SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Introduction 2013.01.26 SD Lab. SOGANG Univ. Gil Yong Song

SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.

Introduction

2013.01.26SD Lab. SOGANG Univ.

Gil Yong Song


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


Introduction


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.


Introduction

MOSFET

IGBT

Thyristor

- Thyristor : low frequency.

IGBT : medium frequency.

MOSFET : high frequency.


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.


Ideal and Typical Power Device Characteristics

• 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

uniform

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.


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.


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.


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


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,


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.


Materials Properties and Technology


Fundamental Properties

- Only the properties for 4H-SiC have been included here


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

- 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.

Room temperature


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.


Impact Ionization Coefficients

- The main advantage of wide band gap semiconductor :

reduced impact ionization coefficients

- of Si : electron>hole, 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


Electron mobility

- . → N-type drift region is favorable rather than P-type.

- Conductivity : doping concentration

- Doping concentration dependence of mobility :

- Fig (2.7) : impurity scattering

- Temperature dependence of mobility :

- Fig (2.8) : lattice scattering


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


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



- 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 : 5x at 2450, p-type aluminum : 3x to 5x 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 1x)

• 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)



• 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 : 3x V/cm

maximum field of SiC : 3x V/cm

the electric field in the oxide can exceed a field of 3x

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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SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Introduction 2013.01.26 SD Lab. SOGANG Univ. Gil Yong Song