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What is IGBT
IGBT (insulated gate bipolar transistor) is a composite fully controlled voltage driven power semiconductor device composed of BJT (bipolar triode) and MOS (insulated gate field effect transistor). It has the advantages of high input impedance of MOSFET and low on voltage drop of GTR. The saturation voltage of GTR decreases, the current carrying density is high, but the driving current is large; MOSFET has small driving power and fast switching speed, but it has large conduction voltage drop and low current carrying density.
IGBT combines the advantages of the above two devices, with low driving power and low saturation voltage. It is very suitable for converter systems with DC voltage of 600V and above, such as AC motor, frequency converter, switching power supply, lighting circuit, traction drive and other fields.
The left side of the IGBT structure diagram shows an n-channel enhanced insulated gate bipolar transistor structure. The N + region is called the source region, and the electrode attached to it is called the source. The P + region is called the drain region. The control area of the device is the gate area, and the electrode attached to it is called the gate. The channel is formed close to the boundary of the gate region. The p-type region (including P + and p-regions) between drain and source (channels are formed in this region) is called subchannel region. The P + region on the other side of the drain region is called the drain injector. It is a unique functional region of IGBT. Together with the drain region and sub channel region, it forms a PNP bipolar transistor, which acts as an emitter, injects holes into the drain for conductive modulation, so as to reduce the on state voltage of the device. The electrode attached to the drain injection region is called a drain.
The switching function of IGBT is to form a channel by adding forward gate voltage to provide base current to PnP (formerly NPN) transistor to turn on IGBT. On the contrary, add the reverse gate voltage to eliminate the channel, cut off the base current and 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. When the channel of the MOSFET is formed, it is injected into the hole (minority carrier) of the n-layer from the P + base to modulate the conductivity of the n-layer and reduce the resistance of the n-layer, so that the IGBT also has a low on state voltage at high voltage.
The static characteristics of IGBT mainly include volt ampere characteristics, transfer characteristics and switching characteristics.
The volt ampere characteristic of IGBT refers to the relationship between drain current and gate voltage when the gate source voltage UGS is taken as a parameter. The output drain current ratio is controlled by the gate source voltage UGS. The higher the UGS, the greater the ID. It is similar to the output characteristics of GTR. It can also be divided into saturation region 1, amplification region 2 and breakdown characteristics. For IGBT in cut-off state, the forward voltage is borne by J2 junction and the reverse voltage is borne by J1 junction. If there is no n + buffer, the forward and reverse blocking voltage can reach the same level. After adding n + buffer, the reverse shutdown voltage can only reach tens of volts, which limits some application ranges of IGBT.
The transfer characteristic of IGBT refers to the relationship 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 on voltage UGS (th), the IGBT is in the off state. In most drain current range after IGBT conduction, 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.
The switching characteristics of IGBT refer to the relationship between drain current and drain source voltage. When IGBT is in the on state, because its PNP transistor is a wide base transistor, its b value is very low. Although the equivalent circuit is Darlington structure, the current flowing through MOSFET becomes the main part of the total current of IGBT. At this time, the on state voltage UDS (on) can be expressed by the following formula
Uds(on) = Uj1 + Udr + IdRoh
Where uj1 is the forward voltage of Ji junction, and its value is 0.7 ~ 1V; UDR -- voltage drop on the expansion resistance RDR; Roh - channel resistance.
The on state current IDS can be expressed by the following formula:
Ids=(1+Bpnp)Imos
Where, IMOs - current flowing through MOSFET.
Due to the conductivity modulation effect in the N + region, the on state voltage drop of IGBT is small, and the on state voltage drop of IGBT with withstand voltage of 1000V is 2 ~ 3V. When IGBT is in off state, only a small leakage current exists.
Dynamic characteristics
During the turn-on process of IGBT, it operates as MOSFET for most of the time. Only in the later stage of the drop process of drain source voltage UDS, the PNP transistor increases a delay time from amplification region to saturation. TD (on) is the on delay time and tri is the current rise time. The drain current on time ton, which is often given in practical application, is the sum of TD (on) tri. The drop time of drain source voltage consists of tfe1 and tfe2.
The triggering and shutdown of IGBT requires the addition of positive voltage and negative voltage between its grid and base. The grid voltage can be generated by different driving circuits. When selecting these driving circuits, it must be based on the following parameters: requirements for device off bias, requirements for gate charge, requirements for solid resistance and power supply. Because the gate emitter impedance of IGBT is large, MOSFET driving technology can be used for triggering. However, since the input capacitance of IGBT is larger than that of MOSFET, the off bias of IGBT should be higher than that provided by many MOSFET driving circuits.
During IGBT shutdown, the waveform of drain current changes into two segments. After the MOSFET is turned off, the stored charge of the PNP transistor is difficult to be eliminated quickly, resulting in a long tail time of the drain current. TD (off) is the turn off delay time and TRV is the rise time of the voltage UDS (f). The falling time TF of drain current often given in practical application consists of T (F1) and t (F2) in the figure, and the off time of drain current
T (off) = TD (off) + TRV ten t (f)
Where, the sum of TD (off) and TRV is also called storage time.
The switching speed of IGBT is lower than that of MOSFET, but significantly higher than that of GTR. IGBT does not need negative gate voltage to reduce the off time, but the off time increases with the increase of parallel resistance between gate and emitter. The on voltage of IGBT is about 3 ~ 4V, which is equivalent to that of MOSFET. The saturation voltage drop of IGBT is lower than that of MOSFET and close to that of GTR. The saturation voltage drop decreases with the increase of gate voltage.
The voltage and current capacity of officially commercial IGBT devices are still very limited, which is 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 be more than 10kV. At present, high-voltage applications can only be realized by IGBT high-voltage series technology. Some foreign manufacturers, such as abb of Switzerland, have developed 8Kv IGBT devices based on the principle of soft penetration. 6500v / 600A high-voltage and high-power IGBT devices produced by eupec of Germany have been put into practical application, and Toshiba of Japan has also been involved in this field. At the same time, major semiconductor manufacturers continue to develop IGBT technologies with high withstand voltage, high current, high speed, low saturation voltage drop, high reliability and low cost, mainly using manufacturing processes below 1um, and some new progress has been made in research and development.
IGBT principle and method
IGBT is the natural evolution of vertical power MOSFET for high current, high voltage applications and fast terminal equipment. Because a high breakdown voltage bvdss requires a source drain channel, which has high resistivity, the manufacturing success rate MOSFET has the characteristics of high RDS (on) value. IGBT eliminates these main disadvantages of existing power MOSFET. Although the latest generation of power MOSFET devices have greatly improved the RDS (on) characteristics, the power on loss is still much higher than that of IGBT technology at high levels. The lower voltage drop, the ability to convert to a low VCE (SAT), and the structure of IGBT can support higher current density and simplify the schematic diagram of IGBT Driver compared with a standard bipolar device.
Conduction
The structure of IGBT silicon wafer is very similar to that of power MOSFET. The main difference is that IGBT adds P + substrate and an N + buffer layer (NPT non through IGBT technology does not add this part). As shown in the equivalent circuit diagram (Fig. 1), one MOSFET 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 inverts the p-base region below the gate, an n-channel is formed, an electron flow appears at the same time, and a current is generated completely in the way of power MOSFET. If the voltage generated by this electron flow is in the range of 0.7V, J1 will be in the forward bias, some holes will be injected into the N-region, and the resistivity between the anode and cathode will be adjusted. This method reduces the total loss of power conduction and starts the second charge flow. The final result is that two different current topologies temporarily appear in the semiconductor level: one electron flow (MOSFET current); Hole current (bipolar).
Turn off
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 in the switching phase, the collector current decreases gradually, because there are still a few carriers (minority carriers) in the N layer after the commutation begins. The reduction of residual current value (wake) depends entirely on the charge density when turning off, and the density is related to several factors, such as the number and topology of impurities, layer thickness and temperature. The attenuation of minority carriers makes the collector current have characteristic wake waveform. The collector current causes the following problems: the power consumption increases; The cross conduction problem is more obvious, especially in the equipment using 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, the hole mobility closely related to IC and VCE. Therefore, it is feasible to reduce this undesirable effect of current acting on the design of terminal equipment according to the reached temperature.
Blocking and latching
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. Due to too much reduction of the thickness of this layer, an effective blocking ability will not be obtained. Therefore, this mechanism is very important. On the other hand, if the area size is increased too much, the pressure drop will be continuously increased. The second point clearly explains why the pressure 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.
IGBT has a parasitic pnpn thyristor between collector and emitter, as shown in Figure 1. Under special conditions, this parasitic device will turn on. This phenomenon will increase the current between collector and emitter, reduce the control ability of equivalent MOSFET, and usually cause device breakdown. The thyristor conduction phenomenon is called IGBT latch. Specifically, the causes of this defect are different from each other and closely related to the state of the device. In general, static and dynamic latches have the following main differences:
When the thyristors are all turned on, the static latch appears. Dynamic latches occur only when turned off. This special phenomenon seriously limits the safe operation area. In order to prevent the harmful phenomenon of parasitic NPN and PNP transistors, it is necessary to take the following measures: prevent the NPN from being partially turned on, and change the layout and doping level respectively. Reduce the total current gain of NPN and PNP transistors. In addition, the latch current has a certain influence on the current gain of PNP and NPN devices, so it is also closely related to the junction temperature; With the increase of junction temperature and gain, the resistivity of p-base region will increase, which destroys the overall characteristics. Therefore, device manufacturers must pay attention to maintaining a certain ratio between the maximum collector current and the latch current, usually 1:5.
Development history in 1979, MOS gate power switching device, as the pioneer of IGBT concept, has been introduced to the world. The device is a thyristor like structure (composed of four layers p-n-p-n), which is characterized by the formation of V-shaped slot grid by strong alkali wet etching process.
In the early 1980s, DMOS (metal oxide semiconductor formed by double diffusion) process for power MOSFET manufacturing technology was adopted in IGBT. [2] At that time, the structure of silicon chip was a thicker NPT (non through) design. Later, a significant improvement in parameter compromise was obtained by using Pt (through) structure, which was developed with the technological progress of epitaxy on silicon wafer and the use of N + buffer layer designed corresponding to a given blocking voltage [3]. In recent years, the design rules of DMOS planar gate structure prepared on epitaxial wafer designed by PT have advanced from 5 microns to 3 microns.
In the mid-1990s, the grooved gate structure returned to a new concept of IGBT. It is a new etching process realized by silicon dry etching technology borrowed from large-scale integration (LSI) process, but it is still a through (PT) chip structure. [4] In this trench structure, a more important improvement in the tradeoff between on-state voltage and off time is realized.
The heavy straight structure of silicon chip has also been dramatically changed. First, the non through (NPT) structure is adopted, and then the weak through (LPT) structure is changed, which makes the safe working area (SOA) similar to the evolution of surface gate structure.
This is the most basic and significant conceptual change from through (PT) technology to non through (NPT) technology. This is: the through (PT) technology will have a relatively high carrier injection coefficient, and its transport efficiency will deteriorate because it requires to control the life of a few carriers. On the other hand, non through (NPT) technology has good transport efficiency based on not killing minority carrier lifetime, but its carrier injection coefficient is relatively low. Furthermore, the non through (NPT) technology is replaced by the soft through (LPT) technology, which is similar to the so-called "soft through" (SPT) or "electric field cut-off" (FS) technology, which further improves the comprehensive effect of "cost performance".
In 1996, cstbt (carrier stored grooved gate bipolar transistor) realized the 5th generation IGBT module [6]. It adopts weak through (LPT) chip structure and more advanced design with wide cell spacing
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