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Introduction
2013.01.26
SD Lab. SOGANG Univ.
Gil Yong Song
SOGANG
UNIVERSITY
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UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
Contents
1. Introduction
ideal and typical power device characteristics
Unipolar power devices
Bipolar power devices
MOS-Bipolar power devices
Ideal drift region for unipolar power device
2. Material Properties and Technology
Fundamental properties
Other properties relevant to power devices
Fabrication Technology
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Introduction
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Introduction
1950’s : The industry relied upon silicon bipolar devices,
such as bipolar power transistors and thyristors.
1970’s : The advent of MOS technology for digital electronics enabled the
creation of a new class of devices in the 1970s for power switching applications.
1980’s : The merger of MOS and bipolar physics enabled creation of yet another
class of devices (IGBT).
- Power devices are required for systems that
operate over a broad spectrum of power
levels and frequencies.
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Introduction
Thyristor
- Thyristor : low frequency.
MOSFET
IGBT : medium frequency.
MOSFET : high frequency.
IGBT
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Ideal and Typical Power Device Characteristics
• Ideal case
- zero on-state voltage drop.
- zero leakage current.
- zero switching time.
• Practical case
- finite voltage drop when carrying current on the
on-state leading to ‘conduction’ power loss.
- finite leakage current when blocking voltage in the
off-state creating power loss.
- Doping concentration and thickness of drift region control the
breakdown voltage.
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Ideal and Typical Power Device Characteristics
uniform
• Ideal transistor conducts
- on-state with zero voltage drop
- off-state with zero leakage current
- active region controlled by gate bias
• Practical transistor conducts
- finite resistance
- finite leakage current while operating in the off-state
- finite breakdown voltage
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non-uniform
Unipolar Power Devices
• Bipolar power devices
- Power loss problem that degrade the power
management efficiency. -> use unipolar power diode!
• Schottky rectifier (example of unipolar power diode)
- utilize a metal-semiconductor barrier to produce
current rectification.
- drift region : to support the reverse blocking voltage.
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Unipolar Power Devices
- The most commonly used unipolar power transistor is MOSFET.
• D-MOSFET
- planar-gate structure
- p-base, N+ source region
- the voltage blocking capability is determined by the doping and thickness
of drift region.
• U-MOSFET
- It has a gate structure embedded within a trench etched into the Si surface.
- The N-type channel is formed on the sidewall of the trench.
- The Si U-MOSFET structure was developed to reduce the on-state resistance.
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Bipolar Power Devices
- Widely used for power switching until 1970s
- Injection of minority carriers into the drift region modulates
its conductivity reducing the on-state voltage drop.
→ this charge must be removed during switching resulting in turn-off
power losses.
- When using SiC, large junction potential for SiC results in a relatively
high on-state voltage drop when compared with Si devices.
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MOS-Bipolar Power Devices
- IGBT(insulated gate bipolar transistor) :
the most widely used Si high voltage device for power switching
- The structure is similar to that for the power MOSFET by combining
the physics of bipolar and MOSFET.
- four-layer parasitic thyristor
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Ideal Drift Region for Unipolar Power Devices
- Drift region is designed to support the blocking voltage.
- Assumption : abrupt junction
- Specific resistance of the ideal drift region is given by
- The depletion width under breakdown conditions is given by,
- The doping concentration in the drift region is given by,
- Combining by these relationships,
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Ideal Drift Region for Unipolar Power Devices
- The improvement in drift region resistance for GaAs
in comparison with silicon is largely due to its much greater
mobility for electrons.
- The improvement in drift region resistance for SiC
in comparison with silicon is largely due to its much larger
critical field for breakdown.
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Materials Properties
and Technology
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Fundamental Properties
- Only the properties for 4H-SiC have been included here
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Energy Band Gap
• Energy Bang Gap
Si : 1.12eV, 4H-SiC : 3.26eV (3 times larger than Si)
- Large band gap → small generation in the depletion regions
→ small leakage current
- Intrinsic carrier concentration
- For silicon,
- For 4H-SiC
Room temperature
- The intrinsic carrier concentration can be calculated as
a function of temperature.
- The bulk generation current is negligible for the determination
of the leakage current in SiC.
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Energy Band Gap
- Built in potential
- For Si, ionized impurity concentrations are equal to the doping
concentration because of the small dopant ionization energy levels.
But, this doesn’t apply to SiC because of the much larger
dopant ionization levels.
- (fig 2.4) The built in potential for SiC is much larger than that for Si
due to the far smaller values for the intrinsic carrier concentration.
- (fig 2.5) The zero bias depletion width for SiC is much larger than
for Si for the same doping concentration.
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Impact Ionization Coefficients
- The main advantage of wide band gap semiconductor :
reduced impact ionization coefficients
- 𝛼 of Si : electron>hole, 𝛼 of SiC : electron<hole.
- a and b are constants that depends upon the material and
the temperature
- 4H-SiC :
- 6H-SiC :
- Critical field : 4H-SiC > Si
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Electron mobility
- 𝜇𝑛 > 𝜇𝑝 . → N-type drift region is favorable rather than P-type.
- Conductivity :
doping concentration 𝑁𝐷
- Doping concentration dependence of mobility :
- Fig (2.7) : impurity scattering
- Temperature dependence of mobility :
- Fig (2.8) : lattice scattering
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Donor and Acceptor Ionization Energy levels
- In Si, the donor and acceptors ionization energy is small (50mV).
- SiC have a large ionization energies for donors and acceptors.
- The most widely used Donor of 4H-SiC : nitrogen (100meV)
- Acceptor of 4H-SiC : aluminum (200meV)
Recombination Lifetimes
- The recombination process is band-to-band but is usually assisted by the deep levels.
- Deep levels can produce leakage current by the generation of carriers in the depletion region.
- SiC (unipolar device) has a low lifetime and diffusion length → fast switching speed
- but bipolar devices have a higher bulk lifetime → slow switching speed
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Metal-Semiconductor Contacts
- Ohmic contact : metal-semiconductor contacts with low barrier height
and high doping level in semiconductor (tunneling process)
- The contact resistance determined by the tunneling process is
dependent upon the barrier height and doping level
- The barrier heights of metal-SiC contact are large due to large band gap.
- Schottky barrier rectifier : Titanium, Nikel
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Fabrication Technology
- SiC can be fabricated using the same equipment used for Si.
However, high temperature are needed for the annealing.
• Diffusion Coefficients and Solubility of Dopants
- In SiC, the diffusion coefficients are very small even at relatively high temperatures.
n-type nitrogen : 5x10−12 𝑐𝑚2 /𝑠 at 2450℃, p-type aluminum : 3x10−14 𝑐𝑚2 /𝑠 to 5x1012 𝑐𝑚2 /𝑠 at 1800-2000℃
Due to the on-set of dissociation at these temperatures with attendant generation of defects,
it is no practical to drive dopants in SiC.
- The solid solubility of dopants in SiC is similar to that of dopants in Si. (up to 1x1020 )
• Ion Implantation and Annealing
- annealing : to activate the dopants and remove lattice damage.
- hot-implantation produces a higher degree of impurity activation and a smaller number of defects.(500℃)
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Fabrication Technology
• Gate oxide formation
- advantage of MOS-gated device : voltage controlled operation.
- gate dielectric issue :
→ quality of the oxide-semiconductor interface determines not
only channel mobility but impacts the threshold voltage.
→ ability of the oxide to withstand higher electric fields than Si?
maximum field of Si : 3x105 V/cm
maximum field of SiC : 3x106 V/cm
the electric field in the oxide can exceed a field of 3x106
solution : use gate dielectric material with large permittivity!
- thermal oxidation : growth rate of SiC is much lower than Si.
much higher temperature and longer time is required for the growth of oxides on SiC.
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