A power MOSFET is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) designed to handle significant power levels. Compared to the other power semiconductor devices, such as an insulated-gate bipolar transistor (IGBT) or a thyristor, its main advantages are high switching speed and good efficiency at low voltages. It shares with the IGBT an isolated gate that makes it easy to drive. They can exhibit low gain, sometimes to the extent that the gate voltage needs to be higher than the voltage being controlled.
The design of power MOSFETs was made possible by the evolution of MOSFET and CMOS technology, used for manufacturing integrated circuits since the 1960s. The power MOSFET shares its operating principle with its low-power counterpart, the lateral MOSFET. The power MOSFET, which is commonly used in power electronics, was adapted from the standard MOSFET and commercially introduced in the 1970s.
The power MOSFET is the most common power semiconductor device in the world, due to its low gate drive power, fast switching speed, easy advanced paralleling capability, wide bandwidth, ruggedness, easy drive, simple biasing, ease of application, and ease of repair. In particular, it is the most widely used low-voltage (less than 200 V) switch. It can be found in a wide range of applications, such as most power supplies, DC-to-DC converters, low-voltage motor controllers, and many other applications.
History
The MOSFET was invented at Bell Labs between 1955 and 1960. It was a breakthrough in power electronics. Generations of MOSFETs enabled power designers to achieve performance and density levels not possible with bipolar transistors.
In 1969, Hitachi introduced the first vertical power MOSFET, which would later be known as the VMOS (V-groove MOSFET). The same year, the DMOS (double-diffused MOSFET) with self-aligned gate was first reported by Y. Tarui, Y. Hayashi and Toshihiro Sekigawa of the Electrotechnical Laboratory (ETL). In 1974, Jun-ichi Nishizawa at Tohoku University invented a power MOSFET for audio, which was soon manufactured by Yamaha Corporation for their high fidelity audio amplifiers. JVC, Pioneer Corporation, Sony and Toshiba also began manufacturing amplifiers with power MOSFETs in 1974. Siliconix commercially introduced a VMOS in 1975.
The VMOS and DMOS developed into what has become known as VDMOS (vertical DMOS). John Moll's research team at HP Labs fabricated DMOS prototypes in 1977, and demonstrated advantages over the VMOS, including lower on-resistance and higher breakdown voltage. The same year, Hitachi introduced the LDMOS (lateral DMOS), a planar type of DMOS. Hitachi was the only LDMOS manufacturer between 1977 and 1983, during which time LDMOS was used in audio power amplifiers from manufacturers such as HH Electronics (V-series) and Ashly Audio, and were used for music and public address systems. With the introduction of the 2G digital mobile network in 1995, the LDMOS became the most widely used RF power amplifier in mobile networks such as 2G, 3G, and 4G.
Alex Lidow co-invented the HexFET, a hexagonal type of Power MOSFET, at Stanford University in 1977, along with Tom Herman. The HexFET was commercialized by International Rectifier in 1978. The insulated-gate bipolar transistor (IGBT), which combines elements of both the power MOSFET and the bipolar junction transistor (BJT), was developed by Jayant Baliga at General Electric between 1977 and 1979.
The superjunction MOSFET is a type of power MOSFET that uses P+ columns that penetrate the N− epitaxial layer. The idea of stacking P and N layers was first proposed by Shozo Shirota and Shigeo Kaneda at Osaka University in 1978. David J. Coe at Philips invented the superjunction MOSFET with alternating p-type and n-type layers, for which a US patent was awarded in 1988.US Patent 4,754,310
Applications
The power MOSFET is the most widely used power semiconductor device in the world. , the power MOSFET accounts for 53% of the power transistor market, ahead of the insulated-gate bipolar transistor (27%), RF power amplifier (11%) and bipolar junction transistor (9%). , over 50 billion power MOSFETs are shipped annually. These include the trench power MOSFET, which sold over 100 billion units up until February 2017, and STMicroelectronics' MDmesh (superjunction MOSFET) which has sold 5 billion units .
Power MOSFETs are commonly used for a wide range of consumer electronics as well as transportation technology, including a wide range of vehicles. Power MOSFETs are widely used in automotive electronics.
RF DMOS, also known as RF power MOSFET, is a type of DMOS power transistor designed for radio-frequency (RF) applications. It is used in various radio and RF applications.
Power MOSFETs (including DMOS, LDMOS and VMOS) are commonly used for a wide range of other applications.
Basic structure
Several structures had been explored in the 1970s, when the first commercial power MOSFETs were introduced. However, most of them have been abandoned (at least until recently) in favour of the Vertical Diffused MOS (VDMOS) structure (also called Double-Diffused MOS or simply DMOS) and the LDMOS (laterally diffused MOS) structure.
The cross section of a VDMOS (see figure 1) shows the "verticality" of the device: it can be seen that the source electrode is placed over the drain, resulting in a current mainly vertical when the transistor is in the on-state. The "diffusion" in VDMOS refers to the manufacturing process: the P wells (see figure 1) are obtained by a diffusion process (actually a double diffusion process to get the P and N+ regions, hence the name double diffused).
Power MOSFETs have a different structure from the lateral MOSFET: as with most power devices, their structure is vertical and not planar. In a planar structure, the current and breakdown voltage ratings are both functions of the channel dimensions (respectively width and length of the channel), resulting in inefficient use of the "silicon real estate". With a vertical structure, the voltage rating of the transistor is a function of the doping and thickness of the N epitaxial layer (see cross section), while the current rating is a function of the channel width. This makes it possible for the transistor to sustain both high blocking voltage and high current within a compact piece of silicon.
LDMOS are power MOSFETs with a lateral structure. They are mainly used in high-end audio power amplifiers, and RF power amplifiers in wireless cellular networks, such as 2G, 3G, and 4G. Their advantage is a better behaviour in the saturated region (corresponding to the linear region of a bipolar junction transistor) than the vertical MOSFETs. Vertical MOSFETs are designed for switching applications, so they are only used in on- or off-states.
On-state resistance
When the power MOSFET is in the on-state (see MOSFET for a discussion on operation modes), it exhibits a resistive behaviour between the drain and source terminals. It can be seen in figure 2 that this resistance (called RDSon for "drain-to-source resistance in on-state") is the sum of many elementary contributions: RS is the source resistance. It represents all resistances between the source terminal of the package to the channel of the MOSFET: resistance of the bond wires, of the source metallisation, and of the N+ wells; Rch. This is the channel resistance. It is inversely proportional to the channel width, and for a given die size, to the channel density. The channel resistance is one of the main contributors to the RDSon of low-voltage MOSFETs, and intensive work has been carried out to reduce their cell size in order to increase the channel density; Ra is the access resistance. It represents the resistance of the epitaxial zone directly under the gate electrode, where the direction of the current changes from horizontal (in the channel) to vertical (to the drain contact); RJFET is the detrimental effect of the cell size reduction mentioned above: the P implantations (see figure 1) form the gates of a parasitic JFET transistor that tend to reduce the width of the current flow; Rn is the resistance of the epitaxial layer. As the role of this layer is to sustain the blocking voltage, Rn is directly related to the voltage rating of the device. A high voltage MOSFET requires a thick, low-doped layer, i.e., highly resistive, whereas a low-voltage transistor only requires a thin layer with a higher doping level, i.e., less resistive. As a result, Rn is the main factor responsible for the resistance of high-voltage MOSFETs; RD is the equivalent of RS for the drain. It represents the resistance of the transistor substrate (the cross section in figure 1 is not at scale, the bottom N+ layer is actually the thickest) and of the package connections.
Breakdown voltage/on-state resistance trade-off
When in the off-state, the power MOSFET is equivalent to a PIN diode (constituted by the P+ diffusion, the N− epitaxial layer and the N+ substrate). When this highly non-symmetrical structure is reverse-biased, the space-charge region extends principally on the lightly-doped side, i.e., over the N− layer. This means that this layer has to withstand most of the MOSFET's off-state drain-to-source voltage.
However, when the MOSFET is in the on-state, this N− layer has no function. Furthermore, as it is a lightly-doped region, its intrinsic resistivity is non-negligible and adds to the MOSFET's on-state drain-to-source resistance (RDSon) (this is the Rn resistance in figure 2).
Two main parameters govern both the breakdown voltage and the RDSon of the transistor: the doping level and the thickness of the N− epitaxial layer. The thicker the layer and the lower its doping level, the higher the breakdown voltage. On the contrary, the thinner the layer and the higher the doping level, the lower the RDSon (and therefore the lower the conduction losses of the MOSFET). Therefore, it can be seen that there is a trade-off in the design of a MOSFET, between its voltage rating and its on-state resistance. This is demonstrated by the plot in figure 3.
Body diode It can be seen in figure 1 that the source metallization connects both the N+ and P+ implantations, although the operating principle of the MOSFET only requires the source to be connected to the N+ zone. However, if it were, this would result in a floating P zone between the N-doped source and drain, which is equivalent to a NPN transistor with a non-connected base. Under certain conditions (under high drain current, when the on-state drain-to-source voltage is in the order of some volts), this parasitic NPN transistor would be triggered, making the MOSFET uncontrollable. The connection of the P implantation to the source metallization shorts the base of the parasitic transistor to its emitter (the source of the MOSFET) and thus prevents spurious latching. This solution, however, creates a diode between the drain (cathode) and the source (anode) of the MOSFET, making it able to block current in only one direction.
Body diodes may be utilized as freewheeling diodes for inductive loads in configurations such as H bridge or half bridge. While these diodes usually have rather high forward voltage drop, they can handle large currents and are sufficient in many applications, reducing part count, and thus, device cost and board space. To increase efficiency, synchronous rectification is often used to minimize the amount of time that the body diode conducts current.
Switching operation