Electronic Circuits and Projects Forum

The transistor is the one device most responsible for the advances in the field of electronics. Due to its small size, high efficiency and low cost, it has replaced the vacuum tube in most applications. Many of the items we take for granted today, such as pocket radios, high speed computers, communication satelites, etc would not be possible without the transistor.

Transistors, like semiconductor diodes, are made of silicon or germanium material. As in a semiconductor diode, the silicon or germanium material is doped to produce N- or P-type semiconductor material. While the diode serves as a switch (open or closed circuit), the transistor functions as a current control device. That is, the current can be varied from near zero to some maximum value.

Let's briefly review the action in a diode. The P-type material contains Holes, or an absence of electrons, and the N-type material contains an excess of Electrons. When the diode is formed, the holes and electrons near the PN Junction combine. The recombination at the junction makes the P-type material slightly negative near the junction and the N-type material slightly positive near the junction. This action sets up a Potential Hill or Barrier to prevent further recombination. When the diode is Forward Biased (N-type material negative and P-type material positive), the barrier is overcome and the diode conducts current. However, when the diode is Reverse Biased (N-type material positive and P-type material Negative), the barrier is aided and only a small reverse current flows. The term Majority Carriers is used to describe the holes that flow in the P-type material and the electrons that flow in the N-type material during forward bias conditions. During reverse bias conditions, holes flow in the N-type material, and electrons flow in the P-type material. Under these conditions, the holes and electrons are referred to as Minority Carriers.

Bipolar Junction Transistor
The basic construction of the Bipolar Junction Transistor, like the diode is constructed of P- and N-type semiconductor material. However, the transistor employs Three sections of semiconductor material. This results in two PN Junctions. The connections to the semiconductor regions are referred to as the Collector, Base, and Emitter. The PN junction between the base and collector regions is referred to as the Collector Junction. The PN junction between the base and emitter regions is referred to as the Emitter Junction.

Current does not flow in through the emitter and collector unless there is voltage bias applied to the base.

The key to the operation of the transistor is the thinness of the base region. If the base region were too thick, the holes from the emitter region would not be able to pass through the base region into the collector region. They would recombine with electrons before reaching the collector region. The holes injected into the N-type base region are minority carriers. Thus, bipolar junction transistor operation is based on Minority Carrier Injection.

Bipolar Junction Transistors may be constructed in either a PNP or an NPN sandwich of semiconductor material. The major difference between the two arrangements is the polarity of the voltage. Remember that the emitter junction must be forward biased. In the NPN arrangement, the P-type base must be positive with respect to the emitter. This causes electrons to be injected into the P-type base region. By also making the collector region positive, the injected electrons pass through the base region into the collector region.

To obtain proper transistor operation (base current controlling collector current). The emitter junction must be forward biased; the collector junction must be reversed biased.

In summary, we can say that the transistor is basically a current control device. The value of base current controls collector current. That is, the transistor is an Electronically Controlled Variable Resistor.

Transistor Testing
An ohmmeter's internal voltage supply can be used to bias the junctions in a transistor, and the resistance may then be noted. First, connect the ohmmeter to the base and emitter. Note the reading, reverse the leads and again note the reading. If the base-to-emitter junction is good, you should obtain one high reading and one low reading. If both readings are low, the junction is probably shorted. An open junction is indicated if both readings are high. The collector-to-base junction can be checked in a similar manner. Again, one reading should be high and the other low if the junction is good.

Sometimes, transistors (especially germanium power types) develope emitter-to-collector shorts. This type of short circuit is not revealed by the base-to-emitter or collector-to-base measurements. Therefore, it is helpful to also perform an ohmmeter check between the emitter and collector. This resistance is rather low in both directions. However, it is not as low as the lower resistance measurement of either the base-to-emitter or the collector-to-base junctions.

When testing power transistors with an ohmmeter, you may get lower readings in both directions than would be obtained with small-signal transistors This is normal for power transistors.

Phototransistors
A phototransistor is a bipolar junction transistor which has a special light sensitive base region. A lens in the transistor case focuses light on this special base region. The light striking the lens controls the value of collector current by changing the emitter-to-collector resistance in proportion to the amount of light shining on the base region.

When it is not exposed to light, the phototransistor exhibits a high emitter-to-collector resistance, which permits only a small collector current, and therefore, a very low output voltage. When light does strike the base material, collector current increases because of a decrease in the emitter-to collector resistance.

In some phototransistors, an electrical connection to the base may be used for forward bias to assist the light-induced current in low-level light conditions. Most phototransistors are currently used in switching applications because of their high-speed operations. When pulsed by an LED (light-emitting- diode), a typical phototransistor will switch on and off again in less than 10 microseconds.