GAN HEMT
ON
SI
CONTENTS

What’s HEMT?

2DEG

Polarization

Contacts

Summary
1. WHAT’S HEMT?
1. HEMT: High Electron Mobility Transistor
2. GaN HEMT
- High electron mobility with heterojunction structure
(Prevent Coulomb scattering with no doping materials)
- High thermal stability
- High Breakdown voltage
3. Vertical Structure
- Shown electron path on the right figure.
- Source/Drain → Ohmic contact,
Gate → Schottky contact: control current
- Formed 2DEG from AlGaN/GaN heterostructure
- High intrinsic carrier density from polarization
effect(Spontaneous + Piezoelectric)
2. 2DEG
1. Principles
AlGaN
GaN
- Discontinuity through the conduction
band of the two semiconductors
determines a charge transfer, creating a
triangular potential.
- Electrons are confined in the triangular
potential in discrete quantum state.
- Mobility of the electrons in 2DEG is
higher than in a bulk.
2. Advantage
- Coulomb scattering was occurred in
doped material because probability of
interruption increased by different atomic
size. → decreased electron mobility
- Carrier mobility of hetero-structure is
higher than doped semiconductor.
- It is possible high frequency devices.
3. POLARIZATION
1. Spontaneous Polarization
Remained
Canceled
Canceled
2. Piezoelectric Polarization
- Non-symmetric structure.
- Remained charge caused
spontaneous polarization.
3. POLARIZATION
2. Piezoelectric Polarization
- When AlGaN growed on the GaN, tensile strain was occurred due to difference of lattice
constant for AlGaN.(AlGaN a=3.112Å, GaN a=3.189Å)
- Strain applied not only x,y axis but z axis. Most of x and y component was canceled, so
only z component influenced on polarization.
3. Total Polarization
- AlGaN/GaN HEMTs transistor don’t need
doping to obtain a high electron density.
- Spontaneous + Piezoelectric polarization
= about 1013 (cm2/Vs) carrier concentration
4. CONTACTS
1. Schottky contact
- General Metal-Semiconductor contact.
- Work fuction of metal is higher than semiconductor. If we contact metal and semiconductor,
energy band is made just like below right figure.
- Forward bias: semiconductor → metal electron diffused,
metal → semiconductor could not diffuse because of energy barrier: Rectifying
contact
2. Ohmic contact
- High doping or selected metal
(Work function of metal is smaller than S.C.)
- Due to tunneling effect, electron could move
free Metal ↔ Semiconductor.
4. CONTACTS
3. Metal – Semiconductor contact of HEMT
- Source / Drain : Ohmic contact
Carrier could move free Metal ↔ Semiconductor.
- Gate : Schottky contact, controlled transistor to turn on / off.
4. Transistor Turn on / off
Applied Voltage
- Applied Gate → (-) voltage, Substrate → (+) voltage
- In GaN area, it influence same as forward bias of p-n junction.
- It lower energy barrier, electrons were diffused to GaN and depleted.
→ No electrons on the channel → Transistor turn off.
5. SUMMARY
1. Total Band Diagram
 HEMT transistor are widely used in electronic application
 AlGaN/GaN structure looks promising because of High power, High efficiency
 Still in research