Power MOSFET with an anode region
This patent introduces a new type of Power MOSFET what they call power MOSFET with an Anode region. A power MOSFET consists of four regions 1. N+ ( source) 2. P ( body) 3. N- ( Epi) 4. N+ ( drain). In the power MOSFET presented in this patent the drain region has been replaced by a P+ region. The P+ region injects minority carriers into N- region. This enhances device performances in power Semiconductors.
The power MOSFET described in this patent has since been modified and improved and it is known as IGBT ( Insulated Gate Bipolar Transistor). IGBT represents the true MOSFET-Bipolar integration in the sense of combining the physics of MOSFET with the physics of bipolar transistor. Currently there are three different IGBT structures available a. NPT ( Non-Punch Through) b. PT (Punch Through) c. FT ( Field Stop). The choice of different structure mainly depends on applications.
IGBT has three operation modes 1. On-state 2. Reverse Blocking State 3. Forward Blocking state. IGBT does not have strong reverse blocking capability. Therefore in applications it operates in on-state and forward blocking state.
One of the advantages of IGBT compared to MOSFET is low voltage drop in N- (Epi) region due to carrier injection from P+ region. This results in smaller power losses for IGBT compared with MOSFET for the same application. IGBT is used for applications with breakdown voltage > = 300V.
Since the IGBT structure is similar to that for the MOS-gated thyristor, it contains a parasitic P-N-P-N thyristor structure between the source and the drain terminals. If this thyristor latches up the current cannot be controlled by the MOS gate and it damages the device (In the patent this problem has been discussed in terms of the current gains of two bipolar transistors). It is important to design IGBT in such a way to suppress this thryistor action.
For a more comprehensive description of IGBT please see the following references
- J. Baliag, Fundamental of Power Semiconductor Devices, 1st edition, Springer Verlag, 2008.
- V. K. Khanna, IGBT Theory and Design, 1st edition, Wiley-IEEE press, 2003.