The unijunction transistor(UJT)

The unijunction transistor(UJT) is a three terminal device with characteristics very different from the conventional 2 junction, bipolar transistor. It is a pulse generator with the trigger or control signal applied at the emitter . This trigger voltage is a fraction (n) of interbase voltage, Vbb.The UJT circuit symbol, junction schematic, and characteristic curve are shown below.

The emitter terminal does not inject current into the base region until its voltage reaches Vp. Once Vp is reached the base circuit conducts and a postive pulse appears at the B1 terminal and a negative pulse at B2. The UJT incorporates a negative resistance region, a low emitter current, and a high output pulse current at terminals B1 and B2, making it an ideal pulse trigger. A simple RC timer circuit using a UJT is shown below.

The very basic specifications of a UJT are:

(a) Vbb(max) - The maximum interbase voltage that can be applied to the UJT
(b) Rbb-the interbase resistance of the UJT
(c) n - The intrinsic standoff ratio which defines Vp.
(d) Ip - The peakpoint emitter current

Here are a few of our most requested UJT's:

PART NO.	RBBO Interbase Resistance @VBB=3V IE=0(KOhms)	h Intrinsic Standoff Ratio @VBB=10V	Iv Valley Current Min. (mA)	Ip Peak Point Emitter Current Max. (µA)	VOB1 Base One Peak Pulse Voltage Min. (V)	Package Style
2N489	4.7 - 6.8	.51 - .62	8	12	--	T0-5
2N490	6.2 - 9.1	.51 - .62	8	12	--
2N491	4.7 - 6.8	.56 - .68	8	12	--
2N1671	4.7 - 9.1	.47 - .62	8	25	--
2N2160	4.0 - 12.0	.47 - .80	8	25	3
2N2646	4.7 - 9.1	.56 - .75	4	5	3	TO-18
2N2647	4.7 - 9.1	.68 - .82	8	2	6

Programmable Unijunction Transistors (PUT) supplied by American Microsemiconductor

PART NO	Gate to Anode Reverse Voltage Max. (V)	DC Anode Current Max. (mA)	IGAO Leakage Current @40V Max. (nA)	IV Valley Current Min. @RG =10k (µA)	Vo Output Voltage Min. (V)
2N6027	40	150	10	70	6
2N6028	40	150	10	25	6