Understanding Power MOSFETs October 1999 Application Note Introduction /Title 7500) Subect Under tandng ower OSETs, nteril orpoation) Autho () Keyords ) Cretor () DOCI FO dfark Pageode Useutines DOCIEW dfark Power MOSFETs (Metal Oxide Semiconductor, Field Effect Transistors) differ from bipolar transistors in operating principles, specifications, and performance. In fact, the performance characteristics of MOSFETs are generally superior to those of bipolar transistors: significantly faster switching time, simpler drive circuitry, the absence of or reduction of the second-breakdown failure mechanism, the ability to be paralleled, and stable gain and response time over a wide temperature range. This note provides a basic explanation of general MOSFET characteristics, and a more thorough discussion of structure, thermal characteristics, gate parameters, operating frequency, output characteristics, and drive requirements. AN-7500 is a term that defines the average velocity of a carrier in terms of the electrical field imposed on it) makes a MOSFET more resistive at elevated temperatures, and much more immune to the thermal-runaway problem experienced by bipolar devices. A useful by-product of the MOSFET process is the internal parasitic diode formed between source and drain, Figure 1(c). (There is no equivalent for this diode in a bipolar transistor other than in a bipolar darlington transistor.) Its characteristics make it useful as a clamp diode in inductive-load switching. ALUM SOURCE n+ GATE n+ n+ n+ p General Characteristics p CONVERTED TO n CHANNEL p n- A conventional n-p-n bipolar power transistor is a currentdriven device whose three terminals (base, emitter, and collector) are connected to the silicon by alloyed metal contacts. Bipolar transistors are described as minority-carrier devices in which injected minority carriers recombine with majority carriers. A drawback of recombination is that it limits the device's operating speed. And because of its currentdriven base-emitter input, a bipolar transistor presents a lowimpedance load to its driving circuit. In most power circuits, this low-impedance input requires somewhat complex drive circuitry. By contrast, a power MOSFET is a voltage-driven device whose gate terminal, Figure 1(a), is electrically isolated from its silicon body by a thin layer of silicon dioxide (SiO2). As a majority-carrier semiconductor, the MOSFET operates at much higher speed than its bipolar counterpart because there is no charge-storage mechanism. A positive voltage applied to the gate of an n-type MOSFET creates an electric field in the channel region beneath the gate; that is, the electric charge on the gate causes the p-region beneath the gate to convert to an n-type region, as shown in Figure 1(b). This conversion, called the surface-inversion phenomenon, allows current to flow between the drain and source through an n-type material. In effect, the MOSFET ceases to be an n-p-n device when in this state. The region between the drain and source can be represented as a resistor, although it does not behave linearly, as a conventional resistor would. Because of this surfaceinversion phenomenon, then, the operation of a MOSFET is entirely different from that of a bipolar transistor, which always retain its n-p-n characteristic. By virtue of its electrically-isolated gate, a MOSFET is described as a high-input impedance, voltage-controlled device, whereas a bipolar transistor is a low-input-impedance, current-controlled device. As a majority-carrier semiconductor, a MOSFET stores no charge, and so can switch faster than a bipolar device. Majority-carrier semiconductors also tend to slow down as temperature increases. This effect, brought about by another phenomenon called carrier mobility (where mobility ©2002 Fairchild Semiconductor Corporation n+ DRAIN (a) D N G N + P – N + + D + G S S – + (b) (c) FIGURE 1. THE MOSFET, A VOLTAGE-CONTROLLED DEVICE WITH AN ELECTRICALLY ISOLATED GATE, USES MAJORITY CARRIERS TO MOVE CURRENT FROM SOURCE TO DRAIN (A). THE KEY TO MOSFET OPERATION IS THE CREATION OF THE INVERSION CHANNEL BENEATH THE GATE WHEN AN ELECTRIC CHARGE IS APPLIED TO THE GATE (B). BECAUSE OF THE MOSFETs CONSTRUCTION, AN INTEGRAL DIODE IS FORMED ON THE DEVICE (C), AND THE DESIGNER CAN USE THIS DIODE FOR A NUMBER OF CIRCUIT FUNCTIONS Structure Fairchild Power MOSFETs are manufactured using a vertical double-diffused process, called VDMOS or simply DMOS. A DMOS MOSFET is a single silicon chip structured with a large number of closely packed, hexagonal cells. The number of cell varies according to the dimensions of the chip. For example, a 120-mil2 chip contains about 5,000 cells; a 240-mil2 chip has more than 25,000 cells. One of the aims of multiple-cells construction is to minimize the MOSFET parameter rDS(ON), or resistance from drain to source, when the device is in the on-state. When rDS(ON) is minimized, the device provides superior power-switching Application Note 7500 Rev. A1 Application Note 7500 performance because the voltage drop from drain to source is also minimized for a given value of drain-to-source current. Since the path between drain and source is essentially resistive, because of the surface-inversion phenomenon, each cell in the device can be assumed to contribute an amount, RN, to the total resistance. An individual cell has a fairly low resistance, but to minimize rDS(ON), it is necessary to put a large number of cells in parallel on a chip. In general, therefore, the greater the number of paralleled cells on a chip, the lower its rDS(ON) value: rDS(ON) = RN/N, where N is the number of cells. 3000 REXT rDS(ON) (mΩ) 1000 rDS(ON) RCHAN RBULK rDS(ON) 100 RBULK RCHAN 10 100 1000 BVDSS (V) FIGURE 2. THE DRAIN-TO-SOURCE RESISTANCE, rDS(ON) , OF A MOSFET IS NOT ONE BUT THREE SEPARATE RESISTANCE COMPONENTS TABLE 1. PERCENTAGE RESISTANCE COMPONENTS FOR A TYPICAL CHIP BVDSS Two conclusions, inherent consequences of the laws of semiconductor physics, and valid for any DMOS device, can be drawn from the preceding discussion: First, rDS(ON) obviously increases with increasing breakdown-voltage capability of a MOSFET. Second, minimum rDS(ON) performance must be sacrificed if the MOSFET must withstand ever-higher breakdown voltages. The significance of RBULK in devices with a high voltage capability is due to the fact that thick, lightly doped epi layers are required for the drain region in order to avoid producing high electric fields (and premature breakdown) within the device. And as the epi layers are made thicker and less resistive to support high voltages, the bulk component of resistance rapidly increases (see Figure 2) and begins to dominate the channel and external resistance. The rDS(ON) therefore, increases with increasing breakdown voltage capability, and low rDS(ON) must be sacrificed if the MOSFET is to withstand even higher breakdown voltages. There is a way around these obstacles. The rDS(ON) in Figure 2 holds only for a given cell and chip size. Using a larger chip results in a lower value for rDS(ON) because a large chip has more cells (see Figure 3), shifting the vertical axis for each of the constituent parts. REXT 30 Note in Figure 2 that R CHAN and REXT are completely independent of voltage, while RBULK is highly dependent on applied voltage. Note also that below about 150 volts, rDS(ON) is dominated by the sum of RCHAN and REXT. Above 150 volts, rDS(ON) is increasingly dominated by RBULK. Table 1 gives a percentage breakdown of the contribution of each resistance for three values of voltage. The penalty for using a larger chip, however, is an increase in cost, since chip size is a major cost factor. And because chip area increases exponentially, not linearly, with voltage, the additional cost can be substantial. For example, to obtain a given rDS(ON) at a breakdown voltage twice as great as the original, the new chip requires an area four or five times larger than the original. Although the cost does not rise exponentially, it is substantially more than the original cost. 40V 150V 500V RCHANNEL 50% 23% 2.4% Effects of Temperature RBULK 35% 70% 97% REXTERNAL 15% 7% <1% The high operating temperatures of bipolar transistors are frequent cause of failure. The high temperatures are caused by hot-spotting, the tendency of current in a bipolar device to concentrate in areas around the emitter. Unchecked, this hot-spotting results in the mechanism of thermal runaway, and eventual destruction of the device. MOSFETs do not suffer this disadvantage because their current flow is in the form of majority carriers. The mobility of majority carriers (where, again, mobility is a term that defines the average velocity of a carrier in terms of the electrical field imposed on it) is temperature dependent in silicon: mobility decreases with increasing temperature. This inverse relationship dictates that the carriers slowdown as the chip gets hotter. In effect, the resistance of the silicon path is increased, which prevents the concentrations of current that lead to hot spots. In fact, if hot spots do attempt to form in a MOSFET, the local resistance increases and defocuses or spreads out the current, rerouting it to cooler portions of the chip. In reality, rDS(ON) is composed of three separate resistances. Figure 2 shows a curve of the three resistive components for a single cell and their contributions to the overall value of rDS(ON). The value of rDS(ON) at any point of the curve is found by adding the values of the three components at that point: rDS(ON) = RBULK + RCHAN + REXT where RCHAN represents the resistance of the channel beneath the gate, and REXT includes all resistances resulting from the substrate, solder connections, leads, and the package. RBULK represents the resistance resulting from the narrow neck of n material between the two layers, as shown in Figure 1(a), plus the resistance of the current path below the neck and through the body to the substrate region of the device. ©2002 Fairchild Semiconductor Corporation Application Note 7500 Rev. A1 Application Note 7500 10 6 4 2 range of tens of nanoamperes, does flow, and is identified on data sheets as a leakage current, IGSS. Because the gate current is so small, the input impedance of a MOSFET is extremely high (in the megohm range) and, in fact, is largely capacitive rather than resistive (because of the isolation of the gate terminal). SMALLEST CHIP rDS(ON) (Ω) 1 0.6 0.4 0.2 Figure 5 illustrates the basic input circuit of a MOSFET. The elements are equivalent, rather than physical, resistance, R, and capacitance, C. The capacitance, called CISS on MOSFET data sheets, is a combination of the device's internal gate-tosource and gate-to-drain capacitance. The resistance, R, represents the resistance of the material in the gate circuit. Together, the equivalent R and C of the input circuit will determine the upper frequency limit of MOSFET operation. LARGEST CHIP 0.1 0.06 0.04 0.02 0.01 0 100 200 300 400 500 600 BVDSS (V) FIGURE 3. AS CHIP SIZE INCREASES, r DS(ON) DECREASES D Because of the character of its silicon structure, a MOSFET has a positive temperature coefficient of resistance, as shown by the curves of Figure 4. R G S CISS 4 ID = 4A VGS = 10V rDS(ON) (Ω) 3 FIGURE 5. A MOSFETs SWITCHING SPEED IS DETERMINED BY ITS INPUT RESISTANCE R AND ITS INPUT CAPACITANCE CISS 2 1 Operating Frequency 0 -50 0 50 100 150 200 JUNCTION TEMPERATURE - TJ (oC) FIGURE 4. MOSFETs HAVE A POSITIVE TEMPERATURE COEFFICIENT OF RESISTANCE, WHICH GREATLY REDUCES THE POSSIBILITY OF THERMAL RUNAWAY AS TEMPERATURE INCREASES The positive temperature coefficient of resistance means that a MOSFET is inherently more stable with temperature fluctuation, and provides its own protection against thermal runaway and second breakdown. Another benefit of this characteristic is that MOSFETs can be operated in parallel without fear that one device will rob current from the others. If any device begins to overheat, its resistance will increase, and its current will be directed away to cooler chips. Gate Parameters To permit the flow of drain-to-source current in an n-type MOSFET, a positive voltage must be applied between the gate and source terminals. Since, as described above, the gate is electrically isolated from the body of the device, theoretically no current can flow from the driving source into the gate. In reality, however, a very small current, in the ©2002 Fairchild Semiconductor Corporation Most DMOS processes use a polysilicon gate structure rather than the metal-gate type. If the resistance of the gate structure (R in Figure 5) is high, the switching time of the DMOS device is increased, thereby reducing its upper operating frequency. Compared to a metal gate, a polysilicon gate has a higher gate resistance. This property accounts for the frequent use of metal-gate MOSFETs in high-frequency (greater than 20MHz) applications, and polysilicon-gate MOSFETs in higher-power but lower-frequency systems. Since the frequency response of a MOSFET is controlled by the effective R and C of its gate terminal, a rough estimate can be made of the upper operating frequency from datasheet parameters. The resistive portion depends on the sheet resistance of the polysilicon-gate overlay structure, a value of approximately 20 ohms. But whereas the total R value is not found on datasheets, the C value (CISS) is; it is recorded as both a maximum value and in graphical form as a function of drain-to-source voltage. The value of CISS is closely related to chip size; the larger the chip, the greater the value. Since the RC combination of the input circuit must be charged and discharged by the driving circuit, and since the capacitance dominates, larger chips will have slower switching times than smaller chips, and are, therefore, more useful in lower-frequency circuits. In general, the upper frequency limit of most power MOSFETs spans a fairly broad range, from 1MHz to 10MHz. Application Note 7500 Rev. A1 Application Note 7500 Output Characteristics Drive Requirements When considering the Vgs level required to operate a MOSFET, note from Figure 6, that the device is not turned on (no drain current flows) unless Vgs is greater than a certain level (called the threshold voltage). In other words, the threshold voltage must be exceeded before an appreciable increase in drain current can be expected. Generally Vgs for many types of DMOS devices is at least 2V. This is an important consideration when selecting devices or designing circuits to drive a MOSFET gate: the gate-drive circuit must provide at least the threshold-voltage level, but preferably, a much higher one. VG = 20V VG = 8V 30 VG = 10V 25 DRAIN CURRENT - ID (A) Probably the most used MOSFET graphical data is the output characteristics or a plot of drain-to-source current (IDS) as a function of drain-to-source voltage (VDS). A typical characteristic, shown in Figure 6, gives the drain current that flows at various VDS values as a function of the gate-tosource voltage (Vgs). The curve is divided into two regions: a linear region in which VDS is small and drain current increases linearly with drain voltage, and a saturated region in which increasing drain voltage has no effect on drain current (the device acts as a constant-current source). The current level at which the linear portion of the curve joins with the saturated portion is called the pinch-off region. VG = 7V 20 PULSE TEST PULSE DURATION = 80µS DUTY CYCLE ≤ 2% TC = 25oC VG = 6V 15 10 VG = 5V 5 VG = 4V 0 0 2 4 6 8 10 12 14 DRAIN-TO-SOURCE VOLTAGE - VDS (V) FIGURE 6. MOSFETs REQUIRE A HIGH INPUT VOLTAGE (AT LEAST 10V) IN ORDER TO DELIVER THEIR FULL RATED DRAIN CURRENT As Figure 6 shows, a MOSFET must be driven by a fairly high voltage, on the order of 10V, to ensure maximum saturated drain-current flow. However, integrated circuits, such as TTL types, cannot deliver the necessary voltage levels unless they are modified with external pull-up resistors. Even with a pull-up to 5V, a TTL driver cannot fully saturate most MOSFETs. Thus, TTL drivers are most suitable when the current to be switched is far less than the rated current of the MOSFET. CMOS ICs can run from supplies of 10V, and these devices are capable of driving a MOSFET into full saturation. On the other hand, a CMOS driver will not switch the MOSFET gate circuit as fast as a TTL driver. The best results, whether TTL or CMOS ICs provide the drive, are achieved when special buffering chips are inserted between the IC output and gate input to match the needs of the MOSFET gate. ©2002 Fairchild Semiconductor Corporation Application Note 7500 Rev. A1 TRADEMARKS The following are registered and unregistered trademarks Fairchild Semiconductor owns or is authorized to use and is not intended to be an exhaustive list of all such trademarks. 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