide substrate of 7.3mΩ•cm with an N concentration of 3.5×10^19cm-3 and an Al concentration of 9×10^18cm-3 can be obtained, and the carrier mobility is 2.8×10^19cm-3.