JPS59181722A - Electrostatic breakdown protecting circuit - Google Patents

Abstract

PURPOSE:To prevent a signal input stage of a bipolar type integrated circuit from electrostatic breakdown without dropping breakdown strength, by connecting the collector of a transistor and the emitter to an input terminal and an earth, respectively, and connecting a resistance of a specified value between the bas and the emitter. CONSTITUTION:An input clamp diode D becomes on when a negative voltage is applied to an input termianl, and prevents a fact that an abnormal negative voltage is applied to an input stage of an integrated circuit. When a positive voltage of an input IN rises excessively, breakdown is generated in a p-n junction between the collector and the base of an npn type TR Q, a part of a current flows to an earth through a resistance R, and the remaining current flows between the base and the emitter. By ths current, the TR Q is turned on, and in that case, a current flowing between the collector and the emitter in hFE times a current flowing between the collector and the base, therefore, by setting the hFE large, an abnormal current by overvoltage is by passed to an earth, and to say nothing of the collector - the base of the TR Q, electrostatic breakdown of the diode D, etc. is prevented. A value of the resistance R is related to input breakdown strength and electrostatic breakdown voltage, and set to 3-10KOMICRON.

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a circuit that protects a signal input stage of a bipolar semiconductor integrated circuit from electrostatic damage.

Prior Art and Problems Diodes connected with opposite polarities are widely used in the signal input stage of semiconductor devices manufactured using a bipolar process. For example, in the diode input type DTL LogiRec shown in FIG. 1, diodes DI and D2 are connected between the input terminals INK and IN2 and the ground so that their anodes are connected to the ground and their cathodes are connected to the input terminal. This circuit is a diode F'D3.

D4 and transistor Q1. Q2 constitutes an open collector output type two-man powered NAND gate.

The diodes DI and D2 are input clamp diodes provided for the purpose of preventing abnormal operation of the circuit by becoming conductive when a negative voltage is applied to the inputs INI and IN2.

The problem with such a circuit is that when an abnormally positive high voltage is applied to the input, the diodes DI, D2 . This means that the diode is likely to be thermally destroyed because D3 and D4 break down and a current flows through the PN junction of the diode.

OBJECTS OF THE INVENTION The present invention seeks to provide an electrostatic discharge protection circuit that is not easily destroyed and does not require a reduction in the withstand voltage of the input stage.

Structure of the Invention The electrostatic discharge protection circuit of the present invention connects the collector of a transistor to the signal input terminal of a bipolar semiconductor integrated circuit, connects the base of the transistor to the ground, and further connects the collector between the base and emitter of the transistor. This is characterized in that a resistor of 3 to IOKΩ is connected to the resistor, and this will be explained in detail below with reference to the illustrated embodiment.

Embodiment of the Invention Figure 2 is a circuit diagram showing an embodiment of the present invention, where Q is an input terminal connected between the collector and the emitter between the input terminal IN and the ground.
A pn type transistor, R is an input resistor connected in parallel between its bottom and emitter. The transistor Q and the resistor R constitute a protection circuit for preventing electrostatic damage to the input stage of the integrated circuit. D is the same manual clamp diode as DI and D2 in FIG. 1, which turns on when a negative voltage is applied to the input terminal to prevent an abnormal negative voltage from being applied to the input stage of the integrated circuit. The protection circuit of the present invention consisting of transistor Q and resistor R performs a protection function when a positive overvoltage is applied. In other words, in this circuit, when the positive voltage at the input IN increases excessively, a breakdown occurs in the pn junction between the collector and base of transistor Q, and part of the current flows to ground through resistor R, and the remaining current flows between base and emitter. flows to When current flows between the base and emitter, the transistor Q
turns on, allowing current to flow between its collector and emitter. The current flowing between the collector and emitter is 11 pB times the current flowing between the collector and base (hFE is the current amplification factor of transistor Q), so by setting hFE to a large value (for example, 100), abnormal current caused by overvoltage can be grounded. This can prevent electrostatic damage not only between the collector and base of the transistor Q but also between the clamp diode D and the like.

The value of the resistor R is related to the input breakdown voltage and the electrostatic breakdown voltage. The input withstand voltage refers to the withstand voltage between the input terminal IN and the ground until a bypass current starts flowing through the protection circuit, and the smaller the value of R, the closer it approaches the withstand voltage between the collector and the base of the transistor Q. An extreme example is the case of R-0, in which case no base current flows, so the transistor Q becomes equivalent to a mere diode, and the input withstand voltage becomes the withstand voltage of the transistor Q (7) C, B itself. On the other hand, when the value of R is large, the input breakdown voltage decreases. This is because a part of the current due to the breakdown between the collector and the base flows to the base and emitter of the transistor Q, and the main current flows between the collector and the emitter of the transistor.

For this reason, once a high voltage is applied and C and B of transistor Q break down, the withstand voltage will drop, but if there is a charge at the base, this charge will cause the base current to flow. C, B A decrease in breakdown voltage occurs even if breakdown does not occur. FIG. 3 shows experimental data proving this, where the horizontal axis is the value of the resistance R, and the vertical axis is the input breakdown voltage BV. The breakdown voltage drop is most significant in R-■.

Electrostatic breakdown voltage is the voltage that causes thermal breakdown of circuit elements when a high voltage pulse due to static electricity is applied to the input terminal.
, this electrostatic discharge damage is lowest when the base and emitter of the transistor Q are short-circuited with R=O. This is because the transistor Q can only function as a diode, and the thermal breakdown voltage at its collector-base junction becomes the electrostatic breakdown voltage. On the other hand, if R-■, even if a breakdown current flows between the collector and the base, it will all become the base current, so most of the input current due to the high voltage pulse will flow between the collector and emitter of the transistor Q. , collector, and base junctions and destroy the junctions can be avoided. When the resistance value of R is increased to several Ω or more, the breakdown voltage E suddenly increases. FIG. 4 shows experimental data proving this, where the horizontal axis is the resistance value of R and the vertical axis is the electrostatic breakdown voltage E.

As is clear from Figures 3 and 4, the input breakdown voltage BV
The breakdown voltage E increases and decreases in the opposite direction to the value of the resistance R.

However, by setting R=3 to IOKΩ, it is possible to increase the electrostatic breakdown voltage E and avoid a decrease in the input breakdown voltage BV.

Therefore, as the base resistance R of the protection transistor Q, R=
3 to IOKΩ is appropriate.

Effects of the Invention As described above, according to the present invention, the collector of the transistor is connected to the input terminal, the emitter is connected to the ground, and a resistor R is connected between the base and the emitter, and the value of the resistor R is set to 3 to IOKΩ. This has the advantage that the signal input stage of the bipolar integrated circuit can be protected from electrostatic damage without lowering the input withstand voltage.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an example of an internal gate provided in the input stage of a bipolar integrated circuit, Fig. 2 is a circuit diagram showing an embodiment of the present invention, and Figs. 3 and 4 show input breakdown voltage and static FIG. 3 is a characteristic diagram showing the dependence of electrostatic breakdown voltage on input resistance value. In the figure, Q is a transistor and R is an input resistance. Application/Person Minoru Aoyagi, Fujitsu Ltd. Representative Patent Attorney Figure 1 cc Figure 2

Claims (1)

[Claims] A bipolar semiconductor integrated circuit, characterized in that the collector of the transistor is connected to the signal input terminal of the bipolar semiconductor integrated circuit, the emitter of the transistor is connected to the ground, and a resistor of 3 to IOKΩ is further connected between the base and emitter of the transistor. Electrostatic damage protection circuit for the signal input stage of a type semiconductor S-product circuit.