6.2.2 Dependence of Recess Geometry
on the E-Field Distribution
The following investigations on breakdown will be carried out on the
distribution of the electric field
in the channel for V_{DS} = 10 V depending on V_{GS}
and geometry parameters. The electric field distribution reflects the resistance
along a current path. The highest field is reached were the resistivity
is high. The intrinsic transistor action is based on controlling the conductivity
of the channel by the gate. As the series resitances are almost constant
the relative resistivity along the current path is changed and thus the
distribution of
for a constant V_{DS}.
In Figure 6.24 a cross section of through the channel is shown for different bias points. For V_{GS} < V_{T} a peak is reached at the drain side of the gate because between gate and drain the largest voltage drop of V_{GD} = 10.4 V occurs compared to V_{GS} = 0.4 V between gate and source. But the electric field is still quite high under the inner recess on the drain side which is characterized by L_{R}. As the resistivity decreases significantly under the cap is reduced accordingly. For higher V_{GS} the conductivity in the channel is increased and the resistivity under the gate gets in the order of the resistivity under the inner and the double recess. Therefore the distribution of is spread out accordingly and its maximum is decreased. _{max} increases again in the open channel regime as the main voltage drop occurs only under the double recess area.
The comparison of the electric field distributions for different bias points leads to a design consideration of the recess geometry in respect to breakdown. The breakdown voltage for V_{GS} < V_{T} is mainly governed by the length L_{R} of the inner recess whereas for open channel conditions the recessed cap becomes dominating. The recess in the cap is characterized by the thickness of the remaining highly doped cap layer and the double recess length L_{DR}.
In Figure
6.25 the electric field distribution for the open channel regime is
shown for V_{GS} = 0.8 V. If the remaining thickness d_{DR}
of the highly doped cap is zero basically the length of the inner recess
is increased by L_{DR}. In this case _{max}
is reached at the end of the recess. If d_{DR} is increased
the resistive distribution is changed such that
is spread out more and thus _{max}
is reduced. If d_{DR} is to thick _{max}
is still reached at the end of the inner recess and its magnitude is higher
again.
It should be mentioned that the values for d_{DR} given in Figure 6.25 are unrealistically low. This is another indication that the model for the semiconductor/passivation interface might be not accurate enough. Additionally the depletion depth of the semiconductor surface highly depends on the damage as an inevitable consequence of the etching process.
To reduce _{max} by increasing the series resistance in some areas leads directly to the trade-off between power capability and RF performance. In the following not only the impact of the geometry on the electric field distribution for V_{DS} = 10 V will be investigated but also the consequences of the respective device geometry on g_{m} and f_{T} at V_{DS} = 2.0 V and V_{GS} = 0.4 (i. e. were g_{m max} is reached).
In Figure 6.26 the maximum field in the channel is shown versus the length of the inner recess L_{R} for V_{DS} = 10 V and V_{GS} < V_{T}. As depicted in the Figure _{max} decreases about linearly with L_{R}. This goes along with an almost linear reduction of g_{m max} for V_{DS} = 2.0 V as the series resistance is increased.
However, for the current gain cut-off frequency another effect becomes
significant as already discussed for the low noise HEMTs in Section
6.1.2.2. With increasing L_{R} the coupling between
the contacts and thus C_{G} is reduced. The reduction of
C_{G} is larger for small L_{R} end gets
less significant for larger L_{R}. This overcompensates
the linear reduction of g_{m} for L_{R} <
190 nm as shown in Figure
6.26. The maximum f_{T} of almost 53 GHz is reached
for a significantly larger L_{R} than in the case of the
low noise HEMT. It will be discussed in Section
6.2.3.2 that the C_{G} of the power HEMTs investigated
here is very high such that the impact of a reduction of C_{G}
due to a larger L_{R} is more significant than in the case
of the low noise HEMTs.
When the channel starts to conduct current the double recess in the
cap eventually becomes significant. It was shown in Figure
6.25 that the thickness of the recessed cap d_{DR} determines
the location of _{max}
and thus influences its magnitude. In addition to the dependence of _{max}
on d_{DR} the influence on the recess length L_{DR}
is shown in Figure
6.27. _{max}
can be controlled over a very wide range from above 4.5*10^{7}V/cm
to below 2.0*10^{7}V/cm. For L_{R} > 300 nm _{max}
increases monotonously with the thickness d_{DR}. For L_{R}
< 300 nm a local minimum appears. The minimum is shifted from d_{DR}
= 1 nm for L_{DR} = 300 nm to d_{DR} = 4
nm for L_{DR} = 100 nm.
The mechanism is illustrated in Figure
6.25. _{max}
without a double recess is about 5*10^{7}V/cm. Using a double recess
with L_{R} = 200 nm and d_{DR} = 2 nm _{max}
can be reduced by almost 50 % to 2.8*10^{7}V/cm. Figure
6.27 also indicates that to receive full benefit of the double recess
a tight control of d_{DR} is required. For too large d_{DR} _{max}
is reduced only slightly whereas for too small d_{DR} not
only _{max}
is increased again but additionally a larger series resistance has to be
expected. A larger resistance manifests itself in a reduced g_{m}
as depicted in Figure
6.28. If a double recess with L_{R} = 200 nm and d_{DR}
= 8 nm is used instead of L_{R} = 100 nm with the same d_{DR} _{max}
is already reduced substantially but the reduction in g_{m}
is still negligible. A double recess with L_{R} = 400 nm
and d_{DR} = 2 nm reduces _{max}
greatly but also g_{m} is reduced by more than 10 %.
Similar to the length of the inner recess L_{R} C_{G}
is influenced by the geometry of the recessed cap due to the coupling of
the gate metal with the highly doped cap. The dependence of C_{G}
on d_{DR} and L_{DR} is shown in Figure
6.29. C_{G} is small if the recessed cap is fully depleted.
But C_{G} increases not only with the thickness d_{DR}
but also with decreasing L_{DR} because in this case the
coupling between the gate metal and the non recessed cap increases. The
local maxima in the C_{G} characteristics for L_{DR}
> 100 nm are not clear. It will be shown later in this chapter that the
distance between the gate metal and the highly doped cap is quite small
for this device, therefore, the coupling is very strong. In this case the
uncertainties in the simulation due to effects such as quantization might
become significant.
This is getting even more pronounced in Figure
6.30 where f_{T} based on the data from Figure
6.28 and Figure
6.29 is shown. As both g_{m} and C_{G}
are increased with increasing d_{DR} and decreased with
increasing L_{DR} no large impact has to be expected on
f_{T}.
For L_{DR} = 100 nm again a weak local maximum can be
observed similar to the dependence on the inner recess. The change of f_{T}
is only within 2 GHz. Some slightly larger effects have to be expected
for L_{DR} > 200 nm. The simulations in these cases are
not accurate enough to clearly draw definite conclusions about the differences
between L_{DR} = 200 nm, 300 nm, and 400 nm. But common
to all characteristics is that a local minimum appears for d_{DR}
between 3 nm and 4 nm. Even for variations of L_{DR} and
d_{DR} in a very wide range f_{T} is only
changed within about 4 GHz.
Helmut Brech 1998-03-11