Single junction transistor

Schematic diagrams of single-junction transistors

with n-type base

with p-type base

The structure of the transistor 2T117B:
The large contact is the emitter, the small contact is B1, the lower side of the crystal is B2

A single- junction transistor (double-base diode, OPT) is a semiconductor device with three electrodes and one pn junction . The single-junction transistor belongs to the family of semiconductor devices with a current-voltage characteristic having a section with negative differential resistance .

Content

Design and designation

The base of the transistor is a semiconductor crystal (such as an n type), which is called the base. At the ends of the crystal, there are ohmic contacts B1 and B2, between which there is a region having a rectifying contact E with a p- type semiconductor acting as an emitter.

It was produced in the USSR and had the designation KT 117A (B, C, D). Foreign analogues 2N6027, 2N6028. Available now. See English page option

History

The design of the device relates to alloy structures on germanium bars, first described by Shockley , Pearson and Hines. At that time, such a structure was called a filament transistor. In the process of development, the device had a three-dimensional structure, then diffusion-planar and, finally, epitaxial-planar. Its name changed from “dual-base diode” to the last “single-junction transistor”.

Principle of Operation

The amplifying and switching properties of an OPT are caused by a change in the resistance of the base as a result of injection of minority charge carriers into it ^[1] .

Equivalent Equivalent Scheme

Current-voltage characteristic of a single-junction transistor.

The principle of operation of a single-junction transistor is convenient to consider using the equivalent circuit, where the upper resistance $R_{B1}$ ${\ displaystyle R_ {B1}}$ and lower resistance $R_{B2}$ ${\ displaystyle R_ {B2}}$ - resistance between the corresponding conclusions of the base and the emitter, and the diode shows the emitter pn junction.

The current flowing through the resistance $R_{B1}$ ${\ displaystyle R_ {B1}}$ and $R_{B2}$ ${\ displaystyle R_ {B2}}$ creates a voltage drop on the first of them, biasing the diode D in the opposite direction. If the voltage at the emitter Ue is less than the voltage drop across the resistance $R_{B1}$ ${\ displaystyle R_ {B1}}$ - the diode D is closed, and only the leakage current flows through it. When the voltage Ue becomes higher than the voltage across the resistance $R_{B1}$ ${\ displaystyle R_ {B1}}$ , the diode begins to pass current in the forward direction. In this case, the voltage drop across the resistance $R_{B1}$ ${\ displaystyle R_ {B1}}$ decreases, which leads to an increase in current in the circuit D- $R_{B1}$ ${\ displaystyle R_ {B1}}$ , which in turn causes a further decrease in the voltage drop across $R_{B1}$ ${\ displaystyle R_ {B1}}$ . This process is an avalanche. Resistance $R_{B1}$ ${\ displaystyle R_ {B1}}$ decreases faster than the current increases through the pn junction, as a result, a region of negative resistance appears on the current-voltage characteristic of a single-junction transistor. With a further increase in current, the dependence of the resistance $R_{B1}$ ${\ displaystyle R_ {B1}}$ the current decreases through the pn junction, and for values greater than a certain value I _{off, the} resistance does not depend on the current (saturation region).

With a decrease in the bias voltage U _{cm, the} current – voltage characteristic shifts to the left and, in the absence of it, turns into the characteristic of the open pn junction.

OPT parameters

The main parameters of single-junction transistors are:

interbase resistance $R_{BB}=R_{B1}+R_{B2}$ ${\ displaystyle R_ {BB} = R_ {B1} + R_ {B2}}$
gear ratio $\eta$ ${\ displaystyle \ eta}$ characterizing the switching voltage and is determined by the formula

\eta ={\frac {R_{B1}}{R_{B1}+R_{B2}}}={\frac {R_{B1}}{R_{BB}}}

{\ displaystyle \ eta = {\ frac {R_ {B1}} {R_ {B1} + R_ {B2}}} = {\ frac {R_ {B1}} {R_ {BB}}}}

operating voltage Ucp is the minimum voltage at the emitter junction required for the device to transition from a state with high resistance to a state with negative resistance
turn-on current Ion - the minimum current required to turn on a single-junction transistor, that is, transfer it to the negative resistance region
off current I off - the smallest emitter current holding the transistor in the on state
off voltage U off - voltage at the emitter junction at a current through it equal to I off;
emitter return current Ieo - leakage current of a closed emitter junction

Application

Single-junction transistors have been widely used in various automation devices, pulsed and measuring equipment - generators, threshold devices, frequency dividers, timers, etc. Although the main function of an OPT is a switch, a relaxation generator is mainly a functional unit among most schemes on an OPT.

Due to the relatively large base volume, single-junction transistors are inferior to bipolar transistors in frequency characteristics ^[1] .

Notes

↑ ¹ ² V.V. Pasynkov, L.K. Chirkin Semiconductor Devices: Textbook for High Schools - 4th ed. - M.: Higher School, 1987. - 478 p. silt

Literature

Zi S.M. Physics of Semiconductor Devices. - M .: Energy, 1973.
Z. S. Physics of Semiconductor Devices = Physics of Semiconductor Devices. - 2nd rev. and add. ed. - M .: Mir, 1984. - T. 1. - S. 248-250. - 456 p. (inaccessible link)
Stepanenko I.P. Fundamentals of the theory of transistors and transistor circuits. - M .: Energy, 1977.
Nefyodov A.V. Domestic semiconductor devices and their foreign analogues, 1980.
Transistors for equipment of wide application. Ed. B. L. Perelman, 1981.
Dyakonov V.P. Single-junction transistors and their analogues. Theory and application. M .: SOLON-Press, 2008 .- 240 p.
Dyakonov V.P. Avalanche transistors and thyristors. Theory and application. M .: SOLON-Press. 2008.- 384 p.
Pasynkov V.V., Chirkin L.K. Semiconductor devices: Textbook for high schools. - 4th rev. and add. ed. - M .: Higher school, 1987 .-- S. 272-275. - 479 p.

[uth-1] ¹ ² V.V. Pasynkov, L.K. Chirkin Semiconductor Devices: Textbook for High Schools - 4th ed. - M.: Higher School, 1987. - 478 p. silt