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Circuits Based on Semiconductor Devices |
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Figure 8.5 IMPATT diode: (a) different structures; and (b) encapsulated diode.
in opposite phase, that is, its resistance is negative. An IMPATT diode must be encapsulated so that the heat generated in the diode is effectively transferred away.
8.2.2 Transistors
The most common transistor in RF applications up to a few gigahertz is the bipolar transistor. In a bipolar transistor both electrons and holes act as current carriers. A bipolar transistor is usually made of silicon. Figure 8.6 shows an n-p-n type bipolar transistor. Between the emitter (E) and collector
(C) there is a thin base (B) layer. In RF applications the transistor is usually in common-emitter connection, that is, the emitter is grounded. Proper bias voltages are applied to the base-emitter and collector-base junctions; then a small change in the base current, DIB , causes a large change in the collector current, DIC . The small-signal gain is
Figure 8.6 Bipolar transistor.
178 Radio Engineering for Wireless Communication and Sensor Applications
b = |
DIC |
(8.6) |
DIB |
Parasitic capacitances and resistances and the drift times of carriers limit the highest usable frequency of the bipolar transistor.
A heterojunction bipolar transistor (HBT) is a faster version of the bipolar transistor. Here heterojunction means an interface of two different semiconductors; for example, the emitter is of Si and the base of SiGe, or the emitter is of AlGaAs and the base of GaAs. Because of the heterojunction, the base can be doped very heavily, and therefore the base resistance is small and the transistor is operational at high frequencies.
Metal-oxide-semiconductor field-effect transistors (MOSFETs) and metalsemiconductor field-effect transistors (MESFETs) are field-effect transistors for RF and microwave applications. MOSFETs fabricated using complementary metal-oxide-semiconductor (CMOS) technology, commonly used for digital microcircuits, are applicable for analog RF circuits up to several gigahertz.
GaAs MESFETs are useful up to millimeter wavelengths. Figure 8.7 shows a cross section of a MESFET and its small-signal equivalent circuit. There is a thin n -type layer on an undoped substrate. This layer forms the transistor channel, where electrons act as carriers. On the surface of the channel layer there are two ohmic contacts, the source (S) and the drain (D), and between them a short gate (G) contact, which forms with the semiconductor a reverse-biased Schottky junction. As in the Schottky diode, there is a depletion layer in the channel under the gate; the width of the depletion layer depends on the gate voltage. Therefore, the gate voltage VGS can be used to control the current between the source and drain, IDS . The ratio of the changes in IDS and VGS with a constant VDS is called the transconductance
Figure 8.7 Metal-semiconductor field-effect transistor (MESFET): (a) structure; (b) equivalent circuit.
Circuits Based on Semiconductor Devices |
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g m = |
∂IDS |
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(8.7) |
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∂VGS |
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The frequency at which the short-circuit current gain is 1, is approximately
f T ≈ |
g m |
≈ |
vs |
(8.8) |
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2pCgs |
2pL |
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where Cgs is the capacitance between the gate and source, vs is the saturation velocity of carriers, and L is the gate length. The maximum oscillation frequency, or frequency at which the power gain is unity, is
f max ≈ |
f T |
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R ds |
(8.9) |
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2 |
√R g + R i + R s |
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The cutoff frequency can be made high if the gate is made short. Typically L is below 1 m m.
A high electron mobility transistor (HEMT) or heterojunction field-effect transistor (HFET) is a MESFET based on a heterojunction. In the HEMT shown in Figure 8.8 an interface between n -type AlGaAs and undoped GaAs forms the heterojunction. At the interface, on the side of GaAs, a very thin potential well is formed, due to the mismatch of energy bands. The potential well is so thin that the electrons attracted by the lower potential form a twodimensional electron gas in the well. Because the electrons drift in the undoped semiconductor, they are not experiencing collisions with impurity ions and therefore their mobility is higher than in a doped semiconductor. Thus, a HEMT is faster than a conventional MESFET. HEMTs made using InP technology are operational up to 200 GHz.
Figure 8.8 Structure of the HEMT.