ENHANCEMENT-MODE MOSFETs

An enhancement-mode MOSFET is normally off because it requires a gate bias signal to cause current flow because of the high impedance of its substrate source-to-drain channel.In the N-channel enhancement-mode MOSFET shown in Fig. 1a, the substrate is P-type silicon and both the source and drain regions are heavily doped N-type silicon. The metal gate, the insulation layer, and the channel act like a capacitor, so if a bias is placed on the gate, a charge of opposite polarity will appear in the channel below it. For example, if the drain voltage is positive with respect to the source voltage, and the bias on the gate is zero, no current will flow.

But, if the gate is then made positive, negative charge carriers (electrons) are induced in the channel between the source and drain regions. Further increases in positive bias induce more electrons into the channel, where they accumulate to form an N-type channel between source to the drain. The value of drain current depends on channel resistance, so gate voltage controls drain current. Because channel conductivity is enhanced by a positive gate bias, the transistor is called an enhancement-mode MOSFET.

Figure 1b shows the schematic symbol for an N-type enhancement-mode MOSFET. The vertical line connected to the gate pin represents the gate, and the broken lines connected to the drain and source pins indicate that a channel does not exist until a gate voltage is applied. The arrowhead representing conventional current points from the P-type substrate to the induced N-type channel

A P-channel enhancement-mode MOSFET has the same geometry as the N-channel enhancement-mode MOSFET except that both the material dopants and the applied voltage polarities are reversed. Its schematic symbol is identical except that the direction of the arrowhead is reversed.


Figure 1 Enhancement-mode N-channel MOSFET: (a) section view, and (b) symbol.

METAL-OXIDE SEMICONDUCTOR FETs (MOSFETs)

The metal-oxide semiconductor FET (MOSFET) offers a higher input impedance than a JFET. A section view of an N-channel MOSFET is shown in Fig. 1a. An insulating layer of silicon dioxide is grown on top of the region between the N-type source and the N-type drain. The gate is electrically isolated from the source and gate contacts and the source to-drain channel beneath it. The schematic symbol for an N-channel MOSFET is shown in Fig. 1b. The two kinds of MOSFETs are enhancement mode and depletion mode. The depletion-mode MOSFET has a lightly doped source-to-drain channel, whereas the enhancement-mode version does not.



Figure 1 Metaloxide semiconductor FET (MOSFET): (a) section view, and (b) symbol.

JUNCTION FETS (JFETs)

The N-channel junction FET (JFET), shown in section view Fig.1a, has an N channel diffused into a P-type substrate and a P-type region diffused or implanted into the N channel to form the P-type gate. Metal deposited directly on the gate, source, and drain regions forms their contacts. Because a JFET has a symmetrical structure, the drain and source are interchangeable. Thus, depending on the location of the ground and the +V power source, the JFET will work in either direction.



Figure 1 Junction field-effect transistors (JFETs): (a) N-channel section view

If a positive voltage is applied at the drain contact and a negative voltage is applied at the source contact with the gate contact open, a drain current flows. If the gate is then biased positive, channel resistance decreases and drain current increases. However, if the gate is biased negative with respect to the source, the PN junction is reverse biased and a depletion region depleted of charge carriers is formed. Because the N-type channel is more lightly doped than the P-type silicon, the depletion region penetrates into the channel, effectively narrowing it and increasing its resistance. If the gate bias voltage is made even more negative, drain current is cut off completely. A gate bias voltage value that will cut off the drain current is called the pinch-off or gate cutoff voltage. The schematic symbol for an N-channel JFET is shown in Fig. 1b. The arrow points from the P-type gate to the N-type channel.


(b) N-channel symbol, (d) P-channel symbol

The P-channel JFET, shown in Fig. 1c, has characteristics similar to those of the N-channel JFET except that the polarities of the voltage and current are reversed. A P-type channel is diffused into an N-type substrate and then an N-type gate region is diffused or implanted into the P-channel to form the N-type gate. If a negative voltage is applied to the drain and a positive voltage is applied to the source, current flows between source and drain. But if the gate is made more negative more current will flow, while if it is made positive with respect to the source, current will be cut off.


(c) P-channel section view

The schematic symbol for a P-channel JFET is shown in Fig. 1d. The arrow points from the P channel to the N gate.

Field-Effect Transistors

A field-effect transistor (FET) is a voltage-operated transistor. Unlike a BJT, a FET requires very little input current, and it exhibits extremely high input resistance. There are two major classes of field-effect transistors: junction FETs (JFETs) and metal-oxide semiconductor FETs (MOSFETs), also known as insulated-gate FETs (IGFETs). FETs are further subdivided into P- and N-type devices. FETS are unipolar transistors because, unlike the BJT, the drain current consists of only one kind of charge carrier: electrons in N-channel FETs and holes in P-channel FETs.

FETs and MOSFETs are both made as discrete transistors, but MOSFET technology has been adopted for manufacturing power FETs (see “Power Transistors” later in this section) and ICs. There are both NMOS and PMOS ICs. When both P- and N-channel MOSFETs are integrated into the same gate circuit, it is a complementary MOS (CMOS).

Darlington Transistor Pairs

A Darlington pair, as shown in the schematic Fig.1, is a pair of BJTs in which the emitter of the first transistor is connected to the base of the second transistor. This configuration provides far higher current gain than a single transistor through direct coupling. The pair can be made on a single die and it is packaged in a three-terminal transistor case. The pairs are often used in linear ICs, such as operational amplifiers, and in power amplifier output stages. Its most common application is that of an emitter follower. The output is taken across a resistor from the emitter of the second transistor to ground. The input resistance at the base of the first transistor is raised to a higher value than that of a singletransistor emitter-follower circuit.



Figure 1 Darlington transistor pair symbol.

Bipolar Junction Transistors (BJTs)

The term transistor implies a silicon bipolar junction transistor (BJT) unless modified by an adjective such as JFET or MOSFET. BJTs can be can be made in two different configurations: NPN and PNP. Figure 1 shows a section view of an NPN BJT transistor. Here the letter N indicates silicon doped with an N-type material, which, by convention, means that it contains an excess of negatively charged electrons. The letter P indicates silicon doped with a P-type material, which means it has an excess of positively charged holes.


Figure 1 NPN bipolar junction transistor (BJT) structure.

A voltage applied to the P-type base in the NPN transistor causes electrons to flow from the N-type emitter through the base to the N-type collector. (Conventional current is considered to flow in the opposite direction). This BJT has vertical topology, so its metal base contact is deposited on the P-type base next to the metal emitter contact on the N-type emitter, while the collector contact is a metal layer on the bottom of the N-type collector.

Electrons in an NPN transistor cannot flow from the emitter to the collector through the P-type base unless a positive bias is placed on the base contact and a positive voltage is applied to the collector contact. Then holes, repelled by the positive bias, enter the emitter region while electrons flow from the emitter region to the base region. Most of the injected electrons complete the transit through the base region into the N-type collector region and are collected at its contact.



Figure 2 NPN bipolar junction transistor: (a) section view, and (b) symbol.

Figure 2a shows a simplified section view of the NPN BJT, and Fig. 2b shows its schematic symbol. The direction of the arrow represents conventional current flow directed from its P-type base to its N-type emitter.



Figure 3 PNP bipolar junction transistor: (a) section view, and (b) symbol.

Figure 3a shows a simplified section view of a PNP BJT, and Fig. 3b shows its schematic symbol. It can be seen that the polarities and doping of NPN and PNP transistors are reversed. The PNP BJT schematic symbol has its arrow directed from its P-type emitter to its N-type base.

Signal-Level Transistors

A transistor is a three-terminal semiconductor device capable of amplification and switching. It is essentially the solid-state analogy of the triode vacuum tube. There are two principal classes of transistors: bipolar junction transistors (BJTs) and field-effect transistors (FETs). These transistors are made as discrete small-signal and power devices. Variations of them are integrated into digital and analog or linear ICs. Small-signal discrete BJTs remain popular in low-frequency circuits, while small-signal discrete FETs meet the requirements for high-input impedance transistors. Discrete power BJTs are still popular in low-frequency and linear circuits, but discrete metal-oxide semiconductor (MOSFET) transistors are preferred for high-frequency switching.

Rectifier Diodes

A rectifier diode is a diode capable of converting AC into DC. It can conduct 1 A or more or dissipate 1 W or more of power. Most rectifier diodes are now made from silicon. The dies have large PN junctions to eliminate or minimize damage from heat produced by power dissipation. Typically packaged as discrete devices, the rectifiers can be paralleled to increase their power-handling ability. Rectifiers rated for less than 6 A are usually packaged in axial-leaded glass or plastic cases. However, those with 8- to 20-A ratings are usually packaged in flat plastic cases with copper tabs that can act as heat sinks or metal-to-metal interfaces with larger heat-dissipating busbars. Rectifiers rated from about 12 to 75 A are usually packaged in metal cases. Some have threaded base studs for fastening the case directly to a larger heat-dissipating surface.

The most important electrical ratings for rectifier diodes are:

• Peak repetitive reverse voltage VRRM
• Average rectified forward current IO
• Peak repetitive forward surge current IFSM

Standard PN junction rectifiers are specified for linear power supplies operating at input frequencies up to 300 Hz, but they are inefficient in switching power supplies that switch at frequencies of 10 kHz or higher because of their slow recovery time. This is the finite amount of time required for the minority and majority carriers—electrons and holes—to recombine after a polarity change of the input signal. The minority carriers must be removed before full blocking voltage is obtained.

Despite their slow recovery time, standard PN junction rectifiers have lower reverse currents, can operate at higher junction temperatures, and can withstand higher inverse voltages than faster rectifiers designed to overcome this speed limitation.

Three types of fast silicon rectifiers perform more efficiently at the higher-frequency switching rates:

1. Fast-recovery rectifiers.
2. Ultrafast- or superfast-recovery rectifiers.
3. Schottky rectifiers.

FAST-RECOVERY RECTIFIERS

A fast-recovery rectifier is a PN junction rectifier made by diffusing gold atoms into a silicon substrate. The gold atoms accelerate the recombination of minority carriers to reduce reverse recovery time. These rectifiers can be switched in 200 to 750 ns. They have current ratings of 1 to 50 A and voltage ratings to 1200 V. Forward voltage drop is typically 1.4 V, higher than the 1.1 to 1.3 V of the standard PN junction. The maximum allowable junction temperature is about 25°C. This value is lower than that for a standard PN junction. The maximum reverse voltage for a fast-recovery rectifier is about 600 V.

ULTRAFAST- OR SUPERFAST-RECOVERY RECTIFIERS

An ultrafast- or superfast-recovery diode is a PN junction rectifier whose reverse recovery time is between 25 and 100 ns. Gold or platinum is also diffused into the silicon wafers from which the rectifier is made to speed up minority carrier recombination. These rectifiers are specified for power supplies with output voltages of 12, 24, and 48 V.

SCHOTTKY RECTIFIERS

A Schottky rectifier has a metal-to-semiconductor junction rather than a PN junction, so it does not have minority charge carriers. The die is in direct contact with one metal electrode, so recovery time, although not specified, is typically less than 10 ns. Recovery current is principally caused by junction capacitance. Schottky rectifiers provide lower forward voltages (VF) than the PN rectifiers (0.4 to 0.8 V vs. 1.1 to 1.3 V). Hence power dissipation is lower and efficiency is higher. One drawback of the Schottky rectifier is its low blocking voltage, typically 35 to 50 V. However, Schottky rectifiers with maximum blocking voltages of 200 V are available. These rectifiers require transient protection, and they have inherently higher leakage current (IRRM) than PN junction rectifiers. This makes them more susceptible to destruction by overheating (thermal runaway). Schottky rectifiers can be paralleled in the output stages of switching power supplies, where they are usually used with output terminals rated for 5 V or less.

VARACTOR DIODES

A varactor diode, also known as a voltage-variable capacitor diode or varicap, is a reversebiased PN junction whose operation depends on the variation of junction capacitance with reverse bias. Special dopant profiles are grown in the depletion layer to enhance this capacitance variation and minimize series resistance losses.

The varactor is made from a semiconductor material whose dopant concentration is graded throughout the device, with the heaviest concentration in the regions adjacent to the junction. The junction region is small to take advantage of the variation of junction capacitance with reverse voltage. Varactor diodes have very low internal resistance so that the PN junction, when reverse biased, acts as a pure capacitor. Because the junction is abrupt, junction capacitance varies inversely as the square root of the reverse voltage.

Most varactor diodes are made from silicon, but gallium-arsenide varactors offer higherfrequency response. Low-power varactors serve as voltage-variable capacitors in electronic tuners, and do phase shifting and switching in the VHF and microwave circuits. They also function as very low frequency multipliers in solid-state transmitters and do limiting and pulse shaping.

Standard varactors can provide 12 W at 1 GHz, 7 W at 2 GHz, 1 W at 5 GHz, and 50 mW at 20 GHz. Efficiencies of 70 to 80 percent have been obtained at 1 and 2 GHz. The dimensions of a varactor’s package depend on its operating frequency and power dissipation.

SCHOTTKY BARRIER DIODES

A Schottky barrier diode is a semiconductor diode formed by a semiconductor layer and a metal contact that provides a nonlinear rectification characteristic. Hot carriers (electrons for N-type materials or holes for P-type materials) are emitted from the Schottky barrier of the semiconductor and move to the metal coating that is the diode base. Majority carriers predominate, but there is essentially no injection or storage of minority carriers to limit switching speeds. These diodes are also called hot-carrier or Schottky diodes.

Schottky-clamped transistors used in some transistor-transistor logic (TTL) IC families include Schottky barrier diodes to prevent transistor saturation, thereby speeding up transistor switching. Also, the gates of gallium-arsenide MESFET transistors are actually Schottky barrier diodes.

ZENER DIODES

A zener or reference diode is a silicon PN junction made to operate only under reverse bias or voltage conditions. At a known reverse voltage an avalanche breakdown occurs, indicated by the knee in the curve shown on the left side of Fig. 1a. Beyond that point the reverse voltage remains constant enough to serve as a useful reference voltage. Zener diodes exhibit sharp reverse knees at less than about 6 V. Large quantities of electrons within the depletion region break the bonds with their atoms, causing a large reverse current to flow, as indicated by the vertical dropoff of the curve.



Figure 1 Characteristic curves for a PN diode: (a) forward bias (right) and reverse bias (left), and (b) symbol for zener diode.


Zener diodes are stable voltage references because the voltage across the diode remains essentially constant for wide variations of current. These diodes are used as generalpurpose voltage regulators and for clipping or bypassing voltages that exceed a specified level. Variations of the zener diode called transient voltage suppressors (TVSs) serve as circuit-protective devices because of their ability to bypass unwanted high-input voltage transients.

The schematic symbol for a zener diode is shown in Fig. 1b. It differs from the conventional diode schematic symbol because of its S-shaped anode representation. Zener diodes have nominal reference voltage values from 1.8 to 200 V and power ratings from 250 mW to as high as 50 W. They are packaged in a variety of glass, metal, and plastic cases, some for surface mounting. TVS diodes have ratings from 5 to 300 V, and can handle up to 5 W steady-state or 1,500 W peak power. Although both of these diodes can operate in the small-signal region, they are considered to be regulator and suppressor diodes rather than small-signal diodes.

Small-Signal Diodes

A small-signal diode is a two-terminal silicon PN junction that can rectify and clip signals. Rated to handle up to 1 W, these diodes are made by growing an N-type region on a P-type wafer so that there is a direct interface or junction between the two different materials. The wafer is then diced and packaged with terminals attached to both sides of the die. The P-type material is the anode and the N-type material is the cathode, as shown in the section view Fig.1a. The P-type anode contains a surplus of “holes,” or vacant sites that can be filled by electrons to conduct current, and the N-type cathode contains a surplus of electrons. The schematic symbol for a diode is shown in Fig.1b. The arrowhead indicates the direction of conventional current flow, but this is opposite to electron flow, indicated by the arrow pointed in the opposite direction.



Figure 1 PN diode: (a) functional diagram, and (b) schematic symbol.

If a positive voltage is applied to the anode and a negative voltage is applied to the cathode, or it is connected to ground, the diode is forward biased. Electrons flow from the cathode across the PN junction to the anode, but conventional current is considered to flow in the opposite direction. However, if a negative voltage is applied to the anode and a positive voltage is applied to the cathode, or it is connected to ground, the diode is reverse or back biased, as shown in Fig. 2. Under these conditions there will be little or no electron flow across the PN junction. A reverse-biased diode effectively becomes an insulator with resistance measurable in megohms because of the expansion of the highly resistive depletion region that forms around the PN junction.



Figure 2 Depletion region of PN junction diode.

Surface Acoustic Wave (SAW) Filters

A surface acoustic wave (SAW) filter is a solid-state filter that can replace a conventional passive inductive-capacitive LC filter. It offers excellent amplitude and phase response over wide bandwidths and frequency ranges. SAW filters are made from piezoelectric materials such as lithium niobate (LiNbO3) and quartz. A filter made from quartz offers excellent temperature stability over wide temperature ranges, and a lithium-niobate filter simplifies electromagnetic-to-acoustic coupling. These filters have relatively high insertion losses, so they typically require an amplifier in series with the SAW to recover lost signal strength.



Figure 1 Crystal in holder.

Crystal Frequency Standards

Crystals used as frequency standards are made from piezoelectric materials that resonate at high frequencies when subjected to an alternating current. Selectively cut quartz crystals generate more stable frequencies than coil-and-capacitor tank circuits. Crystals for generating frequencies for timing or other purposes are packaged in radial-leaded metal cases, as shown in Fig. 1.

Quartz wafers are ground to precise thicknesses, and metal-film electrodes are deposited on both sides. The electrodes are connected to the leads that extend through the base. When powered by AC, the quartz wafer vibrates at a frequency determined by its thickness. Thin crystals resonate at higher frequencies than thick crystals. The highest fundamental frequency of a quartz crystal wafer is 15 to 20 MHz. Harmonics or multiples of this frequency provide higher radio frequencies. Quartz crystals in holders serve as oscillator tank circuits. Crystals can also serve as selective filters because of their high Q factors.