A programmable UJT (PUT) is not a unijunction transistor at all but.a four-layer device. Its name is derived from the fact that its characteristic curve and many of its applications are similar to those of a UJT. Figure 18-53 shows the constructionand schematic symbol of a PUT, which resembles an SCR more than a UJT. Like an SCR, its three terminals are designated anode (A), cathode (K), and gate G). However, note thut the gate is connected to the N region below the uppermost PN junction, like the anode gate in all SCS (Figure 11’\-(6).
In applications, the gate of the PUT is biased positive with respect to the cathode. If the anode is then made about 0.7 V more positive than the gate, the device regeneratively switches on. Thus. the PUT is “programmed” by its gate-tocathode bias voltage. Figure IH-S4(a) shows a typical bias arrangement, where the gate voltage is obtained from a resistive voltage divider. Note that VAK and Vou must be separate voltage sources, since V(i, which is derived from Villi, must remain constant, while v,,~.varies. Figure IX-54(b) shows the characteristic curve. Note the resemblance of the characteristic to that of a UJT. The peak and valley points have the same significance as in a UJT. The advantage of a PUT is that it has a smaller on resistance than a UJT and can handle heavier currents.
A tunnel diode, also called an Esaki diode after its inventor, is constructed with very heavily doped P and N materials, usually germanium or gallium arsenide. the doping level in these highly conductive regions may be from 100 to severel ttiousnnt) times that of a conventional diode. As a consequence, the depletion region at the PN junction is extremely narrow. Recall that forward conduction in a conventional diode occurs only if the forward bias is sufficient to give charge carriers the energy necessary to overcome the barrier potential opposing their passage through the depletion region. When a tunnel diode is only slightly forward biased, many carriers are able to cross through the very narrow depletion region without acquiring that. energy. These carriers are said to tunnel through the depletion region because their energy level is lower than the level of those that overcome the barrier potential.
Because of the tunneling phenomenon, there is a substantial flow of current “through a tunnel diode at the onset of forward bias, lhc same i~true when the diode is reverse biased. This behavior is evident in the J- V characteri.tic of a tunnel diode. shown in Figure 18-56. Notice the steep slope (small resistance) of the characteristic in the region around the origin. For comparison purposes. the dashed line shows the characteristic of a conventional diode. With increasing forward bias, the tunneling phenomenon continues, until it reaches a maximum at Ip in the figure. The forward bias at this point is VI’. Because of higher energies acquired by carriers on the. N side, current due to tunneling ..begins to diminish rapidly with further increase in forward bias. As a result, the characteristic displays a negatiue-resistance region, similar to that of a UJT, where increasing voltage is accompanied toy decreasing current. Wh ~n the forward bias becomes large enough to supply earners with the energy required to surmount the barrier potential, forward current begins to increase again. As shown in the figure, the fo-ward characteristic beyond that point coincides with the characteristic of a conventional diode
Figure 18-57 shows the equivalent circuit of a tunnel diode in its negativere sistance region and several commonly used schematic symbols. The device ‘is most often used in high-frequency or high-speed switching circuits, so the junction capacitance C” and the inductance of the connecting leads, L.r, arc important device parameters. C, may range from 5 to 100 pF, while L. is on the order or a few r-ace icarics. ote the negative resistance labeled – R”, typically -10 n to -200 n. R. is the resistance of the leads and semiconductor material, on the order ~ln ‘.. The negative-resistance characteristic of a tunnel diode has fostered some applications that are unusual among two-terminal devices, including oscillators, amplifiers. and high-speed electronic switches. In the latter application, the tunnel diode is switched between .its peak and valley points with nano- or picosecond switching times. The peak voltage is typically rather small, less than 200 mY. but th,\peak current may range from I·to 100 mA.
VOLTAGE·VARIABLE CAPACITORS (VARACTOR DIODES)
A uoltagc-uariable capacitor (YYC), also called a uaractor: oaricap, or tuning diode. is a diode constructed for use in high-frequency circuits where it is desired to control or adjust capacitance values by varying a de voltage level. I3y virtue of its construction, a reverse-biased diode has all the essential ingredients of a capacitor: t vo conducting regions (P and N regions) separated by a dielectric (the depiction region). Recall that the capacitance of such a structure is given by
where e is the permittivity of the dielectric. A is the cross-sectional surface area ot the conducting regions, and tI is the distance separating the regions (the thickness, or width, of the dielectric). Also recall that the width of the depiction region of ” a reverse-biased diode increases as the reverse-biasing voltage increases. Thus. increasing the reverse bias on a diode causes d to increase and the capacitance f. C ~ e”Ald to decrease. This behavior is the fundamental principle governing the .. operation of a varacior diode. The value of the capacitance obtained from a varactor is small. on the order of 100 pF or less, and it ·is used in practice only to alter the ac impedance it presents to a high-frequency signal. as, for example. in u tuned LC network. It would not be useful for applications such as a low-frequency bynass capacitor, a filter capacitor in a power supply, or a coupling capacitor in an audio amplifier.
An important characteristic of a varactor diode is the ratio of its largest to its smallest capacitance when the voltage across it is adjusted through a specified range. Sometimes called the capacitance tuning ratio, this value is governed by the doping profile of the semiconductor material used to form the P and N regions, that is, the doping density in the vicinity of the junction. In diodes having an abrupt junction, the P and N sides arc uniformly doped and there is an abrupt transition from P to N at the junction. Abrupt-junction var ctors exhibit capacitance ratios from about 2: 1 to 3: 1. In the hyperabrupt (extremely abrupt) junction, the doping level is increased as the junction is approached from either side, so the P material becomes more heavily P near the junction, and the N material becomes more heavily N. The hyperabrupt junction is very sensitive to changes in reverse voltage and this type of varactor may have a capacitance ratio up to 20: I. Figure 18-58 shows typical plots of abrupt and hypcrabrupt varactor capacitance versus reverse voltage. The figure also show the schematic symbol for a varactor diode
Figure 18-59 shows a tuned amplifier with a fixed inductor and a varactor diode that form an LC tank circuit driven by the transistor. The frequency of maximum amplification is the resonant frequency of the tank, which is closely approximated by
where C,I is the varactor-diode capacitance. Capacitor <;1: is a coupling capacitor that .blocks the flow of de current between the transistor and the diode circuitry. Cr is much larger than C, and does not affect resonance. The RI-Rz voltage divider is used to adjust the reverse-biasing voltage across the varactor diode and thus to adjust the resonant frequency of the amplifier. Resistor R, is a large resistance used to prevent the voltage divider from loading the tank network. Since the de current in R, is the very small reverse current through the varactor diode, the de drop across R, is negligible and the varactor voltage is. for all practical purposes, the
voltage VH shown across the divider.
The varactor diode in Figure 18-59 has the abrupt-junction characteristic shown in Figure 18-58. If Vcc = 10 V, L = 80 #LH, I?I = I kH. and R! is a lO-kH potentiometer, rind the frequency range over which the amplifier can he tuned.
The value of VH is minimum when the wiper arm of the potentiometer is at its topmost position in Figure 18-59. In that casccby voltage-divider action,