Introduction to 2n2646

Introduction to 2n2646

• 2n2646 is a three terminal semiconductor device that contains only one junction that behaves like an electrically controlled switch.
• This uni-junction transistor (UJT) can not be used as a linear amplifier but it is widely used in triggered oscillators, free-running oscillators, and pulse generation circuits of low frequencies.
• It is composed of n-type semiconductors where p-type material is embedded along its length that helps in fixing the parameter like intrinsic stand off ratio which can be denoted as ?.
• Typical uni-junction 2n2646 contains three terminals named as emitter(E), base 1(B1) and base 2(B2).
• Sometimes it is also referred as a double base diode.
• The base is composed of lightly doped n-type silicon. The emitter is heavily doped and contains p-type material.
• The name of the device is based on the single junction formed between n-type and p-type material.
• When the emitter is behaved as open-circuit, the resistance between B1 and B2 is called inter-base resistance.

• The emitter junction is closed to B2 than B1, so this device doesn't encompass a symmetrical structure.
• Only a fraction of current will flow from B1 to B2 when there is no potential difference between emitter and base terminals.
• However, if large voltage is applied at the emitter terminals then a very large current will flow from B1 to B2 which ultimately generates larger B2 output current.
• The structure of this UJT is quite similar to N-Type JFET, but the gate surface in N-Type JFET is much larger than the emitter junction in UJT.
• This UJT normally functions when emitter junction is forward biased while JFET normally functions when gate junction is reversed biased.
• It is also referred as a current controlled with negative resistance.

2n2646 Pinout

• This 2n2646 is designed for industrial applications where circuit economy is taken into consideration.
• It is an ideal choice for firing circuits that are widely used in silicon controlled rectifiers.

Working of 2n2646

• When this device is triggered it allows the emitter current to increase until it is limited by the help of emitter power supply.
• It can be used as an oscillator because it exhibits a negative resistance.
• This UJT gets biased when positive voltage is applied between the two bases. It will cause a potential drop in the device.
• The current will start to flow from emitter to base region, when emitter voltage is exceeded one diode voltage above the voltage where diffusion of P-type material occurs.
• This additional current at the base region allows the resistance between emitter and B2 lead to decrease, hence overall improves the conductivity of the device.
• This resistance drop will make the emitter junction forward biased which helps in injecting the more current.
• This drop in resistance makes the device operate over negative resistance that's why it is useful for oscillator circuits.

Absolute Maximum Ratings

• Stresses above than the given values can damage the device.
• Also if stresses are applied for extended period of time above normal operating values, they can effect the device reliability.

Electrical Characteristics

• It is important to note that these electrical characteristics don't effect the performance of the transistor.
• The 2n2646 is mainly manufactured for the purpose where low peak point emitter current, and low emitter leakage current is required.

Applications

• 2n2646 is mainly used in pulse and timing circuits.
• It is a building block of thyristor triggering and sensing circuits.
• Free-running oscillators and pulse generation circuits with low frequencies use this UJT because it exhibits negative resistance.