The well known definition for the current gain cut-off frequency
is given in (4.10).

This definition of
has various approximations in terms of intrinsic small-signal
equivalent elements. The most important approximations for a HEMT are (assuming the parasitic
inductances and capacitances to be stripped off) according to Baeyens in [23]:

and even simpler in (4.12) neglecting the parasitic resistances and the output conductance:

On the contrary, the inclusion of the pad capacities and the fringe part from (4.4) in (4.11) yield an extrinsic charging time and extrinsic , respectively.

The intrinsic transconductance and the extrinsic transconductance are then related by:

where is the extrinsic output conductance. Using (4.11)-(4.13) is precisely defined for different levels of deembedding.

The quantity
is defined in several manners depending on the invariants used for
its definition [111]. Defining the Unilateral Power Gain allows for the highest values
of
in a device representing the maximum gain in a lossless reciprocal deembedding
[111].

Second, the Maximum Available Gain (MAG) and the Maximum Stable Gain (MSG) can be used for defining :

for the stability factor 1. is then determined as:

The MAG drops with a slope of -20 dB/dec as a function of frequency near =1. Kurokawa's stability factor is defined from the S-parameters as [111]:

The Maximum Stable Gain (MSG) is used for 1 is defined as:

The MSG drops with -10 dB/dec as a function of frequency. The transition between the two
for = 1 defines the frequency
, from which
based on MAG/MSG eventually can be
extrapolated with a slope of -20 dB/dec for a given gate width
. The frequency:

defines the stability point and is expressed as a function of small-signal elements [162]. depends the single finger gate width and is a critical quantity for amplifier design, especially for mm-wave applications. It will be analyzed in Chapter 7 with respect to statistical changes of mm-wave devices.

2001-12-21