June 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Like most people familiar
with electronics, when negative resistance semiconductors are mentioned, I immediately
think of tunnel diodes. Negative resistance is
the characteristic where in increase in voltage across the p-n junctions results
an a decreased current. Although the tunnel diode was invented by by Leo Esaki (Sony)
in 1957, it is not mentioned anywhere in this 1969 article. Instead author Wesley
Vincent (Motorola) describes the theoretical operation of 4-layer (3 junction) semiconductors
and how they can be biased to mimic true negative resistance devices. Given that
one of the most common applications of tunnel diodes is to construct relaxation
oscillators, knowing which configurations of standard BJTs can act like negative
resistance devices might help explain unintended high frequency oscillations in
some amplifier circuits.
See also "The
Tunnel Diode" from a 1960 issue of Popular Electronics.
Using Transistors as Negative-Resistance Devices
By Wesley A. Vincent / Advanced Development Section
Government Electronics Div., Motorola Inc.
By using simple transistor-resistor combinations,
characteristics of four-layer diodes, SCR's, and UJT's may be readily simulated.
Several rather unusual negative-resistance devices have become available to the
circuit enthusiast during the past few years. These devices include four-layer diodes,
silicon controlled rectifiers (SCR's), and unijunction transistors (UJT's) to name
a few of the most popular. Industrial competition and improved manufacturing and
production techniques have resulted in price reductions, allowing those with limited
budgets to use them in circuit projects. However, ordinary transistors are more
likely to be readily available for circuit experiments. By using the simple transistor-resistor
combinations presented in this article, the characteristics of four-layer diodes,
SCR's, and UJT's may be simulated.
A relaxation oscillator using the analog circuit of the unijunction transistor,
which is quite similar in operation to the unijunction relaxation oscillator, is
also described. Although the test results presented were obtained using silicon
transistors, low-leakage germanium transistors may be substituted.
An advantage gained in simulating these negative-resistance devices is that the
more important device parameters may be determined by selecting transistors and
resistors used in the substitute combinations. The basic building block for the
circuits discussed is based on the operation and theory of the four-layer or p-n-p-n
diode. The SCR and UJT are then presented as extensions of the four-layer diode
in theory and in synthesizing their characteristics, using transistor-resistor combinations.
Four-Layer Diode Theory
A four-layer diode consists of four alternate regions of p- and n-doped semiconductor,
as shown in Fig. 1A. The p-region terminal is called the anode while the n-terminal
is referred to as the cathode. The V-I characteristics and parameter definitions
associated with the device are shown in Fig. 1B. With the anode biased positively
with respect to the cathode, a negative-resistance (current increases as the voltage
decreases) region exists. The four-layer diode may also be represented by a regenerative
transistor feedback arrangement consisting of a p-n-p and n-p-n transistor, as shown
in Fig. 1C.
Using this model, the mathematical expression for the terminal current IE,
may be expressed in terms of the transistor parameters as follows:
are the common-base current gains; Icop and Icon are the collector-to-base
leakage currents; Mp and Mn are the multiplication factors
which account for carriers created by impact ionization in a reverse-biased junction
Fig. 1A - Four-layer diode has four alternate "p, n" regions
and three "p-n" junctions. B - V-I characteristics
Fig. 1C - Transistor model
The p and n subscripts refer to the parameters associated with the p-n-p and
n-p-n transistors, respectively, in Fig. 1C. Usually Mp and Mn
are assumed to be equal and are designated simply as M. If the leakage terms are
combined, the previous equation may be expressed in a slightly more simplified form
This expression may be used to briefly explain the forward V-I characteristics
of the four-layer diode as follows: For anode-to-cathode voltage less than the breakover
voltage, only a small leakage current flows. The current-gain parameters
are complex functions of injection efficiency, the base transport factor, and surface
conditions. For small anode-to-cathode voltages, the combined values of
are much less than 1. Since no multiplication takes place at low voltages, M is
equal to unity. The denominator in the above expression is only slightly less than
unity so that IE is approximately equal to Ico.
The current gains
increase with increasing current as the forward voltage is increased. Thus, the
forward current increases slightly with increasing voltage. As the forward voltage
is continually increased, the condition occurs where M (αp +
) = 1. When this occurs, the current increases sharply over the previous small leakage
current as shown in Fig. 1B. This voltage is known as the breakover voltage.
At the breakover voltage, multiplication (M) is greater than unity since avalanche
breakdown is occurring in the reverse-bias junctions. Therefore, the combined value
is less than 1.
As the current increases beyond the breakover current,
increase due to their current dependence. A lower multiplication (M) is then required
to maintain the breakover voltage. As a result, the forward bias across the diode
begins to decrease with a negative-resistance region occurring. The current increases
and voltage decreases until the holes injected at the anode of the p-n-p transistor
equal the electrons injected at the emitter of the n-p-n transistor. This is a result
of current continuity conditions and results in forward bias of the center junction
of the four-layer diode. The transistors in the model are then in their "on" or
In the reverse operating mode, the four-layer diode acts like two reverse-biased
diodes in series. A small reverse current exists until the breakdown condition finally
Equivalent Circuit for Four-Layer Diode
When silicon transistors are connected in the manner shown in Fig. 1C, the
forward V-I characteristics resemble those of an ordinary p-n junction rather than
those of a four-layer diode. This occurs because the discrete transistor current
gains are much higher than the current gains in a four-layer diode. The breakover
condition of M (αp
= 1 is reached at a few tenths of a volt when current injection for the transistor
Large transistor leakage currents can also cause low breakover voltages. One
method of reducing the transistor current gain is to place a resistor between its
base and emitter terminals. A shunt path exists for the emitter current with the
result that very little injection takes place until the voltage across the shunt
resistor begins to forward-bias the base-emitter junction.
In the transistor equivalent model shown in Fig. 1C, several possibilities
exist for reducing the combined values of
Resistors can be inserted between the base and emitter of the p-n-p or n-p-n, or
both, transistors. Shown in Figs, 2A and 2B are the forward V-I characteristics
for a transistor-resistor equivalent circuit with a 1000-ohm resistor inserted between
the base and emitter of the n-p-n transistor. It can be seen from these curve-tracer
photographs that the forward V-I characteristics are similar to those of the four-layer
Fig. 2 - (A, B) The transistor-resistor equivalent circuit
for a four-layer diode. (C) Same but with inverted "n-p-n" transistor and (D) with
an inverted "p-n-p" transistor.
In the configurations shown, the breakover voltage is determined by the BVCEO
parameter of the p-n-p transistor. In general, the breakover voltage will be determined
by the transistor with the lower breakdown parameter. Shunt resistors, used to reduce
will increase the transistor breakdown voltage from BVCEO to BVCER.
In Figs. 2A and 2B, BVCEO for the n-p-n transistor is approximately 50
V. However, with the shunt resistor of 1000 ohms, the BVCER voltage is
greater than 100 volts; hence, the breakover voltage for the circuit is determined
by the p-n-p transistors with BVCEO voltages of 54 and 64 volts, respectively.
For breakover voltages less than the BVCEO voltage of the transistors,
either the p-n-p or n-p-n transistor may be operated in an inverted mode. Results
for such a circuit are shown in Fig. 2C where the n-p-n transistor has been
inverted. Even though alpha for an inverted transistor is severely reduced, it is
still necessary to reduce the alpha of either the n-p-n or p-n-p transistor with
a shunt resistor. As with the previous circuit, the breakover voltage is determined
by the lower breakdown of the two devices. In this configuration the breakover point
is determined by the BVECO voltage of the n-p-n transistor. (A close
approximation of the breakover voltage is obtained by knowing the more commonly
specified BVEBO voltage of the n-p-n transistor.)
Another four-layer diode equivalent circuit with its forward V-I characteristics
is shown in Fig. 2D, where the p-n-p transistor has been inverted. The BVECO
voltage of the p-n-p transistor determining the breakover voltage is 6.5 volts.
By selecting the transistor breakdown voltage, the experimenter can simulate
four-layer diode characteristics with a breakover voltage of 5 to 100 volts or more.
The holding current for these configurations is determined by the transistor
current gains and shunt resistors. The holding current may be selected from a few
microamps to 10 or 20 mA or more. Decreasing the value of the shunt resistor (and
hence decreasing the transistor current gain) increases the holding current.
The reverse breakdown voltage for the four-layer diode equivalent circuit is
similar to that of a four-layer diode and is determined by the junction breakdown
of the transistors in the specific configuration. Note that if shunt resistors are
used to reduce both
the reverse breakdown voltage will be only approximately 0.65 volt, the voltage
of one forward-biased diode.
The most noticeable temperature effect for these configurations is that the holding
current decreases with increasing temperature. If the transistor alphas are not
reduced sufficiently by shunt resistors, it is possible for premature firing to
occur with increasing temperature as
increase with temperature.
The test results are not unique for any particular transistor type. Similar results
have been obtained using other silicon and low-leakage germanium transistors.
Simulation of SCR Characteristics
Fig. 3. (A) An SCR is four-layer diode with gate terminal.
(B) V-I characteristics, and (C) equivalent circuit for SCR.
Fig. 4. Characteristics of simulated SCR shown here (A)
in blocking state and (B) in "on" state due to 1 V on gate.
Fig. 5. (A) Symbol and circuit used to explain operation
of UJT. (B) Forward V-I characteristics. (C) Equivalent circuit.
The SCR consists basically of a four-layer diode with the addition of a third
terminal called the gate. The gate is usually attached to the p-region near the
cathode, as shown in Fig. 3A. The gate terminal is used to switch the SCR from
the blocking or "off" state to a low-impedance or "on" state . As the gate current
increases, the breakover voltage decreases, as shown in Fig. 3B.
With a minimum gate current, which is dependent on the particular SCR construction,
the negative-resistance region no longer occurs and the V-I characteristics resemble
those of an ordinary p-n diode. In theory, the gate current causes the individual
current gains αp
to increase so that the condition for breakover, M (αp +
= 1, occurs prior to four-layer junction breakdown.
The transistor-resistor equivalent circuit for the four-layer diode may be adapted
to obtain SCR characteristics by simply adding the gate terminal with a series resistor
to the base of the n-p-n transistor shown in Fig. 3C. The series gate resistor
insures that the collector current of Q1 causes Q2 to turn on, leading to regenerative
action and the low-impedance state. Otherwise the collector current (electron current)
of Q1 would flow into the gate terminal. A diode may also be used to replace the
series gate resistor in Fig. 3C.
Curve-tracer results for the simulated SCR are shown in Fig. 4, where Q1
and Q2 were 2N3906 and 2N3904, respectively. Both resistors used in the equivalent
circuit were 1000 ohms.
Fig. 4B shows the equivalent SCR turned "on" by an applied gate voltage
of 1 volt. The exact gate voltage and current necessary for switching the simulated
SCR will depend on the transistors and resistors used in the equivalent circuit.
A gate current of 1 milliamp should be sufficient to switch silicon or germanium
Simulation of the Unijunction Transistor
The unijunction transistor is another device with negative-resistance characteristics.
It is used in oscillators, timing circuits, pulse and sawtooth generators, and special
triggering applications. The unijunction equivalent circuit can be considered to
be a resistive n-type silicon bar with a p-n junction formed between the terminals,
as shown in Fig. 5A. The end terminals of the bar are referred to as bases
while the anode of the p-n junction is called the emitter. The resistance between
the two bases is known as the interbase resistance. The geometry of the first unijunctions
consisted of the bar structure although newer UJT's include cube, planar, and p-base
In operation, the interbase resistors form a voltage divider with the voltage
applied between base terminals. When the voltage at the emitter forward-biases the
p-n junction, the unijunction enters the negative-resistance region. Forward V-I
characteristics for the unijunction, showing the negative-resistance region, are
illustrated in Fig. 5B.
The unijunction transistor can be considered to be a four-layer diode with the
addition of biasing resistors as shown in Fig. 5C. The biasing resistors replace
the interbase resistors of the unijunction and are used to set the break over point.
However, in unijunction terminology, this voltage is known as the peak-point voltage.
Also, the minimum holding current is referred to as the valley current (IV)
for the unijunction.
For commercially available UJT's, the ratio of interbase resistors setting the
peak-point voltage is determined by the semiconductor manufacturer. Using the transistor-resistor
equivalent circuit in Fig. 5C, the peak-point voltage may be selected by choosing
the appropriate resistive dividers, RI and R2.
Fig. 6. (A) UJT oscillator. (B) Equivalent-circuit version.
A relaxation oscillator is one of the most common applications for the UJT. The
basic configuration appears in Fig. 6A. A positive pulse appears at base 1,
a negative pulse at base 2, and a sawtooth waveform at the emitter. The frequency
of oscillation is controlled by the R1C1 time constant; R2 is selected for minimum
frequency change over a given temperature range, and R3 limits the capacitor discharge
The basic operation of the relaxation oscillator is as follows: When "B+" is
applied, all of this voltage immediately appears across the timing resistor, R1.
The voltage across C1 then increases at a rate determined by the time constant R1C1
as C1 begins to charge toward the applied voltage. When the voltage across the capacitor
increases to the emitter firing voltage, the emitter-base 1 junction becomes forward-biased
and the UJT enters the negative-resistance region. C1 then discharges through R3
and the emitter of the unijunction, this continues until the voltage across C1 falls
sufficiently and causes the UJT to turn off. When the UJT turns off, the applied
voltage minus the turn-off voltage appears across R1. The capacitor begins to charge
again and the cycle is repeated. Positive and negative output pulses are produced,
as shown on the diagram, as a result of the pulse of current flow through the UJT.
During firing, negative resistance occurs between the emitter and base 1 due to
carriers injected across the junction. This mechanism is known as conductivity modulation
of the bulk silicon.
The configuration of the equivalent-circuit relaxation oscillator and its similarity
to the UJT oscillator are shown in Fig. 6B. The operation is similar to the
unijunction oscillator except that the capacitor is discharged through the low impedance,
resulting from the four-layer action of Q1 and Q2 when firing occurs. Component
values in Fig. 6B are for an oscillator with a frequency of approximately 1
The firing point for this circuit is:
or approximately 5.5 + 0.6 = 6.1 volts. A positive pulse can be obtained by dividing
R2 into two separate resistors and taking the output between them.
In summary then, we have shown that it is possible to simulate the characteristics
of four-layer diodes, SCR's, and UJTs by simply wiring together a pair of transistors
and some resistors.
Posted July 3, 2017