The Structure and Working Principle of IGBT
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The Structure and Working Principle of IGBT

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The Structure and Working Principle of IGBT

Figure 1 shows an N-channel enhancement type insulated gate bipolar transistor structure, the N+ region is called the source region, and the electrode attached to it is called the source. The N+ region is called the drain region. The control region of the device is the gate region, and the electrode attached to it is called the gate. A channel is formed in close proximity to the gate region boundary. The P-type region between the drain and source (including the P+ and P- regions, where the channel is formed) is called the sub-channel region.



The P+ region on the other side of the drain region is called the drain injection region, which is a unique functional region of the IGBT. Together with the drain region and the sub-channel region, it forms a PNP bipolar transistor, which acts as an emitter and injects air into the drain. hole, conduct conduction modulation to reduce the on-state voltage of the device. The electrode attached to the drain implanted region is called the drain.

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The switching function of the IGBT is to form a channel by adding a forward gate voltage, provide a base current to the PNP transistor, and make the IGBT turn on. On the contrary, the reverse gate voltage is applied to eliminate the channel, and the reverse base current flows to turn off the IGBT.



The driving method of IGBT is basically the same as that of MOSFET. It only needs to control the input pole 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 into the N layer, and the conductance of the N layer is modulated to reduce the resistance of the N layer, so that the IGBT also has a low voltage at high voltage. on-state voltage.


IGBT operating characteristics


1. Static characteristics

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 curve between the drain current and the 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. It can also be divided into saturation region 1, amplification region 2 and breakdown characteristics 3 parts. In the off-state 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 voltages can reach the same level. After adding the N+ buffer, the reverse turn-off voltage can only reach the level of several tens of volts, thus limiting some applications of the IGBT.



·The transfer characteristic of IGBT refers to the relationship curve between the output drain current Id and the gate-source voltage Ugs. It is the same as the transfer characteristic of MOSFET, when the gate-source voltage is less than the turn-on voltage Ugs(th), the IGBT is in the off state. Id has a linear relationship with Ugs over most of the drain current range after the IGBT is turned on. The maximum gate-source voltage is limited by the maximum drain current, and its optimum value is generally about 15V.



·The switching characteristics of an IGBT refer to the relationship between the drain current and the 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 a major part of the total IGBT current.


At this time, the on-state voltage Uds(on) can be expressed by the following formula:

Uds(on) = Uj1 + Udr + IdRoh

In the formula, Uj1 - the forward voltage of the JI junction, its value is 0.7 ~ 1V; Udr - the voltage drop on the expansion resistor Rdr; Roh - the channel resistance.

The on-state current Ids can be expressed as:

Ids=(1+Bpnp)Imos

Where Imos - 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, and the on-state voltage drop of the IGBT with a withstand voltage of 1000V is 2-3V. When the IGBT is in the off state, only a small leakage current exists.


2. Dynamic characteristics


During the turn-on process of the IGBT, it operates as a MOSFET most of the time, but at the later stage of the decline of the drain-source voltage Uds, the PNP transistor goes from the amplifying region to saturation, adding a delay time. td(on) is the turn-on delay time, and tri is the current rise time. The drain current turn-on time ton often given in practical applications is the sum of td(on) and tri. The fall time of the drain-source voltage consists of tfe1 and tfe2.



Triggering and turn-off of IGBT requires adding forward voltage and negative voltage between its gate and base, and the gate voltage can be generated by different driving circuits. When selecting these driver circuits, it must be based on the following parameters: device turn-off bias requirements, gate charge requirements, robustness requirements, and power supply conditions.



Because of the high gate-emitter impedance of the IGBT, MOSFET drive techniques can be used for triggering, but 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.



The switching speed of IGBT is lower than that of MOSFET, but significantly higher than that of GTR. The IGBT does not require a negative gate voltage during turn-off to reduce the turn-off time, but the turn-off time increases with the gate and emitter parallel resistances. The turn-on voltage of IGBT is about 3 to 4V, which is equivalent to that of MOSFET. When the IGBT is turned on, the saturation voltage drop 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 official commercial high-voltage and high-current IGBT device has not yet appeared, and its voltage and current capacity are still very limited, which is far from meeting the needs of the development of power electronic application technology, especially in many applications in the high-voltage field, the voltage level of the device is required to reach 10KV above. At present, high-voltage applications can only be realized through technologies such as IGBT high-voltage series connection.



Some foreign manufacturers, such as ABB in Switzerland, 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 Japan's Toshiba has also set foot in this field. At the same time, major semiconductor manufacturers continue to develop high-voltage, high-current, high-speed, low-saturated 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 working principle of IGBT


The N-channel IGBT works by applying a (positive) voltage above the threshold voltage VTH between the gate and the emitter to form an inversion layer (channel) on the p-layer directly under the gate electrode, starting from the emitter electrode. The lower n-layer injects electrons. The electrons are minority carriers of p+n-p transistors, and they flow into holes from the p+ layer of the collector substrate to conduct conductivity modulation (bipolar operation), so the collector-emitter saturation voltage can be reduced.



An n+pn — parasitic transistor is formed on the emitter electrode side. If n+pn — parasitic transistor works, it becomes p+n — pn+ thyristor again. Current continues to flow until the output side stops supplying current. Control via output signal is no longer possible. This state is generally referred to as a latched state.



In order to suppress the working IGBT of the n+pn-parasitic transistor, the current amplification factor α of the p+n-p transistor is reduced as much as possible as a measure to solve the blocking. Specifically, the current amplification factor α of p+n-p is designed to be 0.5 or less. The blocking current IL of the IGBT is more than three times the rated current (DC). The driving principle of IGBT is basically the same as that of power MOSFET, and the on-off is determined by the gate-emitter voltage uGE.


1. Turn on


The structure of IGBT silicon is very similar to that of power MOSFET. The main difference is that IGBT adds a P+ substrate and an N+ buffer layer (NPT-non-punch-IGBT technology does not add this part), and 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 causes the inversion of the P base region under the gate, an N-channel is formed, and a flow of electrons occurs at the same time, and a current is generated exactly in the manner of a power MOSFET.



If the voltage produced by this electron flow is in the range of 0.7V, then J1 will be forward biased, some holes will be injected into the N-region, and the resistivity between the cathode and anode will be adjusted, which reduces the power conduction total loss and initiates a second charge flow.



The end result is that two different current topologies emerge temporarily within the semiconductor layer: an electron flow (MOSFET current), a hole current (bipolar). When uGE is greater than the turn-on voltage UGE(th), a channel is formed in the MOSFET to provide base current for the transistor and the IGBT is turned on.



2. Turn-on voltage drop


The conductance modulation effect reduces the resistance RN and makes the on-state voltage drop small.



3. Shutdown


When a negative bias is applied to the gate or the gate voltage is lower than the threshold value, the channel is disabled and no holes are injected into the N-region. In any case, if the MOSFET current decreases rapidly during the switching phase, the collector current decreases gradually because there are still minority carriers (minority carriers) in the N-layer after commutation begins.


This reduction in residual current value (wake) is entirely dependent on the charge density at turn-off, which in turn is related to several factors, such as the number and topology of dopants, layer thickness and temperature. The attenuation of minority carriers makes the collector current have a characteristic wake waveform, and the collector current causes the following problems: increased power consumption; cross-conduction problems, especially on devices using freewheeling diodes, the problem is more obvious.


Considering that the wake is related to the recombination of minority carriers, the current value of the wake should be closely related to the hole mobility which is closely related to the temperature of the chip, IC and VCE. Therefore, depending on the temperature reached, it is feasible to reduce the undesired effect of this current on the end equipment design, and the wake characteristics are related to VCE, IC and TC.


When a back pressure is applied between the gate and the emitter or no signal is applied, the channel in the MOSFET disappears, the base current of the transistor is cut off, and the IGBT is turned off.



4. Reverse blocking


When a reverse voltage is applied to the collector, J1 is controlled by the reverse bias, and the depletion layer expands to the N-region. Because the thickness of this layer is reduced too much, an effective blocking ability will not be obtained, so this mechanism is very important. On the other hand, if you increase the size of this area too much, you will continuously increase the pressure drop.



5. Forward blocking


When the gate and emitter are shorted and a positive voltage is applied at the collector terminal, the P/NJ3 junction is controlled by the reverse voltage. At this time, the depletion layer in the N drift region is still subjected to the externally applied voltage.



6. Latch


The IGBT has a parasitic PNPN thyristor between the collector and the emitter, and under special conditions, this parasitic device turns on. This phenomenon increases the amount of current between the collector and the emitter, reduces the controllability of the equivalent MOSFET, and often causes device breakdown problems.


The thyristor turn-on phenomenon is called IGBT latch-up, and specifically, the causes of this defect are different from each other and are closely related to the state of the device. In general, the main difference between static and dynamic latch-up is as follows: Static latch-up occurs when the thyristors are all on. Dynamic latch-up occurs only at shutdown. This particular 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:

· One is to prevent the NPN part from being turned on, changing the layout and doping level, respectively.

· The second is to reduce the total current gain of the NPN and PNP transistors.



In addition, the latch-up current has a certain effect on the current gain of PNP and NPN devices, so it is also very closely related to the junction temperature; when the junction temperature and gain are increased, the resistivity of the P base region will increase, destroying overall characteristics. Therefore, device manufacturers must take care to maintain a certain ratio between the maximum collector current value and the latch-up current, typically a ratio of 1:5.


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