24.4 JFET and MESFET
711
(a)
1.0
0.8
0.6
0.4
0.2
0
0
0
0.4
0.6
0.8 1.0
D
P
D
P
0.1
0.4
0.2
0.3
0.5
0.8
0.6
0.2
(b)
0
0.4
0.6
0.8 1.0
D
P
0.2
0.4
0.2
0
D
P
0
0.1
0.6
0.3
0.8
Fig. 24.18 I –V characteristic a without consideration of drift saturation (z = 0) and b with drift saturation (z = 3) for
various values of (V G + V bi )/V P =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 as indicated at the right side. The intersections of
the dashed line and the solid lines indicate the beginning of saturation. Adapted from [2060]
Fig. 24.19 Decrease of
(saturated) forward
conductance with gate
voltage (according to
(24.49)) and parametric
dependence on z for z = 0,
0.5, 1, 2, 3, 5 and 10.
Adapted from [2060]
Two-Region Model
In order to model the GaAs drift velocity vs. field characteristic, a two-region model is used. In the
front region of the channel (region I), the field is small enough and a constant mobility μ is used. In
the back region of the channel (region II) is the high-field region where a constant drift velocity v s
is used. With increasing source–drain voltage, the region II (I) increases (decreases) in length. The
relative width of region II is also increased with decreasing channel length.
Saturated-Drift Model
Here, the drift velocity is taken everywhere as v s , i.e. complete drift saturation. This is a good approximation for short channels (high fields) that are in current saturation. In this case, the current is given by
I D = −e N D v s [a − h(x)] Z .
(24.53)
Equation (24.53) is valid for homogeneous doping. For other doping profiles, the current is given by
711
(a)
1.0
0.8
0.6
0.4
0.2
0
0
0
0.4
0.6
0.8 1.0
D
P
D
P
0.1
0.4
0.2
0.3
0.5
0.8
0.6
0.2
(b)
0
0.4
0.6
0.8 1.0
D
P
0.2
0.4
0.2
0
D
P
0
0.1
0.6
0.3
0.8
Fig. 24.18 I –V characteristic a without consideration of drift saturation (z = 0) and b with drift saturation (z = 3) for
various values of (V G + V bi )/V P =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 as indicated at the right side. The intersections of
the dashed line and the solid lines indicate the beginning of saturation. Adapted from [2060]
Fig. 24.19 Decrease of
(saturated) forward
conductance with gate
voltage (according to
(24.49)) and parametric
dependence on z for z = 0,
0.5, 1, 2, 3, 5 and 10.
Adapted from [2060]
Two-Region Model
In order to model the GaAs drift velocity vs. field characteristic, a two-region model is used. In the
front region of the channel (region I), the field is small enough and a constant mobility μ is used. In
the back region of the channel (region II) is the high-field region where a constant drift velocity v s
is used. With increasing source–drain voltage, the region II (I) increases (decreases) in length. The
relative width of region II is also increased with decreasing channel length.
Saturated-Drift Model
Here, the drift velocity is taken everywhere as v s , i.e. complete drift saturation. This is a good approximation for short channels (high fields) that are in current saturation. In this case, the current is given by
I D = −e N D v s [a − h(x)] Z .
(24.53)
Equation (24.53) is valid for homogeneous doping. For other doping profiles, the current is given by