GTO Thyristors, Power MOSFETs, and IGBTs: Features and Uses
GTO (Gate Turn-Off Thyristor)
- Conventional thyristors (CTs) are nearly ideal switches for use in power electronic applications.
- These can be easily turned on by a positive gate current.
Once in the on state, the gate loses control.
- CTs can now be turned off by expensive and bulky commutation circuitry.
- This shortcoming of thyristors limits their use up to about 1KHz applications.
- These drawbacks in thyristors have led to the development of GTOs.
- GTO is a more versatile power semiconductor device.
- Like a CT but with added features.
- Can be easily turned off by a negative gate pulse of appropriate amplitude.
- GTO is a PNPN device that can be turned on by a positive gate current and turned off by a negative gate current at its gate-cathode terminal.
- The self-turn-off capability of GTO makes it the most suitable device for inverter and chopper circuits.
Comparison Between GTO and Thyristor
Disadvantages
- The magnitude of latching and holding current is more.
- On-state voltage drop and associated loss are more.
- The triggering gate current required is high.
- Gate drive circuit losses are more.
- Reverse voltage blocking capability is less than forward voltage blocking capability.
Advantages
- Faster switching speed.
- Surge current capability is comparable with an SCR.
- More di/dt rating at turn-on.
- GTO circuit configuration has low size and weight.
- Higher efficiency.
- Reduced acoustical and electromagnetic noise due to the elimination of commutation chokes.
Power MOSFET
Power MOSFET: Output Characteristics
Switching Characteristics
Power MOSFET Comparison with BJT
BJT
- Bipolar
- Input impedance low
- Higher switching losses
- Current controlled device
- Negative temperature coefficient
- Hot spots and secondary breakdown occur in BJT
- Available with a rating up to 1200 V, 800 A
MOSFET
- Unipolar device
- High input impedance
- Lower switching losses and conduction losses
- Voltage-controlled device
- Positive temperature coefficient
- Secondary breakdown does not occur
- High voltage rating, more conduction loss
- 500 V, 140 A
IGBT
This reverse voltage blocking capability is useful in some AC circuit applications.
If the thickness of the drift region is reduced, the depletion layer may touch P+. To avoid that, we keep a buffer layer, an N+ layer.
This type of structure is called an anti-symmetric or punch-through IGBT.
A shorter drift region means lower on-state losses.
The presence of a buffer layer makes the reverse voltage capability quite low.
On-State Operation
When the gate-emitter voltage increases to more than the threshold value, an inversion layer is formed beneath the gate of the IGBT.
This inversion layer shorts the N- drift region to the N+ source region, exactly as in the MOSFET.
An electron current flows through this inversion layer, which in turn causes substantial hole injection from the P+ drain contact layer into the N- drift region, as shown in the figure.
The injected holes move across the drift region by both drift and diffusion, taking a variety of paths, and reach the P-type body region that surrounds the N+ source region.
V-I Characteristics
- Output characteristics are the plot of IC versus VCE.
- Transfer characteristics are the plot of IC versus VGE.
Switching Characteristics
Comparison of IGBT with MOSFET
IGBT
- Three terminals called gate, emitter, and collector.
- High input impedance.
- Voltage-controlled device.
- Can be designed for higher voltage ratings than PMOSFET.
MOSFET
- Three terminals called gate, source, and drain.
- High input impedance.
- Voltage-controlled device.
- On-state voltage drop and losses rise more rapidly than IGBT with a rise in temperature.