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IGBT, Insulated Gate Bipolar Transistor, is a composite fully controlled voltage-driven power semiconductor device composed of BJT (bipolar transistor) and IGFET, (Insulated Gate Field Effect Transistor). It has the advantages of both the high input impedance of MOSFET and the low on-voltage drop of GTR.
IGBT, Insulated Gate Bipolar Transistor, is a composite fully controlled voltage-driven power semiconductor device composed of BJT (bipolar transistor) and IGFET, (Insulated Gate Field Effect Transistor). It has the advantages of both the high input impedance of MOSFET and the low on-voltage drop of GTR. The saturation voltage of GTR is reduced, and the current-carrying density is large, but the driving current is larger. The drive power of MOSFET is very small and the switching speed is fast, but the conduction voltage drop is large, and the current-carrying density is small. IGBT combines the advantages of the above two devices, the driving power is small and the saturation voltage is reduced. It is very suitable to be used in the converter system with a DC voltage of 600V and above, such as an AC motor, inverter, switching power supply, lighting circuit, traction drive, and other fields.
IGBT module is a modular semiconductor product that is packaged by IGBT (insulated gate bipolar transistor) and FWD (freewheeling diode) through a specific circuit bridge. The packaged IGBT module is directly applied to equipment such as inverters and UPS uninterruptible power supplies. IGBT module has the characteristics of energy-saving, convenient installation and maintenance, and stable heat dissipation. Generally, IGBT also refers to the IGBT module. With the advancement of concepts such as energy conservation and environmental protection, such products will become more and more common in the market. IGBT is the core device for energy conversion and transmission, commonly known as the "CPU" of power electronic devices, and is widely used in rail transit, smart grid, aerospace, electric vehicles, and new energy equipment.
The figure shows an N-channel enhanced insulated gate bipolar transistor structure. The N+ region is called the source region, and the electrode on it is called the source (ie emitter E). The N base is called the drain region. The control area of the device is the gate area, and the electrode on it is called the gate (ie, gate G). The channel is formed at the boundary of the gate region. The P-type region between the C and E poles is called the subchannel region. The P+ area on the other side of the drain area is called the drain injector. It is a unique functional area of the IGBT and forms a PNP bipolar transistor together with the drain area and the sub-channel area. It acts as an emitter, injects holes into the drain, conducts conduction modulation, and reduces the on-state voltage of the device. The electrode on the drain injection area is called the drain (ie collector C).
N-channel enhanced insulated gate bipolar transistor structure
The switching function of the IGBT is to form a channel by adding a positive gate voltage to provide base current to the PNP (formerly NPN) transistor to turn on the IGBT. On the contrary, adding reverse gate voltage to eliminate the channel and cutting off the base current will turn off the IGBT. The driving method of IGBT is basically the same as that of MOSFET. It only needs to control the input N-channel MOSFET, so it has high input impedance characteristics. After the channel of the MOSFET is formed, holes (minor carriers) are injected from the P+ base to the N- layer to modulate the conductance of the N- layer and to reduce the resistance of the N- layer. So the IGBT has a low on-state voltage even at high voltage.
IGBT is a three-terminal device. It has gate G, collector c, and emitter E. The structure and simplified equivalent circuit of the IGBT are shown in the figure.
IGBT internal structure and equivalent circuit
The figure shows the cross-sectional schematic diagram of the internal structure of the N-IGBT combined with the N-channel VDMOSFFT and GTR. IGBT has one more layer of P+ implantation area than VDMOSFET, forming a large area PN junction J1. The P+ injection region emits minority carriers to the N base region when the IGBT is turned on, so the conductivity of the drift region is modulated then the IGBT has a strong current flow capability. The N+ layer between the P+ injection region and the N- drift region is called a buffer zone. Whether there is a buffer that determines the different characteristics of the IGBT. The IGBT with N* buffer is called asymmetric IGBT, also called punch-through IGBT. It has the advantages of small forward voltage drop, short turn-off time, and small tail current during turn-off, but its reverse blocking ability is relatively weak. The IGBT without N-buffer is called symmetrical IGBT, also called non-punch-through IGBT. It has strong forward and reverse blocking capability, but its other characteristics are not as good as asymmetric IGBTs.
The simplified equivalent circuit shown in the figure shows that the IGBT is a Darlington structure composed of GTR and MOSFET. Part of this structure is driven by MOSFET, and the other part is a thick base PNP transistor.
The ideal equivalent circuit and actual equivalent circuit of IGBT
From the equivalent circuit, the IGBT can be used as a monolithic Bi-MOS transistor formed by a Darlington connection of a PNP bipolar transistor and a power MOSFET.
Therefore, when a positive voltage is applied between the gate and the emitter to turn on the power MOSFET, the base-collector of the PNP transistor is connected to low resistance, so that the PNP transistor is in a conductive state. Adding a p+ layer, inject holes from the p+ layer to the n base in the on state, thereby triggering a change in conductivity. Therefore, it can get extremely low on-resistance compared with power MOSFET.
After that, when the voltage between the gate and the emitter is 0V, the power MOSFET is in the off state, and the base current of the PNP transistor is cut off, thus being in the off state.
As mentioned above, the IGBT, like the power MOSFET, can control the turn-on and turn-off through the voltage signal.
The static characteristics of IGBT mainly include volt-ampere characteristics and transfer characteristics.
The volt-ampere characteristic of IGBT refers to the relationship curve between drain current and gate voltage when the gate-source voltage Ugs is used as the parameter. The output drain current ratio is controlled by the gate-source voltage Ugs. The higher the Ugs, the larger the Id. It is similar to the output characteristics of GTR, and can also be divided into saturation zone 1, amplification zone 2, and breakdown characteristics. In the off-state of IGBT, the forward voltage is borne by the J2 junction, and the reverse voltage is borne by the J1 junction. If there is no N+ buffer, the forward and reverse blocking voltage can be at the same level. After adding the N+ buffer, the reverse turn-off voltage can only reach a level of tens of volts, which limits the scope of certain applications of IGBTs.
The transfer characteristic of IGBT refers to the relationship curve between the output drain current Id and the gate-source voltage Ugs. It has the same transfer characteristics as MOSFET. When the gate-source voltage is less than the turn-on voltage Ugs(th), the IGBT is in the off state. In most of the drain current range after the IGBT is turned on, Id has a linear relationship with Ugs. The maximum gate-source voltage is limited by the maximum drain current, and its optimal value is generally about 15V.
Dynamic characteristics are also called switching characteristics. The switching characteristics of IGBTs are divided into two parts: one is the switching speed, the main indicator is the time of each part of the switching process; the other is the loss during the switching process.
The switching characteristic of IGBT refers to the relationship between drain current and drain-source voltage. When the IGBT is in the ON state, its B value is extremely low because its PNP transistor is a wide base transistor. Although the equivalent circuit is a Darlington structure, the current flowing through the MOSFET becomes the main part of the total current of the IGBT. At this time, the on-state voltage Uds(on) can be expressed by the following formula:
Uds(on) = Uj1 + Udr + IdRoh
Where Uj1 —— the forward voltage of the JI junction, its value is 0.7 ~ 1V; Udr —— the voltage drop on the extension resistance Rdr; Roh —— the channel resistance.
On-state current Ids can be expressed by the following formula:
Ids=(1+Bpnp)Imos
In the formula, Imos is the current flowing through the MOSFET.
Due to the conductance modulation effect in the N+ region, the on-state voltage drop of the IGBT is small. The on-state voltage drop of an IGBT with a withstand voltage of 1000V is 2 to 3V. When the IGBT is in the off state, only a small leakage current exists.
The IGBT operates as a MOSFET most of the time during the turn-on process. In the late stage of the drain-source voltage Uds falling process, the PNP transistor goes from the amplifying region to saturation, and delay time is added. td(on) is the turn-on delay time, and tri is the current rise time. In practical applications, the drain current turn-on time ton is the sum of td (on) tri, and the drain-source voltage fall time is composed of tfe1 and tfe2.
The triggering and turning off of IGBT requires that a positive voltage and a negative voltage are added between its gate and base, and the gate voltage can be generated by different driving circuits. When selecting these drive circuits, it must be based on the following parameters: device turn-off bias requirements, gate charge requirements, endurance requirements, and power supply conditions. Because the IGBT gate-emitter impedance is large, MOSFET driving technology can be used for triggering. However, since the input capacitance of the IGBT is larger than that of the MOSFET, the turn-off bias of the IGBT should be higher than that provided by many MOSFET drive circuits.
During the turn-off of the IGBT, the waveform of the drain current becomes two segments. Because after the MOSFET is turned off, the stored charge of the PNP transistor is difficult to quickly eliminate, resulting in a long tail time for the drain current. td(off) is the turn-off delay time, and trv is the rise time of the voltage Uds(f). The fall time Tf of the drain current often given in practical applications is composed of t(f1) and t(f2) in the figure, and the turn-off time of the drain current t(off)=td(off)+trv + t(f). In the formula: the sum of td(off) and trv is also called storage time.
The switching speed of IGBT is lower than that of MOSFET, but it is significantly higher than GTR. IGBT does not need negative gate voltage to reduce the turn-off time when it is turned off, but the turn-off time increases with the increase of the parallel resistance of the gate and emitter. The turn-on voltage of IGBT is about 3 to 4V, which is equivalent to that of MOSFET. The saturation voltage drop when the IGBT is turned on is lower than that of the MOSFET and close to the GTR, and the saturation voltage drop decreases with the increase of the gate voltage.
The voltage and current capacity of officially commercial IGBT devices are still very limited, far from meeting the needs of the development of power electronics application technology. In many applications in the high-voltage field, the voltage level of the device is required to reach above 10KV. At present, high-voltage applications can only be achieved through technologies such as IGBT high-voltage series connection. Some manufacturers such as Swiss ABB have developed 8KV IGBT devices using the principle of soft punch-through. The 6500V/600A high-voltage and high-power IGBT devices produced by EUPEC in Germany have been put into practical use, and Toshiba of Japan has also stepped into this field. At the same time, major semiconductor manufacturers continue to develop high withstand voltage, high current, high speed, low saturation voltage drop, high reliability, and low-cost technologies for IGBTs, mainly using manufacturing processes below 1um, and some new progress has been made in research and development.
The structure of the IGBT silicon chip is very similar to that of the power MOSFET. The main difference is that the IGBT adds a P+ substrate and an N+ buffer layer. One of the MOSFETs drives two bipolar devices. The application of the substrate creates a J1 junction between the P+ and N+ regions of the tube body. When the positive gate bias causes the P base region to be reversed under the gate, an N channel is formed. At the same time, a current of electrons appears, and a current is generated in exactly the same way as a power MOSFET. If the voltage generated by this electron flow is in the range of 0.7V, J1 will be forward biased. Some holes are injected into the N-zone and adjust the resistivity between the cathode and anode. In this way, the total loss of power conduction is reduced and a second charge flow is started. The final result is that two different current topologies temporarily appear within the semiconductor hierarchy: an electron flow (MOSFET current); a hole current (bipolar).
When a negative bias is applied to the gate or the gate voltage is lower than the threshold, the channel is prohibited and no holes are injected into the N-region. In any case, if the MOSFET current drops rapidly during the switching phase, the collector current gradually decreases. This is because, after the start of commutation, there are still a few carriers (minor carriers) in the N layer. The reduction of this residual current value (wake current) depends entirely on the density of the charge when it is turned off. The density is related to several factors, such as the amount and topology of dopants, layer thickness, and temperature. The attenuation of the minority carrier causes the collector current to have a characteristic wake waveform. The collector current causes the following problems: 1. Increased power consumption; 2. Cross-conduction problems, especially in devices that use freewheeling diodes.
Since the wake is related to the recombination of minority carriers, the current value of the wake should be closely related to the temperature of the chip and the hole mobility which is closely related to IC and VCE. Therefore, depending on the temperature reached, it is feasible to reduce this undesirable effect of the current acting on the terminal equipment design.
When a reverse voltage is applied to the collector, J1 will be controlled by the reverse bias, and the depletion layer will expand to the N-region. If the thickness of this layer is reduced too much, an effective blocking ability will not be obtained. Therefore, this mechanism is very important. On the other hand, if you increase the size of this area too much, it will continuously increase the pressure drop. The second point clearly explains why the voltage drop of NPT devices is higher than that of equivalent (IC and speed are the same) PT devices.
When the gate and emitter are shorted and a positive voltage is applied to the collector terminal, the P/N J3 junction is controlled by the reverse voltage. At this time, the depletion layer in the N drift region still bears the externally applied voltage.
Under normal circumstances, the main differences between static and dynamic latches are as follows:
When the thyristors are all turned on, static latching occurs. The dynamic latching occurs only when the thyristors turned off. This special phenomenon severely limits the safe operating area. To prevent the harmful phenomena of parasitic NPN and PNP transistors, it is necessary to take the following measures: prevent the NPN part from turning on, change the layout and doping level respectively, and reduce the total current gain of the NPN and PNP transistors. Besides, the latching current has a certain effect on the current gain of PNP and NPN devices. Therefore, it has a very close relationship with the junction temperature; when the junction temperature and gain increase, the resistivity of the P base region will increase and damage the overall characteristics. Therefore, device manufacturers must pay attention to maintaining a certain ratio between the maximum collector current value and the latching current, which is usually 1:5.
In 1979, MOS gate power switching devices were introduced to the world as the pioneer of the IGBT concept. This device appears as a thyristor-like structure (P-N-P-N four-layer composition), which is characterized by the formation of a V-shaped groove gate through a strong alkali wet etching process.
In the early 1980s, the DMOS (Double Diffusion Formed Metal-Oxide-Semiconductor) process used in power MOSFET manufacturing technology was adopted in the IGBT. At that time, the structure of the silicon chip was a thicker NPT (non-punch through) type design. Later, through the use of the PT (punch-through) structure method, a significant improvement in parameter trade-off was obtained. This is due to the technological advancement of epitaxy on silicon wafers and the use of n+ buffer layers designed for a given blocking voltage. In the past few years, the design rules of this DMOS planar gate structure prepared on an epitaxial wafer designed by PT have advanced from 5 microns to 3 microns.
In the mid-1990s, the trench gate structure returned to a new concept of IGBT, which was a new etching process realized by silicon dry etching technology borrowed from the large-scale integration (LSI) process, but it was still punch-through ( PT) type chip structure. In this trench structure, a more important improvement in the trade-off between on-state voltage and off-time is achieved.
Punch-through (PT) technology has a relatively high carrier injection coefficient, and because it requires the control of minority carrier lifetime, its transport efficiency deteriorates. On the other hand, the non-punch-through (NPT) technology is based on the fact that it does not kill the minority carrier lifetime and has a good transport efficiency, but its carrier injection coefficient is relatively low. Furthermore, non-punch-through (NPT) technology has been replaced by soft-punch-through (LPT) technology, which is similar to what some people call "soft punch-through" (SPT) or "electric field cut-off" (FS) type technology, which makes the overall effect of "cost-performance" has been further improved.
In 1996, CSTBT (carrier stored trench-gate Bipolar transistor) enabled the realization of the fifth-generation IGBT module, which adopted a weak punch-through (LPT) chip structure and adopted a more advanced design of wide cell spacing. At present, new concepts including a "reverse blocking type" function or a "reverse conduction type" function of IGBT devices is under study for further optimization.
Now, the high-current and high-voltage IGBT has been modularized, and an integrated dedicated IGBT drive circuit has been manufactured. Its performance is better, the reliability of the whole machine is higher and the volume is smaller.
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