This PPT describe the Transport Mechanism in Thin Oxides of MOS Devices.

1. Charge transport or current conduction in MIS devices
(Semester – IVth )
Course code: MPHYT-D404
Type : Core course, Credit : 3
Dr. Arvind Kumar
Assistant Professor
Department of Physics
Indira Gandhi National Tribal University
Amarkantak (MP) - 484887, INDIA
5/2/2020 1

2.  An ideal gate insulator does not conduct any current, but for real
insulators there can be some leakage current which varies with the
voltage or electric field across the gate oxide.
 By looking at the band diagram of the MOS system, we see that for
electrons in the conduction band, there is a barrier, although
electrons with energy less than this barrier cannot go through the
oxide classically, however from the knowledge of quantum
mechanics one can understand that electrons can tunnel through a
barrier, especially if the barrier thickness is sufficiently small.
 In the MOS structure there are several conduction mechanisms that
have been proposed to describe the leakage current conduction in
dielectric films.
1. Schottky emission (SE)
2. Fowler–Nordheim tunneling (FN)
3. Direct tunneling (DT)
4. Poole–Frenkel emission (PF)
5. Space charge limited conduction (SCLC)
6. Other possible mechanism

3.  Schottky emission is a field-assisted thermionic
emission of an electron over a surface barrier (path 1)
in Fig. It can be modeled according to the following
equation:
 Which is termed as the effective Richardson constant.
Schottky emission (SE)

4. Al
 Due to constant downscaling of gate-dielectric thicknesses in
modern MOS devices the effect of tunneling has drastically
gained relevance.
 Quantum mechanical tunneling describes the transition of
carriers through a classically forbidden energy state.
 This can be an electron tunneling from the semiconductor
through a dielectric, which represents an energy barrier, to the
gate contact of an MOS structure. Even if the energy barrier is
higher than the electron energy, there is quantum mechanically a
finite probability of this transition.
 The reason lies in the wavelike behavior of particles on the
quantum scale where the wave function describes the probability
of finding an electron at a certain position in space.
 As the wave function penetrates the barrier and can even extend
to the other side, quantum mechanics predict a non-zero
probability for an electron to be on the other side.
where C is a parameter affected by the density of states in
conduction band, electric field, and B.
Direct tunneling (DT)

5.  The energy band conditions for Fowler-Nordheim tunneling, which is a
special case of direct tunneling, are depicted in Figure. The electrons do
not tunnel directly to the other side of the barrier. Instead they tunnel from
the silicon inversion layer to the conduction band of the dielectric layer
from where they are transported to the gate contact. The Fowler-
Nordheim regime is significant for thicker dielectrics and sufficiently high
electric fields.
Fowler–Nordheim tunneling (FN)
Al
2
2 3/28 2 *
exp[ ]
8 3
FN B
B
qmq
J E
h hE


 
 

6.  The PF emission is due to the emission of trapped electrons into the
conduction band of insulator.
 Electrons can move (slowly) through an insulator by the following
method. The electrons are generally trapped in localized states and
Occasionally, random thermal fluctuations will give that electron enough
energy to get out of its localized state, and move to the conduction band.
Poole–Frenkel emission (PF)
/
exp[ ]rt
PF
qEq
J CE q
kT kT

  

7.  According to SCLC model, a strong accumulation layer formed in the
Semiconductor as large voltage is applied across MOS device. In most of
the oxide layers, the space charge limited current (SCLC) is dominated at
the higher voltages/field and given by
 Where εi is the permittivity of oxide, is the mobility, V is the voltage and d
is the thickness of the oxide layer.
Space charge limited conduction (SCLC)
2 39
8
SCLC iJ V d  


8.  Trap assisted tunneling (TAT)/Hopping: The trap-assisted tunneling
process of an electron from the cathode to the anode via a trap is
considered as a two-step process. Electrons are captured from the cathode,
relax to the energy level of the trap by emitting one or more phonons , and
are then emitted to the anode. This process is inelastic as the electron
energy is not conserved during the tunneling process.
 Ionic conduction: Ionic conduction consists of the transit of ions (atoms
of positive or negative charge) from one site to another via point defects
called vacancies in the crystal lattice.
 Ballistic Transport: Ballistic transport is carrier transport without
scattering or any other mechanism, which would cause a loss of energy.
 Stress-induced leakage current (SILC):Due to the repeated high-field
stress, defects can arise in the dielectric leading to tunneling currents,
even at low fields.
Other possible mechanism

9.  Leakage current density as a function of applied voltage of Al/ZrO2/p-Si.
9

10. 10
/
exp[ ]rt
PF
qEq
J CE q
kT kT

  
2
2 3/28 2 *
exp[ ]
8 3
FN B
B
qmq
J E
h hE


 
 
2 39
8
SCLC iJ V d  

Arvind et al, Appl. Surf. Sci. 370, 373 (2016).
 Different conduction Mechanism in ZrO2 Thin layer.