Archive for the ‘Semiconductor’ Category

Power Semiconductors: The BJT, MOSFET, and IGBT (6)

March 20, 2009

VI. CONCLUSION

Whereas all of the mentioned devices have certain advantages, the IGBT has proven to make the best use of all of these advantages while minimizing the disadvantages. As such, IGBTs have taken over in popularity and are now being even further optimized to suit the needs of an ideal power semiconductor.

REFERENCES
[1] Vrej Barkhordarian, “Power MOSFET Basics”, (International Rectifier, 2004), http://www.irf.com/technical-info/appnotes/mosfet.pdf.
[2] B. Jayant Baliga, Modern Power Devices (Krieger Publishing, Malabar, Florida, 1987), Chap. 1.
[3] “IGBT Fundamentals,” (Seimens Semiconductor Group, 2004), http://www.infineon.com/cmc_upload/migrated_files/document_files/Application_Notes/lh_ap_18.pdf.
[4] Fraidoon Mazda, Power Electronics Handbook (Newnes, Oxford, 1997), Chap. 1.
[5] D.A. Grant, “Power semiconductors-innovation and improvement continue to challenge the designer,” in New Developments in Power Semiconductor Devices, IEE Colloquium on, May 1991.
[6] “Main Applications for Power Modules,” (Mitsubishi Electronics, Sept. 1998), http://www.mitsubishichips.com/webfiles/pdf/powermos2_0.pdf.

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Power Semiconductors: The BJT, MOSFET, and IGBT (5)

March 20, 2009
V. APPLICATIONS

Because power semiconductors have very wide-ranging applications, the most desirable type for a given application comes down to several factors: the amplification, the switching speed, and the power class. Trends in particular types can be seen in applications in industry, the consumer
market, and transportation.

A. Industrial Applications
Within industry, the two main uses for power semiconductors are for motor control and power supplies. For motor drives, power semiconductors can control all sizes of motors from those found in large mills to simple machine tools. The trend in the method of controlling of these motors has been toward IGBTs. Likewise, due to their versatility, the use of the IGBT has become the trend in constructing power supplies for such applications as battery charging, welding, and induction heating.

B. Consumer Applications
In the consumer market, power semiconductors can be found in audio amplifiers, heat controls, light dimmers, and again in motor controls. Because of the low cost and high amplification of MOSFETs, they have become the preference in audio amplifier construction. On the other hand, the IGBT is dominating in heat controls, dimmers, and motor drives.
An example consumer application of the IGBT can be seen in Figure 4. Here, and IGBT modules is used to control an induction coil that is used to heat a pan for cooking. In this particular case, the IGBT acts as a switch; when voltage is applied to the gate, the current will flow through the induction coil, otherwise no current will flow. Because the IGBT can handle high currents, there is little concern that the semiconductor will overheat and/or be destroyed with the high currents that are necessary for induction heating.

C. Transportation Applications
Within transportation, motor control with the IGBT is again most prevalent. IGBTs are utilized in drives for electric cars and trains. However, BJTs can be found in simpler applications such as electronic ignition and vehicle voltage regulation.
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Power Semiconductors: The BJT, MOSFET, and IGBT (4)

March 20, 2009
IV. THE INSULATED GATE BIPOLAR TRANSISTOR (IGBT)
A. Physical Characteristics
The IGBT is the most popular power semiconductor currently used today. It combines the MOS gate structure with the bipolar current conduction to create a device that is the best of both the MOSFET and the BJT. For terminals, it is a hybrid between the BJT and the MOSFET.

It has three terminals: the collector, the gate, and the emitter. Schematically, the IGBT is basically a p-n-p BJT powered by an n-channel MOSFET.

B. Operational Advantages and Disadvantages
The IGBT brings together the advantages of a BJT and MOSFET. It has high input impedance, a low power consumption, and a large safe operating area. It also has a remarkably high power handling capability for a given chip size. And, because it is a minority carrier device on a p+ substrate, it has superior conduction to a standard MOSFET. When used in combination with power integrated circuits, one can expect a cost reduction by a factor of ten [2].
The only disadvantage comes in switching speed. While the IGBT can compete with the speed of the BJT, it cannot beat the MOSFET. The faster the switching, the greater the forward voltage drop, and it has a relatively high turn-off time due to the long lifetime of minority carriers. However, these problems recently have been overcome [4], allowing the IGBT to dominate in industrial applications.

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Power Semiconductors: The BJT, MOSFET, and IGBT (3)

March 20, 2009
III. THE METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR (MOSFET)

A. Physical Characteristics
The MOSFET was introduced in the 1970s and, unlike the BJT, is a voltage controlled device. It also has three terminals, though they differ from the BJT: the source, the gate, and the drain. The source and drain diffusions are separated by the gate.

The MOSFET has a p or n channel and can operate in depletion or enhancement mode. In enhancement, no current flows when the gate voltage is zero. In depletion mode, however, a narrow n channel is formed under the gate such that current will still flow when the gate voltage is zero.


B. Operational Advantages and Disadvantages
In comparison to the BJT, the MOSFET is far superior. It has a high input impedance, reducing complexity and cost, and a low input current drive. At low currents, it also has a higher gain than the BJT. To handle higher currents, it is sufficient to simply put several MOSFETs in parallel, and because there is only one breakdown region, the safe operating region is larger. Additionally, because they are free from minority carrier storage times, MOSFETs are faster at switching than the BJTs.
Despite its advantages, the MOSFET has low gain at high currents. Moreover, it was slow to catch on and had an overall greater cost than a BJT of the same power rating. However, in recent years, the prices have come down and MOSFETs have gained in popularity [2].

Power Semiconductors: The BJT, MOSFET, and IGBT (2)

March 20, 2009
II. THE BIPOLAR JUNCTION TRANSISTOR (BJT)

A. Physical Characteristics
One of the first types of power semiconductors, the BJT is a three layered semiconductor consisting of a sandwich of p-n-p or n-p-n materials. In addition, it has three terminals: the emitter, the collector, and the base. The base is lightly doped, whereas the emitter is heavily doped and wider.

The emitter-base region is forward biased so that majority carriers will flow across the junction. On the other hand, the collectorbase region is reverse biased, which results in a small minority carrier flow.


B. Operational Advantages and Disadvantages
When used in a common emitter mode, as it is most often, the BJT acts as a current-controlled switch. The base current is in the input and the collector current is the output. Because it is current-controlled, it has a fairly low saturation voltage, which is desirable. In addition, BJTs are able to handle high voltages and currents with few problems.
Of course, there are many drawbacks. The BJT has low gain at high frequencies, so it is not useful for amplification under those conditions. Additionally, it does not have a very high surge rating—the peak current is only about twice the maximum continuous current rating. Unlike MOSFETs, BJTs also have a relatively slow switching speed because it takes time to charge the emitter and collector depletion capacitances, which consequently slows the turn-on time.
There are also two breakdown areas associated with the BJT that reduce its safe operating area. The first is the avalanche breakdown, which causes a rapid rise in current, and a second breakdown can be brought on by inductive loads, which can overheat and destroy the transistor.

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Power Semiconductors: The BJT, MOSFET, and IGBT (1)

March 20, 2009
I. INTRODUCTION

Although power semiconductors were first developed in the late 1940s, they have matured significantly in sixty years. In today’s society, power semiconductors can be found everywhere.

They are essentially the solid-state version of the mechanical relay or the vacuum tube. Some of most common applications include motor drives, uninterruptible power supplies, audio amplifiers, and fluorescent lighting. For the best device suited for the job, it has become a competition between the bipolar junction transistor (BJT), the metal oxide field-effect transistor (MOSFET), and the insulated gate bipolar transistor (IGBT). Each has inherent advantages over the others, but the IGBT has seemed to dominate the industry in recent years.

IGBT Basic (8)

January 8, 2009

3-6. Switching Characteristics

Turn-on
When the device is in the forward blocking mode, and if the positive gate bias (threshold voltage), which is enough to invert the surface of P base region under the gate, is applied, then an n-type channel forms and the current begins to flow.

At this time the anode-cathode voltage must be above 0.7V (potential barrier), so that it can forward bias the P+ substrate / N- drift junction (J1). The electron current flowing from the N+ emitter to the N- drift region through the channel is the base drive current of the vertical PNP transistor, and it induces a minority carrier (hole) injection from the P+ region to the N- base region. The current that flows to the emitter electrode are divided into the electron current (MOS current) flowing through the channel and bipolar current flowing through the P body / N- drift junction (J2). When gate bias falls to near the threshold voltage at on-state, the inversion layer conductivity is reduced, and significant voltage drop that arises from electron current flow occurs across the region as in a MOSFET.
When the voltage drop is equal to the difference between the gate bias and threshold voltage (VGE – Vth), then the channel is pinched off. At this point, the electron current becomes saturated.
Since this limits the base drive current of the PNP transistor, the hole current flowing through the PNP transistor is also limited. As a result, the device operates with saturated current at the active region (gate controlled output current).

Turn-off
The gate must be shorted to the emitter or a negative bias must be applied to the gate. When the gate voltage falls below the threshold voltage, the inversion layer cannot be maintained, and the supply of electrons into the N- drift region is blocked. At this point the turn-off process begins. As illustrated in Fig. 6, the collector current (ICO) falls to zero in two stages. As the electron current supplied through the MOSFET channel during the on-state is stopped, collector
current suffers an initial abrupt fall (ICD). After that, the tail current (ICT) comes from the minority carrier (hole) that was injected through the N- drift region from the P+ substrate during the on-state. The tail current of the IGBT lowers switching characteristics and increases switching loss. Since N- drift region is the base of the PNP transistor, it cannot be approached from outside, so it is not possible to control the tail current from outside. But it can be controlled with the amount of minority carrier (hole) injected through the N- drift region and recombination rate when it is off. In order to reduce the amount of injected minority carrier and increase the recombination rate when it is off, the concentration and the thickness of the N+ buffer layer between the P+ substrate and N- drift region must increase, as well as the dose of electron irradiation (in FSC, electron irradiation is applied to above 600V class except 400V) that takes place after device fabrication. However, improving the switching speed of the IGBT generally accompanies reduced current handling capability. As such, the trade-off between switching speed, which is related to switching loss, and forward voltage drop, which in turn is related to conduction loss, is important. The asymmetric structure is superior in such trade-offs as compared to a symmetric structure, and it can be improved by increasing the doping concentration in the buffer layer. In terms of power loss, the power MOSFET is better suited for
lower blocking voltage and high operating frequency applications, while the IGBT is better suited for higher blocking voltage and lower operating frequency.


High temperature characteristics

The minority carrier lifetime in the drift region increases as the temperature increases. This not only delays recombination process (tail current) of the minority carrier, but it also increases the NP transistor gain. So the portion of the initial abrupt fall (ICD) in the overall collector current reduces. As such, tf(fall time) of the spec is lengthened, and turn-off time increases with an increase in temperature, and the asymmetric structure has a lower rate of increase than the symmetric structure.

to be continued…………..

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IGBT Basic (7)

January 8, 2009
3-5. Forward Conduction Characteristics
Due to its structure, the IGBT is sometimes viewed as a serial connection of the MOSFET and PiN diode, and sometimes it is seen as a wide base PNP transistor driven by the MOSFET in Darlington configuration.
The former description can be used to understand the behavior of the device, but the latter better describes the IGBT.

Fig. 2 is a graph of the IGBT’s static characteristics. Even if a MOSFET channel of the input side is formed, the collector current does not flow if the anode-cathode forward voltage drop does not exceed approximately 0.7V as in the PiN diode. In addition, the current is saturated when the voltage across the MOSFET channel is greater than (VGE – Vth) and has an infinite output resistance, as in a power MOSFET. However, in a symmetrical IGBT, the collector current increases with the increase in collector voltage, and the rate of increase in the collector current also increases with the increase in collector voltage. Such finite output resistance is due to a shortening of the channel due to an increase in the collector voltage, and a secondary decrease in the drain output resistance due to bipolar transistor current flow. In order to increase the collector output resistance, an asymmetrical structure with a N+ buffer layer between the N- drift region and P+ substrate is used to prevent an increase in the bipolar transistor’s current gain with the increase in the collector voltage. In an asymmetrical structure, the width of the undepleted N- drift region does not change rapidly with the increase in the collector voltage due to the high concentration of the buffer layer, but it remains the same width as the N+ buffer layer for all collector voltages. This results in a constant value of the PNP transistor’s current gain. In addition to this, the N+ buffer layer reduces the injection efficiency of the P+ substrate / N+ buffer junction (J1). This reduces the current gain of the PNP transistor. As such, an IGBT with an asymmetrical structure has much superior output characteristics than a symmetrical type. In addition, collector output resistance can be increased with electron irradiation to shorten the minority carrier lifetime, which reduces the diffusion length. The following is the equation for obtaining the saturated collector current of the IGBT:


Transconductance at the active region can be obtained by differentiating the IC,sat with respect
to VGE.

The IGBT’s saturated collector current and transconductance are higher than those of the power MOSFETs of the same aspect ratio (Z/LCH). This is because the PNP transistor’s current gain (αPNP) is less than 1 (0.2 to 0.3 in general).

On-state voltage drop
Forward current-voltage characteristics and the conduction loss of a MOSFET are described as n-resistance. On the other hand, the characteristics of the IGBT are described as voltage drop at rated current, as is the case with the bipolar power transistor. On-state voltage drop is comprised of voltage drop of the forward biased P+ substrate / N- drift junction (J1), the voltage drop of conductivity modulated N- drift region and the voltage drop of MOSFET. Cut-in voltage for forward biased J1 is about 0.7V at room temperature. Cut-in voltage decreases due to a sharp increase in intrinsic carrier concentration as the temperature rises. The voltage drop of the N drift region can be obtained by integrating the electric field of the entire drift region, and it is generally less than 0.1V due to a strong conductivity modulation caused by injected holes from J1. The voltage drop of a MOSFET is the sum of the voltage drops from the channel region, JFET region and accumulation layer. Due to a decrease in drift layer resistance, the portion of JFET resistance and channel resistance is increased in the voltage drop between on-state collector-emitter. Hence, low JFET and channel resistance design are important factors in obtaining the best performance in an IGBT. The voltage drop at the channel is proportional to the channel length, gate oxide thickness. And it is inversely proportional to channel width, electron mobility and gate bias. The channel width can be increased by increasing the concentration of circuits by decreasing the size of each unit cell. But because of this the JFET resistance increases significantly, so the optimal size of the unit cell exists for each voltage rating.
The IGBT decreases the minority carrier lifetime with electron irradiation in order to improve the switching speed, and this increases the on-state voltage drop. Even in IGBTs with the same structure, the IGBT with a fast switching speed has a larger voltage drop, and the IGBT with a slower switching speed has a smaller on-state voltage drop depending on the condition of electron irradiation, which takes place after device fabrication.

High temperature characteristics
One must be aware of the changes in characteristics from changes in temperature, as the IGBT’s input characteristics are similar to a MOSFET, and output characteristics are similar to bipolar transistors. As temperature rises, the energy barrier of the P+ substrate / N- drift region junction (J1: emitter-base junction of PNP transistor) decreases, which leads to a lower cut-in voltage, and the threshold voltage decreases as in a MOSFET. As channel resistance increases, the amount of electron current (MOS current) decreases, which is injected to the Ndrift region. However, current gain, which is the ratio of the hole current (bipolar current) to the electron current, increases. In addition, N- drift region (base of the PNP transistor) resistance increases. Due to these characteristics, changes in cut-in voltage of J1 are larger than those in channel resistance and N- drift region resistance at low collector current level, so the IGBT has negative temperature coefficient similar to the bipolar transistor. On the other hand, channel resistance and N- drift region resistance determine the on-state voltage at high collector current, which results in a positive temperature coefficient similar to a power MOSFET. The crossover point for the two characteristics is different for each product, and the collector-emitter voltage drop is independent from temperature at the crossover point. In real applications, it is used in areas with negative temperature coefficient, and these factors must be considered in parallel application. Figs. 3~5 illustrate these characteristics with graphs from the data sheet.


to be continued…………..

IGBT Basic (6)

January 8, 2009
3-4. Leakage Current
Leakage current is divided into two types. One is leakage current from the depleted drift region, and the other flows on the surface of the junction termination.

Since the IGBT uses a P+ substrate and is irradiated with electrons to improve switching speed, the amount of leakage current from the drift region is greater than that of the power MOSFET. If there is a high voltage between the anode and cathode when the IGBT is off, the depletion region widens to nearly the entire drift region from the P+ body / N- drift junction (J2), and the entire region becomes depleted. The electron hole created by the heat of the depletion region is manifested as leakage current according to the area of the depletion region and minority carrier lifetime. This increases with the increase of the rated voltage and current. In particular, leakage current increases as the minority carrier lifetime shortens. As such, leakage current tends to increase with higher speed IGBTs.

IGBT Basic (5)

January 8, 2009
3-3. Static Blocking Characteristics

Reverse Blocking Capability
When negative voltage is introduced to the collector as shown in Fig. 1, the P+ substrate / Ndrift junction (J1) is reverse biased, and the depletion layer generally expands to the N- drift region.

As such, securing an optimal design in resistivity and thickness for the N- drift region is essential in obtaining desirable reverse blocking capability. As a general guideline, the width of the N-drift region is equivalent to the sum of depletion width at maximum operating voltage and minority carrier diffusion length. It is important to optimize the breakdown voltage while maintaining a narrow N- drift region width, as the forward voltage drop increases with an increase in N- drift region width. The following is an equation for calculating the N- drift region width:

Forward Blocking Capability
When the gate is shorted at the emitter, and a positive voltage is introduced at the collector, the P base / N- drift region junction (J2) is reverse biased, and it is supported upto the rated voltage by the depletion region formed at the N- drift region.


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