01

Structure of IGBT

 

Figure 1 shows the structure of an N-channel enhancement-mode insulated gate bipolar transistor. The N+ region is called the source region, and the electrode attached to it is called the source electrode. 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 electrode. The channel is formed close to the boundary of the gate region. The P-type region (including P+ and P- regions) between the drain and source (where the channel is formed) is called the subchannel region. The P+ region on the other side of the drain region is called the drain injector, which is a unique functional area of the IGBT. Together with the drain region and subchannel region, it forms a PNP bipolar transistor, acting as the emitter to inject holes into the drain, modulating conductivity to reduce the device's on-state voltage. The electrode attached to the drain injector is called the drain electrode.

 

 

The switching function of the IGBT is achieved by applying a positive gate voltage to form a channel, providing base current to the PNP transistor, allowing the IGBT to conduct. Conversely, applying a negative gate voltage eliminates the channel, causing reverse base current to flow, turning off the IGBT. The driving method of the IGBT is fundamentally the same as that of the MOSFET, only requiring control of the input N-channel MOSFET, thus exhibiting high input impedance characteristics. Once the MOSFET channel is formed, holes (minority carriers) are injected from the P+ base into the N-layer, modulating the conductivity of the N-layer, reducing its resistance, allowing the IGBT to maintain a low on-state voltage even at high voltages.

 

02

Working Characteristics of IGBT

 

1. Static Characteristics

 

The static characteristics of the IGBT mainly include volt-ampere characteristics, transfer characteristics, and switching characteristics.

 

The volt-ampere characteristics of the IGBT refer to the relationship curve between the drain current and the gate-source voltage Ugs when Ugs is used as a variable. The output drain current is controlled by the gate-source voltage Ugs; the higher the Ugs, the larger the Id. It is similar to the output characteristics of a GTR and can also be divided into three parts: saturation region 1, amplification region 2, and breakdown characteristics 3. In the cutoff state of the 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 region, the forward and reverse blocking voltages can be at the same level. After adding the N+ buffer region, the reverse blocking voltage can only reach a few tens of volts, thus limiting certain application ranges of the IGBT.

 

The transfer characteristics of the IGBT refer to the relationship curve between the output drain current Id and the gate-source voltage Ugs. It is the same as the transfer characteristics of the 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 is linearly related to Ugs. The maximum gate-source voltage is limited by the maximum drain current, and its optimal value is generally around 15V.

 

The switching characteristics of the IGBT refer to the relationship between the drain current and the drain-source voltage. When the IGBT is in the on state, due to its PNP transistor being a wide-base transistor, its B value is very low. 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 as:

Uds(on) = Uj1 + Udr + IdRoh

 

where Uj1 is the forward voltage of the JI junction, with a value of 0.7 to 1V;

Udr is the voltage drop across the extended resistance Rdr;

Roh is the channel resistance.

 

The on-state current Ids can be expressed as:

Ids = (1 + Bpnp)Imos

 

where Imos is the current flowing through the MOSFET.

 

Due to the existence of the N+ region's conductivity modulation effect, the on-state voltage drop of the IGBT is small. The on-state voltage drop of an IGBT rated for 1000V is 2 to 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 for most of the time, only increasing a delay time when the drain-source voltage Uds decreases in the later stage, transitioning from the amplification region to saturation. 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 often given as the sum of td(on) and tri. The drain-source voltage decrease time consists of tfe1 and tfe2.

 

The triggering and turn-off requirements of the IGBT necessitate applying positive and negative voltages between its gate and base. The gate voltage can be generated by different driving circuits. When selecting these driving circuits, the following parameters must be considered: the requirements for device turn-off bias, gate charge requirements, robustness 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 driving circuits.

 

The switching speed of the IGBT is lower than that of the MOSFET but significantly higher than that of the GTR. The IGBT does not require a negative gate voltage to reduce the turn-off time, but the turn-off time increases with the increase of the parallel resistance between the gate and emitter. The turn-on voltage of the IGBT is about 3 to 4V, comparable to that of the MOSFET. The saturation voltage drop when the IGBT is on is lower than that of the MOSFET and close to that of the GTR, and the saturation voltage drop decreases with the increase of gate voltage.

 

As of now, commercially available high-voltage, high-current IGBT devices have not yet emerged, and their voltage and current capacities remain quite limited, far from meeting the demands of power electronics technology development, particularly in many high-voltage applications where the voltage rating of the devices is required to exceed 10KV. Currently, high-voltage applications can only be realized through technologies such as high-voltage series connection of IGBTs. Some foreign manufacturers, such as Switzerland's ABB, have developed 8KV IGBT devices using the soft turn-on principle, while Germany's EUPEC has produced 6500V/600A high-power IGBT devices that have been practically applied. Japan's Toshiba has also ventured into this field. Meanwhile, major semiconductor manufacturers are continuously developing IGBTs with high voltage resistance, high current, high speed, low saturation voltage drop, high reliability, and low-cost technologies, primarily utilizing processes below 1μm, achieving some new advancements.

 

03

Working Principle of IGBT

 

The N-channel IGBT operates by applying a threshold voltage VTH above the (positive) voltage between the gate and emitter, forming an inversion layer (channel) on the p-layer directly below the gate electrode, which begins to inject electrons from the n-layer under the emitter electrode. These electrons are minority carriers in the p+n-p transistor, flowing from the collector substrate p+ layer into holes, modulating conductivity (bipolar operation), thus reducing the saturation voltage between the collector and emitter. An n+pn-parasitic transistor is formed on the emitter electrode side. If the n+pn-parasitic transistor operates, it transforms into a p+n-pn+ thyristor. The current continues to flow until the output side stops supplying current. Control is no longer possible through the output signal. This state is generally referred to as the latch-up state.

 

To suppress the operation of the n+pn-parasitic transistor, the IGBT employs a strategy to minimize the current gain coefficient α of the p+n-p transistor as a solution to latch-up. Specifically, the current gain coefficient α of the p+n-p is designed to be below 0.5. The latch-up current IL of the IGBT is more than three times the rated current (DC). The driving principle of the IGBT is fundamentally the same as that of the power MOSFET, with the on/off state determined by the gate-emitter voltage uGE.

 

(1) On

 

The structure of the IGBT silicon wafer is very similar to that of the power MOSFET, with the main difference being that the IGBT adds a P+ substrate and an N+ buffer layer (NPT - non-punch-through - IGBT technology does not add this part), where one MOSFET drives two bipolar devices. The application of the substrate creates a J1 junction between the P+ and N+ regions of the device. When a positive gate bias causes inversion in the P base region below the gate, an N-channel is formed, and an electron flow occurs, generating a current entirely in the manner of a power MOSFET. If the voltage generated by this electron flow is within the range of 0.7V, then J1 will be under forward bias, injecting some holes into the N- region and adjusting the resistivity between the anode and cathode, which reduces the total loss during power conduction and initiates a second charge flow. The final result is that two different current topologies temporarily appear at the semiconductor level: one electron flow (MOSFET current); hole current (bipolar). When uGE is greater than the turn-on voltage UGE(th), a channel is formed in the MOSFET, providing base current to the transistor, and the IGBT turns on.

 

(2) On-state voltage drop

 

The conductivity modulation effect reduces the resistance RN, resulting in a smaller on-state voltage drop.

 

(3) Turn-off

 

When a negative bias is applied to the gate or the gate voltage is below the threshold value, the channel is blocked, and no holes are injected into the N- region. In any case, if the MOSFET current rapidly decreases during the switching phase, the collector current gradually decreases, because after the commutation begins, there are still minority carriers (minority) present in the N layer. The reduction of this residual current value (tail current) entirely depends on the charge density at turn-off, which is related to several factors such as the amount of dopants, topology, layer thickness, and temperature. The decay of minority carriers gives the collector current a characteristic tail current waveform, which leads to the following issues: increased power consumption; cross-conduction problems, especially in devices using freewheeling diodes, where the problem is more pronounced.

 

Given that the tail current is related to the recombination of minority carriers, the current value of the tail current should be closely related to the temperature of the chip, IC, and the hole mobility related to VCE. Therefore, it is feasible to reduce the undesirable effects of this action on the current in terminal device design based on the achieved temperature, with tail current characteristics related to VCE, IC, and TC.

When reverse voltage is applied between the gate and emitter or no signal is applied, the channel in the MOSFET disappears, the base current of the transistor is cut off, and the IGBT turns off.

 

(4) Reverse blocking

 

When a reverse voltage is applied to the collector, J1 will be controlled by reverse bias, and the depletion layer will extend into the N- region. If the thickness of this layer is excessively reduced, an effective blocking capability cannot be achieved, so this mechanism is very important. On the other hand, if the size of this region is excessively increased, the voltage drop will continuously increase.

 

(5) Forward blocking

 

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

 

(6) Latch-up

 

The IGBT has a parasitic PNPN thyristor between the collector and emitter. Under special conditions, this parasitic device will turn on. This phenomenon increases the current between the collector and emitter, reducing the control capability of the equivalent MOSFET, and usually also causes device breakdown issues. The thyristor turn-on phenomenon is referred to as IGBT latch-up, specifically, the causes of this defect vary and are closely related to the state of the device. Generally, static and dynamic latch-up have the following main differences: static latch-up occurs when the thyristor is fully on. Dynamic latch-up occurs only during turn-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: first, prevent the NPN part from turning on by changing the layout and doping levels. Second, reduce the total current gain of the NPN and PNP transistors.

 

In addition, latch-up current has a certain impact on the current gain of PNP and NPN devices, so it is also closely related to junction temperature; under conditions of increased junction temperature and gain, the resistivity of the P base region will increase, damaging the overall characteristics. Therefore, device manufacturers must pay attention to maintaining a certain ratio between the maximum collector current value and the latch-up current, usually a ratio of 1:5.

 

 

 

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