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129

Lab Experiments

Experiment-57

F

UJT CHARACTERISTICS B K Uma Prasad Kamaljeeth Instrumentation & Service Unit, Bangalore –560 094, INDIA.

Using 2N2646 Silicon UJT variations in the base resistance (RB1) is studied with emitter current (IE). The emitter characteristic curve is drawn and the values of valley point and peak point voltage and current are determined. Intrinsic standoff ratio (η), resistances RB1, RB2, RBB are determined. The emitter characteristic curve is displayed on the CRO.

Introduction Unijunction transistor (UJT)[1] is a single junction three-terminianal special semiconductor device used in pulse and switching circuits. It exhibits special characteristics; a negative resistance region in the characteristic curve. It consists of a N type silicon bar along one side of which, about 70% distance from the bottom a P type island is doped. The region where the P Island meets with N bar is the uni-pn-junction. The P material is called the emitter and the top of the bar close to the emitter is called Base-2 (B2), the bottom of the bar is called the Base-1 (B1). Figure 1 shows the structure of UJT. B2

Base B2 Emitter Diode Emitter E

p

N

Fixed Resistor RB2

E

Variable Resistor RB1 Base B1

B1

B2 E

B1 Schmatic Symbol

Figure-1, UJT structure, equivalent circuit, and schematic symbol The emitter E is heavily doped having many holes. The N type bar is lightly doped such that the bar resistance is 5 to 10 K Ohm with emitter open. The emitter forms a pn junction equivalent to a diode as shown in the equivalent circuit. The N terminal of the diode is connected to voltage divider formed by the bar. The resistance RB2 of base-2 is of lower value and resistance of the base-1 RB1 is of higher value because of the positioning of the pn doping. Further, by applying a positive voltage to base-2 with respect to base-1, the emitter ejected holes travel down towards the base-1. This makes variations in base resistance RB1. Hence, base-1 resistance is a variable resistance and base-2 resistance is a fixed resistance as shown in Figure-1. The resistance RB1 varies fro 50 Ω to 5KΩ. The base-to-base resistance is given by Vol-3, No-2, JUNE-2003

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Lab Experiments

R BB = R B1 + R B2 When IE = 0

…1

The resistances RB1 and RB2 cannot be measured using multimeter. However, measuring RBB and determining intrinsic standoff ratio RB1 and RB2 can be determined. By applying a dc voltage (VBB) across the two base terminals of the UJT, the voltage at the junction of RB1 and RB2 can be written as VBR1 =

R B1 VBB = ηVBB R B1 + R B2

Where

…2

η is called intrinsic standoff ratio VBB is voltage across the two base terminals.

The voltage VBR1 is the internal voltage, which cannot be measured. However, an estimation of it can be done. The emitter terminal of UJT is the p-type semiconductor, the n-type side forms the voltage divider, and at the junction of two resistors; the voltage is given equation-2. With emitter, open there is no flow of current. To make emitter diode conduct, the emitter voltage VE, VE = VBR1 + 0.6V = VP

…3

Where, 0.6V is the forward voltage drop across the emitter diode. This is minimum voltage required to make UJT conduct and this voltage is called peak-point voltage VP. Once current starts flowing through the UJT the emitter voltage drops and resistance RB1 starts decreasing. In addition, it attains a low value at voltage called valley-point voltage. Between peak-point and valley point, UJT has negative resistance characteristics. Further increase in the emitter voltage increase the emitter current. The emitter voltage increases slightly and in this region, UJT has positive resistance; This region is called saturation region. The decrease in the resistance in the active or the negative resistance region is due to holes injected in to the n-type slab from the p-type, when conduction is established. The increased hole concentration in the n-type slab results in the increase of free electrons in the slab, producing an increase in the conductivity and corresponding drop in the resistance. From equations 2 and 3,the intrinsic standoff ratio is given by

η=

VP − 0.6 VBB

…4 Table-1 Parameters Maximum ratings Power dissipation 300mWatt Emitter Current 50mA Emitter reverse voltage 30V Intrinsic standoff ratio 0.6 to 0.7V Maximum ratings of 2N2646 UJT

Peak-point voltage VP can be determined experimentally for different value of VBB. UJT 2N2646 is available in metal cane and plastic package. Table-1, gives maximum rating of UJT and Vol-3, No-2, JUNE-2003

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Figure-2 shows base diagram. This special semiconductor device is used to generate pulses, which are usefull in triggering circuits. In particular, it is used with SCR and Triacs in power control applications [1]. B2

B1 B2 E B1

E Bottom View

Top View Plastic Package

Metal can Package

Figure-2, Base diagram of 2N2646 UJT

Instruments Used UJT characteristics experimental setup consisting of 0-10V variable dual power supply, digital dc voltmeter 0-20V, digital dc milliammeter 0-20mA, DMM and dual trace CRO.

Components Used UJT 2N2646

Experimental Procedure The experiment consists of three parts namely Part-A, Determination of channel resistance RBB Part-B, IE-VE characteristics and determination of η Part-C, Variation RB1 with emitter current

Part-A, Determination of channel resistance RBB 1. The circuit connections are made as shown in Figure-3. The emitter is left open.

IBB

E

B2

VBB V

0-10V

B1

Figure-3, Circuit connections to determine channel resistance Vol-3, No-2, JUNE-2003

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2. The voltage VBB is set to 1 volt and corresponding current flowing through the channel is recorded in Table-2. 3. Trial is repeated by varying VBB is steps of 1 volt up to a maximum of 10volts. In each case, the current flowing is noted and the channel resistance is calculated. The average value obtained tallyed with value measured (6.2K) using DMM. Table-2

VBB (V) 1 2 3 4 5 6 7 8 9 10

IBB (mA)

R BB =

VBB I BB

0.16 6.25K 0.33 6.06 0.49 6.12 0.65 6.15 0.81 6.17 0.96 6.25 1.10 6.36 1.25 6.40 1.38 6.52 1.61 6.62 Average RBB = 6.29KΩ

Channel resistance calculation

Part-B, IE-VE characteristics and determination of η IE 1K

E

VBB 0-10V

VEE 0-10V

B2

VE

B1 V

Figure-4, Circuit connections to determine IE-VE characteristics

4. Complete circuit connections are made as shown in Figure-4. VBB is set to 8 volts by adjusting VBB power supply. While monitoring voltmeter emitter voltage VE is varied slowly. When VE reaches peak voltage point suddenly, voltage starts dropping. This is the indication of current flow in the UJT. The peak voltage at which conduction starts is noted in Table-3. This procedure is repeated for 2-3 times to confirm the exact peak voltage. VP = 5.6V 5. Further increase in the VE value increases the current. The current and voltages are noted in Table-3.

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6. Trial is repeated by varying VBB to 10 volts and 4 volts. In each case peak-point, voltage is determined and the variations in emitter current with emitter voltage are recorded in Table-3. 7. A graph showing the variation in emitter current with emitter voltage is drawn. This is the IEVE characteristic. From the characteristics curve, the peak-point current, valley-point current and valley-point voltages are noted as shown in Figure-5.

VE(V) 0 5 5.6 1.15 1.16 1.17

VBB = 8V IE(mA) RB1(KΩ) 0 ∞ 0 ∞ 0 ∞ 4.56 1.096 7.09 0.705 8.90 0.561

VE(V) 0 6 6.92 6.99 7.13 1.18

Table-3 VBB = 10V IE(mA) RB1(KΩ) 0 ∞ 0 ∞ 0 ∞ 0 ∞ 0 ∞ 8.88 0.735

VBB = 4V IE(mA) RB1(KΩ) 0 ∞ 0 ∞ 0 ∞ 2.55 1.164 5.30 0.560 8.93 0.332

VE(V) 0 3 3.57 1.08 1.10 1.14

Variation of emitter current with emitter voltage at various VBB

η=

VP − 0.6 5.6 − 0.6 = = 0.625 VBB 8

R B1 =

V RB1 IE

=

ηV BB IE

=

0.625x8 4.56

= 1.096KΩ

6 Vp Negative Resistance Region

Emitter voltage (V)

5 4 3

Valley point

2 Vv

1 0 0

2

4

6

Emitter current (mA)

Figure-5, IE-VE characteristics of 2N2646 UJT From the Figure-5, VP = 5.6V, IP = 0, VV = 0.9V, IV = 5.3mA

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8. Experiment is repeated for VBB = 10volts and 4volts. The corresponding value of η ,VP,, IP , VV are determined. The readings obtained are tabulated in Table-3.

Part-C, Variation RB1 with emitter current

Base resistance RB1(K Ohm)

9. A graph is drawn taking base resistance RB1 along Y-axis and emitter current along X-axis. The graph is shown in Figure-6

0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0

2

4

6

8

10

Emitter current (mA)

Figure-6, Variation of base resistance with emitter current

Results The results obtained are tabulated inTable-4. The characteristics curve is observed on CRO [2] using time varying input to emitter instead of dc input. The characteristic curve observed is shown in Figure-7. Table-4 VBB (V) 10V 8V 4V η 0.653 0.625 0.594 IV (mA) 6.1 5.3 2.5 IP (mA) 0 0 0 VV (V) 1.00 0.91 1.00 VP (V) 7.73 5.60 2.57 RBB (KΩ) 6.29K Various electrical parameters of 2N2646 UJT

Figure-7, Characteristics of 2N2646 Vol-3, No-2, JUNE-2003

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Discussions The negative resistance region is used in pulse generation for triggering thyristors. The UJT can produce three different types of pulses at a time. For a given UJT, only the valley-point voltage (≈ 1V) and channel resistance RBB (6.29K) are constants. The η value is found to vary with base voltage. However, for practical design application the average value (0.612) can be taken. The variations in the η value are due to the n-channel. There is a flow of majority carrier electrons in the channel due to base potential. This adds to the emitter current. Hence, the emitter current is algebraic sum of pn junction diode current and n-channel current.

References [1]

Lious Nasheeisley Robert Boylestad, Discreate and Integrated Devices, Page 291.

[2]

Dr K V Acharya, LE Vol-3, No-1, March-2003, Page-33.

Vol-3, No-2, JUNE-2003