Constructional Features of a Power MOSFET
Constructional Features of Power MOSFET:
- Power MOSFET is a device that evolved from MOS integrated circuit technology. The first attempts to develop high voltage MOSFETs were by redesigning lateral MOSFET to increase their voltage blocking capacity. The resulting technology was called lateral double diffused MOS (DMOS).
- However it was soon realized that much larger breakdown voltage and current ratings could be achieved by resorting to a vertically oriented structure. Since then, vertical DMOS (VDMOS) structure has been adapted by virtually all manufacturers of Power MOSFET.
- A power MOSFET using VDMOS technology has vertically oriented three layer structure of alternating p type and n type semiconductors.
- A large number of such cells are connected in parallel to form a complete device. The two n end layers labeled “Source” and “Drain” are heavily doped to approximately the same level. The p type middle layer is termed the body (or substrate) and has moderate doping level (2 to 3 orders of magnitude lower than n regions on both sides).
- The n- drain drift region has the lowest doping density. Thickness of this region determines the breakdown voltage of the device.
- The gate terminal is placed over the n- and p type regions of the cell structure and is insulated from the semiconductor body be a thin layer of silicon dioxide (also called the gate oxide). The source and the drain region of all cells on a wafer are connected to the same metallic contacts to form the Source and the Drain terminals of the complete device.
- Similarly all gate terminals are also connected together. The source is constructed of many (thousands) small polygon shaped areas that are surrounded by the gate regions. The geometric shape of the source regions, to same extent, influences the ON state resistance of the MOSFET.
- One interesting feature of the MOSFET cell is that the alternating n n- p n structure embeds a parasitic BJT (with its base and emitter shorted by the source metallization) into each MOSFET cell.
- The nonzero resistance between the base and the emitter of the parasitic npn BJT arises due to the body spreading resistance of the p type substrate.
- In the design of the MOSFET cells special care is taken so that this resistance is minimized and switching operation of the parasitic BJT is suppressed. With an effective short circuit between the body and the source the BJT always remain in cut off and its collector-base junction is represented as an anti-parallel diode (called the body diode) in the circuit symbol of a Power MOSFET.