Introduction
In the past years, channel lengths of field-effect transistors in integrated circuits were reduced to arrive at currently about 40 nm
1
. Smaller conventional transistors have been built
2
3
4
5
6
7
8
9
with gate lengths down to 10 nm and below. As well-known with decreasing channel length the desired long-channel behavior of a transistor is degraded by short-channel effects
10
11
12
. One major source of these short-channel effects is the multi-dimensional nature of the electro-static field which causes a reduction of the gate voltage control over the electron channel. A second source is the advent of quantum transport. The most obvious quantum short-channel effect is the formation of a source-drain tunneling regime below threshold gate voltage. Here, the I
D - V
D-traces show a positive bending as opposed to the negative bending resulting for classically allowed transport
13
14
. The source-drain tunneling and the classically allowed transport regime are separated by a close-to linear threshold trace (LTT). Such a behavior is found in numerous MOSFETs with channel lengths in the range of a few tens of nanometers (see, for example,
2
3
4
5
6
7
8
9
).
Starting from a three-dimensional formulation of the transport problem it is possible to construct a one-dimensional effective model
14
which allows to derive scale-invariant expressions for the drain current
15
16
. Here, the quantity
![]()
arises as a natural scaling length for quantum transport where ε
F is the Fermi energy in the source contact and m* is the effective mass of the charge carriers. The quantum short-channel effects were studied as a function of the dimensionless characteristic length l = L/λ of the transistor channel, where L is its physical length.
In this conference contribution, we discuss the physics of the major quantities in our scale-invariant model which are the chemical potential, the supply function, and the scale-invariant current transmission. We specify its range of applicability: generally, for a channel length up to a few tens of nanometers a LTT is definable up to room temperature. For higher temperatures, a LTT can only be found below a channel length of 10 nm. An inspection of the I
D - V
G-traces yields in qualitative agreement with experiments that at low drain voltages transport becomes thermally activated below the threshold gate voltage while it does not for large drain voltages. Though our model reproduces interesting qualitative features of the experiments it fails to provide a quantitative description: the theoretical values are larger than the experimental ones by a little less than a decade. Such a finding is expected for our simple model.
Theory
Tsu-Esaki formula for the drain current
In Refs.
13
14
, the transport problem in a nano-FET was reduced to a one-dimensional effective problem invoking a "single-mode abrupt transition" approximation. Here, the electrons move along the transport direction in an effective potential given
![]()
(see Figure 1b). The energy zero in Equation 1 coincides with the position of the conduction band minimum in the highly n-doped source contact. As shown in
14
<p>Figure 1</p>Generic n-channel nano-field effect transistor
Generic n-channel nano-field effect transistor. (a) Schematic representation. (b) One-dimensional effective potential Veff.
![]()
![]()
where E
k = 1 is the bottom of the lowest two-dimensional subband resulting in the z-confinement potential of the electron channel at zero drain voltage (see Figure 4b of Ref.
13
). The parameter W is the width of the transistor. Finally, V
D = eU
D is the drain potential at drain voltage U
D which is assumed to fall off linearly.
Experimentally, one measures in a wide transistor the current density J, which is the current per width of the transistor that we express as
![]()
Here
![]()
is the number of equivalent conduction band minima ('valleys') in the electron channel and I
0 = 2eε
F/h. In Refs.
15
16
a scale-invariant expression
![]()
was derived. Here, m = μ/ε
F is the normalized chemical potential in the source contact, v
D = V
D/ε
F is the normalized drain voltage, and v
G = V
G/ε
F is the normalized gate voltage. As illustrated in Figure 1(b) the gate voltage is defined as the energy difference μ - V
0 = V
G, i.e., for V
G > 0 the transistor operates in the ON-state regime of classically allowed transport and for V
G < 0 in the source-drain tunneling regime. The control variable V
G is used to eliminate the unknown variable V
0. For the chemical potential in the source contact one finds (see next section)
![]()
where u = kBT/ε
F is the normalized thermal energy. Equation 4 has the form of a Tsu-Esaki formula with the normalized supply function
![]()
Here, F
-1/2 is the Fermi-Dirac integral
![]()
of order -1/2 and
![]()
is the inverse function of F
1/2. The effective current transmission
![]()
depends on
![]()
which is the normalized energy of the electron motion in the y-z-plane while
![]()
is their energy in the x-direction. In the next sections, we will discuss the occurring quantities in detail.
Chemical potential in source- and drain-contact
For a wide enough transistor and a sufficient junction depth a (see Figure 1) the electrons in the contacts can be treated as a three-dimensional non-interacting electron gas. Furthermore, we assume that all donor impurities of density Ni
are ionized. From charge neutrality it is then obtained that the electron density n
0 is independent of the temperature and given by
![]()
Here m
e is the effective mass and N
V is the valley-degeneracy factor in the contacts, respectively. In the zero temperature limit a Sommerfeld expansion of the Fermi-Dirac integral leads to
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Equating 7 and 8 results in
![]()
which is identical with (5) and plotted in Figure 2. As well-known, with increasing temperature the chemical potential falls off because the high-energy tail of the Fermi-distribution reaches up to ever higher energies.
<p>Figure 2</p>Normalized chemical potential vs. thermal energy according to Equation 9 in green solid line and parabolic approximation in red dash-dotted line
Normalized chemical potential vs. thermal energy according to Equation 9 in green solid line and parabolic approximation in red dash-dotted line.
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Supply function
As shown in Ref.
14
the supply function for a wide transistor can be written as
![]()
This expression can be interpreted as the partition function (loosely speaking the "number of occupied states") in the grand canonic ensemble of a non-interacting homogeneous three-dimensional electron gas in the subsystem of electrons with a given lateral wave vector (ky
, kz
) yielding the energy
![]()
in the y-z-direction. Formally equivalent it can be interpreted as the full partition function in the grand canonic ensemble of a one-dimensional electron gas at the chemical potential μ - ε. Performing the limit
![]()
the Riemann sum in the variable
![]()
can be replaced by the Fermi-Dirac integral F
-1/2. It results that
![]()
with the normalized transistor width w = W/λ. For the scaling of the supply function in Equation 11 we define (see Ref.
14
)
![]()
where
![]()
and we use the identity V
0= ε
F = m - v
G. For the source contact we write
![]()
leading to the first factor in the square bracket of the Tsu-Esaki equation 4. In the drain contact, the chemical potential is lower by the factor V
D. Replacing μ → μ - V
D yields
![]()
Below we will show that for transistor operation the low temperature limit is relevant (see Figure 2). Here, one may apply in leading order
![]()
(resulting from a Sommerfeld expansion) and F
-1/2(-x → ∞) → exp (x). Since V
0 > 0 the factor v
G - m is negative and we obtain from (12)
![]()
From Figure 3 it is seen that for ε below the chemical potential the supply function is well described by the square-root dependence in the
![]()
limit. If ε lies above the chemical chemical one obtains the
![]()
limit which is a small exponential tail due to thermal activation.
<p>Figure 3</p>Supply function in the source contact (see Equation 6) for u = 0.1 and vG = 0 (black line), low-temperature limit according to Equation 15 for α < 0 (red dashed line) and α > 0 (green dashed line)
Supply function in the source contact (see Equation 6) for u = 0.1 and vG = 0 (black line), low-temperature limit according to Equation 15 for α < 0 (red dashed line) and α > 0 (green dashed line). Because of the small temperature m(u) ~ 1 so that ![]()
occurs at ![]()
.
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Current transmission
The effective current transmission in Equation 16 is given y
![]()
It is calculated from the scattering solutions of the scaled one-dimensional Schrödinger equation
![]()
with β = 2m*V
0
L
2/ħ
2 = l
2(m - v
G), and ŷ = y/L. The scaled effective potential
![]()
is given by
![]()
,
![]()
, and
![]()
,where
![]()
. As usual, the scattering functions emitted from the source contact
![]()
obey the asymptotic conditions
![]()
and
![]()
with
![]()
and
![]()
.
As can be seen from Figure 4, around
![]()
the current transmission changes from around zero to around one. For weak barriers there is a relatively large current transmission below one leading to drain leakage currents. For strong barriers this remnant transmission vanishes and we can approximate the current transmission by an ideal one.
<p>Figure 4</p>Scaled effective model
Scaled effective model. (a) Scaled effective potential. (b) Effective current transmission at u = 0.1, vD = 0.5, and vG = 0 (![]()
= 0.504 and m = 0.992). The considered characteristic lengths are l = 4 (red, weak barrier, β = 15.87) and l = 25 (green, strong barrier, β = 619.8). The ideal limit (Equation 19) in blue line.
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To a large extent the Fowler Nordheim oscillations in the numerical transmission average out performing the integration in Equation 4.
Parameters in experimental nano-FETs
Heavily doped contacts
In the heavily doped contacts the electrons can be approximated as a three-dimensional non-interacting Fermi gas. Then from (8) the Fermi energy above the bottom of the conduction band is given by
![]()
For n
++-doped Si contacts the valley-degeneracy is N
V = 6 and the effective mass is taken as
![]()
. Here m
1 = 0.19m
0 and m
2 = 0.98m
0 are the effective masses corresponding to the principle axes of the constant energy ellipsoids. In our later numerical calculations we set ε
F = 0.35 eV assuming a level of source-doping as high as N
i = n
0 = 1021 cm-3.
Electron channel
In the electron channel a strong lateral subband quantization exists As well-known
17
at low temperatures only the two constant energy ellipsoids with the heavy mass m
2 perpendicular to the (100)-interface are occupied leading to a valley degeneracy of g
v = 2. The in-plane effective mass is therefore the light mass m* = m
1 entering the relation
![]()
Here ε
F = 0.35 eV was assumed. One then has in Equation 3 I
0 = ~ 27μA and with λ ~ 1 nm as well as
![]()
= 2 one obtains J
0 = 5.4 × 104
μA/μm.
Discussion
We discuss our numerical results on the background of experimental characteristics for a 10 nm gate length transistor
4
5
reproduced in Figure 7. As demonstrated in Sect. "Parameters in experimental nano-FETs" one obtains from Equation 21 a characteristic length of λ ~ 1 nm under reasonable assumptions. For the experimental 10 nm gate length, we thus obtain l = L/λ = 10. Furthermore, Equation 20 yields the value of ε
F = 0.35 eV. The conversion of the experimental drain voltage V into the theoretical parameter v
D is given by
<p>Figure 7</p>Drain characteristics in experiment and theory
Drain characteristics in experiment and theory. (a) Experimental drain characteristics for a nano-transistor with L = 10 nm 45. Our assumption for the LTTis marked with a green dashed line leading to a threshold gate voltage of ![]()
= 0.15V. (b) Theoretical drain characteristics for l = 10 and u = 0.1 (see Fig. 5a) with the green dashed threshold characteristic at ![]()
= -0.05.
![]()
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The maximum experimental drain voltage of 0.75 V then sets the scale for v
D ranging from zero to v
D = 0.75 eV/0.35 eV ~ 2. For the conversion experimental gate voltage V
G to the theoretical parameter v
G we make linear ansatz as
![]()
where
![]()
is the experimental threshold gate voltage (see Figure 8a). The constant β is chosen so that
![]()
converts into
![]()
. In our example, it is shown from Figure 8a
![]()
= 0.15 V and from Figure 8b
![]()
= -0.05, so that β = -0.2 eV. To match the experimental drain characteristic to the theoretical one we first convert the highest experimental value for V
G into the corresponding theoretical one. Inserting in (23) V
G = 0.75 V yields v
G ~ 0.5. Second, we adjust the experimental and the theoretical drain current-scales so that in Figure 7 the curves for the experimental current at V
G = 0.7 and the theoretical curve at v
G = 0.5 agree. It then turns out that the other corresponding experimental and theoretical traces agree as well. This agreement carries over to the range of negative gate voltages with thermally activated transport. This can be gathered from the I
D - V
G traces in Figure 8. We note that the constant of proportionality in Equation 23 given by 1 eV is more then ε
F which one would expect from the theoretical definition v
G = V
G/ε
F. Here, we emphasize that the experimental value of e
V
G corresponds to the change of the potential at the transistor gate while the parameter v
G describes the position of the bottom of the lowest two-dimensional subband in the electron channel. The linear ansatz in Equation 23 and especially the constant of proportionality 1 eV can thus only be justified in a self-consistent calculation of the subband levels as has been provided, e.g., by Stern
18
.
<p>Figure 8</p>Threshold characteristics in experiment and theory
Threshold characteristics in experiment and theory. (a) Experimental threshold characteristics for the nano-transistor in Fig. 7a. (b) Theoretical threshold characteristics for l = 10 and u = 0.1 with the blue dashed lines corresponding to thermal activation.
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The experimental and the theoretical drain characteristics in Figure 7 look structurally very similar. For a quantitative comparison we recall from Sect. "Parameters in experimental nano-FETs" the value of J
0 = 5.4 × 104
μA/μm. Then the maximum value j = 0.15 in Figure 7b corresponds to a theoretical current per width of 8 × 103
μA/μm. To compare with the experimental current per width we assume that in the y-axis labels in Figures 7a and 8a it should read μA/μm instead of A/μm. The former unit is the usual one in the literature on comparable nanotransistors (see Refs.
2
3
4
5
6
7
8
9
) and with this correction the order of magnitude of the drain current per width agrees with that of the comparable transistors. It is found that the theoretical results are larger than the experimental ones by about a factor of ten. Such a failure has to be expected given the simplicity of our model. First, for an improvement it is necessary to proceed from potentials resulting in a self-consistent calculation. Second, our representation of the transistor by an effectively one-dimensional system probably underestimates the backscattering caused by the relatively abrupt transition between contacts and electron channel. Third, the drain current in a real transistor is reduced by impurity interaction, in particular, by inelastic scattering. As a final remark we note that in transistors with a gate length in the micrometer scale short-channel effects may occur which are structurally similar to the ones discussed in this article (see Sect. 8.4 of
10
). Therefore, a quantitatively more reliable quantum calculation would be desirable allowing to distinguish between the short-channel effects on micrometer scale and quantum short-channel effects.