rapidly in the regions beyond 1GHz. Average axial and circuit. If the battery is connected as indicated, a voltage plastic package for use below 1GHz are available gradient is set up in the high resistivity n -type bar. As a from 15s. to £4, with metal encapsulations ranging from result the position of the junction along the bar deter- 30s. to £7. devices are available from £10 mines the positive potential required at the emitter upwards. terminal to enable the p -n junction to conduct. Thus if the battery is 12V and the junction is midway between base 1 and base 2 -the potential required to Unijunction make the diode conduct is (6V +diode drop) As its name implies the unijunction transistor is a This parameter is sometimes quoted in specifying uni- junction and is known as the peak point three -terminal, single-junction device and its symbol is shown in Fig. 8. The circuit operation is however unlike emitter voltage (Vp). The resistance ratio which deter- either a transistor or diode, as indeed is the construction. mines this potential on the equivalent circuit is r R1 \Rl +R2/ and this is known as the intrinsic standoff ratio (O. Hence to turn on the unijunction transistor the emitter voltage must satisfy the following equation: V emitter =Vp- (R E +R2) VB +Vd 81 = nVB +Vd (1).

Since the intrinsic standoff ratio 17 determines Vp for (a) Base one Base two any supply voltage this is always quoted as a parameter (a) Alloy structure and for any given device has a stated tolerance. Oxide layer When the diode is made to conduct by this potential Vp then the base i resistance R1 reduces rapidly to a Emitter Base one low value -thus causing a rapid increase in emitter current and a B2 drop in emitter voltage towards zero volts. This effect is achieved by the electron input through the emitter causing the reduction in resistivity of the base 1 ©81 region of the bar structure. Base two In order 1 (b) B.S. preffered (b) Diffused structure to reduce the base region to low resistivity a minimum current into the emitter is required. This parameter is Fig. 8a, b (left): Unijunction transistor symbols. called the peak point current (Ip) and is Fig. 9a, b (right): Unijunction transistor constructions. clearly shown in the unijunction characteristic Fig. 11. VE 4 Peak The structure consists of a block of resistive semiconduc- tor material with a junction formed at a definite position along the length of the bar. Leads are connected to the ends of the bar and are denoted base 1 and base 2 whilst the connection to the junction is referred to as the Fig. 11: Emitter voltage emitter. Hence the designation double base diode which Unijunction was the original term for this device. At present only transistor Valley n-type silicon is used characteristic. for the bar and the junction is Negative formed by alloying or diffusing a p -type impurity at the resistance Saturation required point along the bar. Typical alloy and diffused region region junction constructions are shown in Fig. 9 and although alloy junction types are the cheapest as well as the most IP IV IE widely used the diffused construction is increasingly Emitter current popular due to the low leakage currents and faster switching times. Encapsulation is usually in the familiar The characteristic also shows the TO -18 and TO -5 transistor casing although plastic is region which causes the switching action and the stable used increasingly for the cheaper devices. At present saturation region which is achieved after Ip is exceeded. terminations are not completely standardised and the Ip is usually of the order of 0.5 to 100v.A and n of the manufacturers' data should be consulted. order of 0.5 to 0.85. The total resistance known as the The circuit operation of the unijunction transistor interbase resistance RBBI varies between 4kSI and 12kû. can be understood by referring to Fig. 10 which shows Other parameters supplied by the manufacturer are peak the essential diagrammatic structure and its equivalent emitter current ('2A), power (Po), maximum base to base voltage (VBZ VBI, X30 -50V) and frequency- which can be in the megacycle region. B2 I B2 It can be seen from the characteristics that until the P -type impurity R2 diode conducts the impedance is very high (1 -10Mû) whilst at on it drops rapidly towards zero. When Resistive VB coupled to a charging it is off until Vp is N -type reached, then rapidly conducts and discharges the

R1 capacitor. When the discharge current falls below Ip it off and the capacitor can recharge to Vp. Thus B1 B1 it is useful as a but can also be used (a) (b) (c) for triggering, voltage sampling and in protec- Structure Equivalent circuit Applied polarity tion circuits. The disadvantages of unijunction transistors are that Fig. 10: Basic unijunction structure. the ratio -q varies considerably as it depends on the

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