Electro-Optics - Applied Optics Fundamentals and Device Applications - page 12

3. Its operating frequency is up to the millimeter region.
4. Its noise figure is lower.
5. Its input resistance is very high, up to several megaohms.
Unipolar FETs come in two basic forms: the p-n junction gate and the
Schottky barrier gate.
The first FET structure to be discussed is the p-n junction FET, or JFET. The
basic physical structure of the JFET is shown in Figure 2.7a [15]. The n-type
material sandwiched between two p + -type material layers acts as the channel
through which the current passes from the source to the drain of the device.
The voltage applied to the gate contacts of the device determines the width
of the depletion region and therefore the width of the channel. A reverse bias
between the p + -type layers causes the depletion regions to cross over into the
n-type channel region. Since the channel region has fixed resistivity due to
its doping profile, the resistance of the channel will vary in response to the
changes in the effective cross-sectional area. With the n-type channel config-
uration shown in Figure 2.7a, the electrons flow from the source to the drain.
This flow would be from the drain to the source for a p-type channel sand-
wiched between n + -type gate regions. Figure 2.7b [16] depicts the restriction
of the conduction channel by an increase in drain voltage. As V D increases,
so does I D , which tends to increase the size of the depletion regions. The
reverse bias, being larger toward the drain than toward the source, generates
the tilt in the depletion region toward the drain. Since the resistance of the
restricted conduction channel increases, the I-V characteristic of the chan-
nel diverges from linear. As V D increases more, there is a point at which the
value of I D levels off and the channel is completely pinched off by the deple-
tion regions. Once the device is in this saturated operating region, the drain
current may be modulated by varying the gate voltage.
The JFET was the first variety of field effect device developed and is still
found in many applications. Its structure, however, requires a multiple-
diffusion process and so makes it a difficult device to integrate. The device is
also limited to frequencies below X-band due to the slow transit times under
the diffused gate regions.
The Schottky barrier gate FET is of most interest for the development of
OMMICs. Schottky suggested in 1938 that a potential barrier could arise
from stable space charges in the semiconductor without the introduction of
a chemical layer. This provided the foundation for the development of the
metal semiconductor FET (MESFET).
The physical structure of the MESFET is shown in Figure 2.8. This particu-
lar GASFET is a low-noise device that has been fabricated using ion implan-
tation techniques. This technique will be compared with others for the mass
production of MMIC devices in Sections 5.10 and 5.11. The device fabrication
process begins with the implantation of donor ions (in this case, Si + ions)
directly into a semi-insulating GaAs substrate wafer. Next, n + contact layers