Bipolar transistors

This type of transistor is one of the most important of the semiconductor devices. It is a bipolar device in that both electrons and holes are involved in the conduction process. The bipolar transistor delivers a change in output current in response to a change in input voltage at the base. The ratio of these two changes has resistance dimensions and is a “transfer” property (input-to-output), hence the name transistor.

A perspective view of a silicon p-n-p bipolar transistor is shown in Figure 4A. Basically the bipolar transistor is fabricated by first forming an n-type region in the p-type substrate; subsequently a p⁺ region (very heavily doped p-type) is formed in the n region. Ohmic contacts are made to the top p⁺ and n regions through the windows opened in the oxide layer (an insulator) and to the p region at the bottom.

An idealized, one-dimensional structure of the bipolar transistor, shown in Figure 4B, can be considered as a section of the device along the dashed lines in Figure 4A. The heavily doped p⁺ region is called the emitter, the narrow central n region is the base, and the p region is the collector. The circuit arrangement in Figure 4B is known as a common-base configuration. The arrows indicate the directions of current flow under normal operating conditions—namely, the emitter-base junction is forward-biased and the base-collector junction is reverse-biased. The complementary structure of the p-n-p bipolar transistor is the n-p-n bipolar transistor, which is obtained by interchanging p for n and n for p in Figure 4A. The current flow and voltage polarity are all reversed. The circuit symbols for p-n-p and n-p-n transistors are given in Figure 4C.

The bipolar transistor is composed of two closely coupled p-n junctions. The emitter-base p⁺-n junction is forward-biased and has low resistance. The majority carriers (holes) in the p⁺-emitter are injected (or emitted) into the base region. The base-collector n-p junction is reverse-biased. It has high resistance, and only a small leakage current will flow across the junction. If the base width is sufficiently narrow, however, most of the holes injected from the emitter can flow through the base and reach the collector. This transport mechanism gives rise to the prevailing nomenclature: emitter, which emits or injects carriers, and collector, which collects these carriers injected from a nearby junction.

The current gain for the common-base configuration is defined as the change in collector current divided by the change in emitter current when the base-to-collector voltage is constant. Typical common-base current gain in a well-designed bipolar transistor is very close to unity. The most useful amplifier circuit is the common-emitter configuration, as shown in Figure 5A, in which a small change in the input current to the base requires little power but can result in much greater current in the output circuit. A typical output current-voltage characteristic for the common-emitter configuration is shown in Figure 5B, where the collector current I_C is plotted against the emitter-collector voltage V_EC for various base currents. A numerical example is provided using Figure 5B. If V_EC is fixed at five volts and the base current I_B is varied from 10 to 15 microamperes (μA; 1 μA = 10⁻⁶ A), the collector current I_C will change from about four to six milliamperes (mA; 1 mA = 10⁻³ A), as can be read from the left axis. Therefore, an increment of 5 μA in the input-base current gives rise to an increment of 2 mA in the output circuit—an increase of 400 times, with the input signal thus being substantially amplified. In addition to their use as amplifiers, bipolar transistors are key components for oscillators and pulse and analog circuits, as well as for high-speed integrated circuits. There are more than 45,000 types of bipolar transistors for low-frequency operation, with power outputs up to 3,000 watts and a current rating of more than 1,000 amperes. At microwave frequencies, bipolar transistors have power outputs of more than 200 watts at 1 gigahertz and about 10 watts at 10 gigahertz.

Thyristors

The thyristors constitute a family of semiconductor devices that exhibit bistable characteristics and can be switched between a high-resistance, low-current “off” state and a low-resistance, high-current “on” state. The operation of thyristors is intimately related to the bipolar transistor, in which both electrons and holes are involved in the conduction processes. The name thyristor is derived from the electron tube called the gas thyratron, since the electrical characteristics of both devices are similar in many respects. Because of their two stable states (on and off) and low power dissipations in these states, thyristors are used in applications ranging from speed control in home appliances to switching and power conversion in high-voltage transmission lines. More than 40,000 types of thyristors are available, with current ratings from a few milliamperes to more than 5,000 amperes and voltage ratings extending to 900,000 volts.

Figure 6A provides a perspective view of a thyristor structure. An n-type wafer is generally chosen as the starting material. Then, a diffusion step is used to form the p1 and p2 layers simultaneously by diffusing the wafer from both sides. (Diffusion is the movement of impurity atoms into the crystalline structure of a semiconductor.) Finally, n-type impurity atoms are diffused through a ring-shaped window in an oxide into the p2 region to form the n2 layer.

A cross section of the thyristor along the dashed lines is shown in Figure 6B. The thyristor is a four-layer p-n-p-n diode with three p-n junctions in series. The contact electrode to the outer p layer (p1) is called the anode, and that to the outer n layer (n2) is designated the cathode. An additional electrode, known as the gate electrode, is connected to the inner p layer (p2).

The basic current-voltage characteristic of a thyristor is illustrated in Figure 6C. It exhibits three distinct regions: the forward-blocking (or off) state, the forward-conducting (or on) state, and the reverse-blocking state, which is similar to that of a reverse-biased p-n junction. Thus, a thyristor operated in the forward region is a bistable device that can switch from a high-resistance, low-current off state to a low-resistance, high-current on state, or vice versa.

In the forward off state, most of the voltage drops across the centre n1-p2 junction, while in the forward on state all three junctions are forward-biased. The forward current-voltage characteristic can be explained using the method of a two-transistor analog—that is, to consider the device as a p-n-p transistor and an n-p-n transistor connected with the base of one transistor (n1) attached to the collector of the other. As the voltage V_AK in Figure 6C increases from zero, the current I_A will increase. This in turn causes the current gains of both transistors to increase. Because of the regenerative nature of these processes, switching eventually occurs, and the device is in its on state. The maximum forward voltage that can be applied to the device prior to switching is called the forward-breakover voltage V_BF. The magnitude of V_BF depends on the gate current. Higher gate currents cause the current I_A to increase faster, enhance the regeneration process, and switch at lower breakover voltages. The effect of gate current on the switching behaviour is shown in Figure 6C (dotted line).

A bidirectional, three-terminal thyristor is called a triac. This device can switch the current in either direction by applying a small current of either polarity between the gate and one of the two main terminals. The triac is fabricated by integrating two thyristors in an inverse parallel connection. It is used in AC applications such as light dimming, motor-speed control, and temperature control. There also are many light-activated thyristors that use an optical signal to control the switching behaviour of devices.