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 VDS = 10 V depending on VGS
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 VDS.
max
is shifted from the drain end of the gate to the end of the double recess
if the conductivity of the channel is increased by increasing VGS.In Figure
6.24 a cross section of 
through the channel is shown for different bias points. For VGS
< VT a peak is reached at the drain side of the gate
because between gate and drain the largest voltage drop of VGD
= 10.4 V occurs compared to VGS = 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 LR.
As the resistivity decreases significantly under the cap 
is reduced accordingly. For higher VGS 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 VGS < VT is mainly governed by the length LR 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 LDR.
In Figure
6.25 the electric field distribution for the open channel regime is
shown for VGS = 0.8 V. If the remaining thickness dDR
of the highly doped cap is zero basically the length of the inner recess
is increased by LDR. In this case 
max
is reached at the end of the recess. If dDR is increased
the resistive distribution is changed such that 
is spread out more and thus 
max
is reduced. If dDR is to thick 
max
is still reached at the end of the inner recess and its magnitude is higher
again.
max
the thickness of the recessed cap has to be such that 
is equally distributed under the recess.It should be mentioned that the values for dDR 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 VDS = 10 V will be investigated but also the consequences
of the respective device geometry on gm and fT
at VDS = 2.0 V and VGS = 0.4 (i. e.
were gm max is reached).
In Figure
6.26 the maximum field in the channel is shown versus the length of
the inner recess LR for VDS = 10 V
and VGS < VT. As depicted in the
Figure 
max
decreases about linearly with LR. This goes along with
an almost linear reduction of gm max for VDS
= 2.0 V as the series resistance is increased.
max
at VDS = 10 V and VGS < VT
is correlated with a reduction of gm at the active bias
point VDS = 2.0 V and VGS = 0.4 V.
A strong increase of CG leads to a reduced fT
for small LR.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 LR the coupling between
the contacts and thus CG is reduced. The reduction of
CG is larger for small LR end gets
less significant for larger LR. This overcompensates
the linear reduction of gm for LR <
190 nm as shown in Figure
6.26. The maximum fT of almost 53 GHz is reached
for a significantly larger LR than in the case of the
low noise HEMT. It will be discussed in Section
6.2.3.2 that the CG of the power HEMTs investigated
here is very high such that the impact of a reduction of CG
due to a larger LR 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 dDR determines
the location of 
max
and thus influences its magnitude. In addition to the dependence of 
max
on dDR the influence on the recess length LDR
is shown in Figure
6.27. 
max
can be controlled over a very wide range from above 4.5*107V/cm
to below 2.0*107V/cm. For LR > 300 nm 
max
increases monotonously with the thickness dDR. For LR
< 300 nm a local minimum appears. The minimum is shifted from dDR
= 1 nm for LDR = 300 nm to dDR = 4
nm for LDR = 100 nm.
The mechanism is illustrated in Figure
6.25. 
max
without a double recess is about 5*107V/cm. Using a double recess
with LR = 200 nm and dDR = 2 nm 
max
can be reduced by almost 50 % to 2.8*107V/cm. Figure
6.27 also indicates that to receive full benefit of the double recess
a tight control of dDR is required. For too large dDR 
max
is reduced only slightly whereas for too small dDR not
only 
max
is increased again but additionally a larger series resistance has to be
expected. A larger resistance manifests itself in a reduced gm
as depicted in Figure
6.28. If a double recess with LR = 200 nm and dDR
= 8 nm is used instead of LR = 100 nm with the same dDR 
max
is already reduced substantially but the reduction in gm
is still negligible. A double recess with LR = 400 nm
and dDR = 2 nm reduces 
max
greatly but also gm is reduced by more than 10 %.
Similar to the length of the inner recess LR CG
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 CG
on dDR and LDR is shown in Figure
6.29. CG is small if the recessed cap is fully depleted.
But CG increases not only with the thickness dDR
but also with decreasing LDR because in this case the
coupling between the gate metal and the non recessed cap increases. The
local maxima in the CG characteristics for LDR
> 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 fT based on the data from Figure
6.28 and Figure
6.29 is shown. As both gm and CG
are increased with increasing dDR and decreased with
increasing LDR no large impact has to be expected on
fT.
For LDR = 100 nm again a weak local maximum can be
observed similar to the dependence on the inner recess. The change of fT
is only within 2 GHz. Some slightly larger effects have to be expected
for LDR > 200 nm. The simulations in these cases are
not accurate enough to clearly draw definite conclusions about the differences
between LDR = 200 nm, 300 nm, and 400 nm. But common
to all characteristics is that a local minimum appears for dDR
between 3 nm and 4 nm. Even for variations of LDR and
dDR in a very wide range fT is only
changed within about 4 GHz.
Helmut Brech 1998-03-11