The leakage current of typical signal diodes (eg. 1N4148) is rather high in the nA range and they only follow the ideal diode law (used in logametric and multiplication circuits) over a small range of currents.

The CB and BE diode junctions in a BJT transistor provide a solution for this and other problems via the following BJT Diodes Modes.

The Base Emitter Junction as a Diode

The base emitter junction provides a low leakage, fast recovery  diode. 

For example, a 2N3904 type transistor can be used as a diode to obtain a very low leakage diode with less than 1pA of reverse leakage using the Base Emitter junction. However, it turns into a zener diode at around 6V. This works great for 5V and lower voltage logic circuits, for high impedance shunt protection  and RF applications.

The down side is high reverse capacitance (5 pF) compared to a signal diode (1pF.)

The Collector Base Junction as a Diode

The collector base junction has a higher reverse voltage (eg. 40 V) capability than the base emitter junction and similar low leakage performance, lower reverse capacitance (3 pF) and somewhat lower conductance at high currents (eg 4 Ohms.) The reverse recovery time is much higher.

The Reinforced Diode or Active Diode

The Reinforced Diode or Diode Connected BJT or Active Diode as it is called is simply a transistor whose collector is connected to the base. Thus the collector-emitter part of the transistor is connected in parallel to its base-emitter junction so we can think of this combination as of a "reinforced diode". The current through this "composed diode" is beta times bigger than the current through the single p-n (base-emitter) junction. So its IV curve is more vertical or, as they say, its differential resistance in this part is lower. That is why the active diode is better than the ordinary diode.

For a fixed input current it creates a precise voltage reference that can be connected to the base of another transistor. For a matched transistor an identical amount of current will flow through its base as through the base of the reference Reinforced Diode transistor.

The "Rubber" Zener Diode

If you apply not the whole collector-emitter voltage to the base-emitter junction but a part of it, VBE will be multiplied (like in the non-inverting amplifier). The "transistor diode" will act as a "transistor Zener diode" with any desired voltage. This network is widely used as a bias circuit in op-amp and power amplifiers.

The transition from the non-conducting to conducting state will be less sharp that with the reinforced diode because of the voltage division at the base by R1 and R2 slowing the rate of change.

The Base Emitter Zener Diode

The base-emitter junction of a bipolar junction transistor (BJT) can act as a Zener diode when it is reverse-biased beyond a certain voltage, known as the base-emitter reverse breakdown voltage (VBE(BO)). This breakdown occurs due to a phenomenon called avalanche breakdown, which is caused by a rapid increase in the number of charge carriers in the junction as the applied voltage increases.  The breakdown voltage is in the range of 3 to 12 V for common transistors, usually in excess of 6 V. 

In common with Zener diodes a lot of noise is produced which can be used as a noise generator.

NPN Transistor -  Emitter Collector Negative Resistance Diode (Negistor) 

The negistor effect in NPN transistors arises from the interplay of several factors, including the transistor's doping profile, the applied voltage, and the temperature. To understand the underlying mechanism, let's delve into the details:

1. Avalanche Breakdown:

At the heart of the negistor effect lies the phenomenon of avalanche breakdown. When a reverse bias is applied to an NPN transistor's base-emitter junction, the electric field across the junction increases. If this electric field becomes sufficiently strong, it can cause impact ionization, where energetic electrons knock off valence electrons, generating more charge carriers.

This rapid increase in charge carriers leads to an exponential rise in current, causing the junction to break down. This breakdown is known as avalanche breakdown, and it marks the onset of the negistor effect.

2. Hot Carriers:

In the avalanche breakdown regime, the base-emitter junction becomes a highly conductive region due to the abundance of free charge carriers. These energetic charge carriers, often referred to as hot carriers, can gain enough kinetic energy to cross the emitter-base barrier, entering the base region.

The presence of hot carriers in the base region alters the transistor's current-voltage (I-V) characteristics. The base current, which typically controls the collector current, becomes less influential due to the hot carriers' ability to bypass the base-emitter junction directly.

3. Negative Resistance:

As the voltage across the base-emitter junction increases beyond the avalanche breakdown point, the collector current decreases counterintuitively. This paradoxical behavior stems from the dominance of hot carriers in the conduction process.

The hot carriers' direct path between the emitter and the collector reduces the transistor's gain, leading to a decrease in collector current despite the increasing voltage. This negative resistance behavior is the hallmark of the negistor effect.

4. Temperature Dependence:

The negistor effect is highly sensitive to temperature. As the temperature increases, the mobility of charge carriers increases, enhancing the avalanche breakdown process and lowering the voltage required to induce negative resistance.

Conversely, at lower temperatures, the mobility of charge carriers decreases, making it more difficult to achieve avalanche breakdown and negative resistance. This temperature dependence plays a significant role in negistor oscillator design and stability.

In summary, the negistor effect seen in many (but not all) NPN transistors arises from the interplay of avalanche breakdown, hot carriers, and the transistor's doping profile. These factors combine to create the peculiar negative resistance behavior, which forms the basis for negistor oscillator circuits and other applications.

Avalanche Transistors

When the voltage across the collector-emitter junction of a transistor exceeds the breakdown voltage, the electric field in the junction becomes strong enough to ionize atoms in the semiconductor material. This ionization creates free electrons and holes, which can then collide with other atoms and create even more free electrons and holes. This process is known as avalanche breakdown, and it results in a rapid increase in the current flowing through the junction.

The avalanche mode is known for its fast switching times, which are on the order of a few nanoseconds. This is because the avalanche breakdown phenomenon occurs very quickly, so the transistor can switch from "off" to "on" very quickly.

Specialized transistors are available for this mode of operation (ZTX415,FMMT411.)