Transistor bipolar

A bipolar transistor is a three terminal device which includes the components of emitter (E), base (B) and collector (C). There are two types of bipolar transistors, npn and pnp. The letters refer to the layers of semiconductor materials used to make the transistors. Fig. 9.3a, b shows the schematic illustrations of current flows and the circuit symbols of npn and pnp transistors. The shaded regions in the figure represent two depletion regions formed at the interfaces of emitter/base and base/collector. 

A bipolar transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on). Bipolar transistors are the quickest devices. However the current flow through the base creates heat. This disadvantage makes bipolar transistors less popular than other devices such as metal oxide semiconductor transistor. 

A schematic diagram of the cross section and dopant distributions in a planar npn bipolar junction transistor in Si are shown in Fig. 9.4a, b. The transistor is made by implanting donors at high concentrations, ND, into the n-type Si to form the emitter, and implanting a lower concentration, NA, of acceptors deeper into the n-type Si to form the p-type base. 

---

Some of tliese problems are avoided in heterojunction bipolar transistors (HBTs) [jU, 38], tlie majority of which are based on III-V compounds such as GaAs/AlGaAs. In an HBT, tlie gap of tlie emitter is larger tlian tliat of tlie base. The conduction and valence band offsets tliat result from tlie matching up of tlie two different materials at tlie heterojunction prevent or reduce tlie injection of tlie base majority carriers into tlie emitter. This peniiits tlie use of... 

Fig. 5. Bipolar transistor (a) schematic and (b) doping profiles of A, arsenic ion implanted into the silicon of the emitter ( -type) B, boron ion implanted into the silicon of the base (p-type) C, antimony ion implanted into the buried layer ( -type) and D, the epi layer...

In addition to its use as a rectifier, the p—n junction (26) is the fundamental building block for bipolar, junction EFT (fFET), and MOSFET transistors. A thorough understanding of p—n junctions explains much of transistor behavior. The theory (5) of the p—n junction and its role in bipolar transistors was presented within a year of the discovery of the point-contact transistor. 

The frequency response or switching speed of the bipolar transistor is governed by the same processes which control the speed of thep—n junction, the capacitance associated with the movement of charge into and out of the depletion regions. To achieve high frequencies the dimensions of the active areas and parasitic circuit elements must be reduced. The two critical dimensions are the width of the emitter contact and the base thickness, W. The cutoff frequency,, is the frequency at which = 57 / - b /t > where is the emitter-to-coUector delay time and is the sum of the emitter... 

The development of this hybrid combination in a bipolar transistor has greatly enhanced the application of power transistors in the field of power conversion and variable-speed drives. It possesses the qualities of both the power bipolar transistor (BJT) and the power MOSFET. Like a power MOSFET, it is a voltagc-eontrollcd switching device... 

This drive loss ean beeome signifieant. A driver transistor ean be added to the pass transistor to inerease the effeetive gain of the pass unit and thus deerease the drive eurrent, or a power MOSFET ean be used as a pass unit that uses magnitudes less de drive eurrent than the bipolar power transistor. Unfortunately, the MOSFET requires up to 10 VDC to drive the gate. This ean drasti-eally inerease the dropout voltage. In the vast majority of linear regulator applieations, there is little differenee in operation between a buffered pass unit and a MOSFET insofar as effieieney is eoneerned. Bipolar transistors are mueh less expensive than power MOSFET and have less propensity to oseillate. 

There are two major types of power switches used today the bipolar power transistor (BJT) and the power MOSFET. The IGBT (integrated gate bipolar transistor) is used in the higher power industrial applications, such as 1 kW power supplies and electronic motor drives. The IBGT has a slower turn-off than does the MOSFET, so it is typically used for switching frequencies of less than 20 kHz. 

The power MOSFET is the most common choice as a power switch. Its cost and saturation loss are comparable to the bipolar transistor in most applications and it switches five to ten times faster. It is also easier to use in a design. 

Certain features in the PR spectra at 300 K from GaAs/Gai j,jAlj heterojunction bipolar transistor structures have been correlated with actual device performance thus PR can be used as an effective screening tool. From the observed FK oscillations it has been possible to evaluate the built-in dc electric fields in the Gai j jAlj emitter, as well as in the n—GaAs collector region. The behavior... 

Figure 4 Photoreflectance spectra for two GaAs/Ga Alj haterojunction bipolar transistor structures fabricated by MBE and MOCVD, respectively, at 300 K.

Secondary Ion Mass Spectroscopy (SIMS). When the p-n junction and the GaAs/GaAlAs heterojunction are not coincident, carrier recombination occurs, reducing the current and the performance of fabricated heterojunction bipolar transistors. 

In real device structures like heterojunction bipolar transistors, certain features in the PR spectrum can be correlated with actual device performance. Thus PR has been employed as an effective contacdess screening technique to eliminate structures that have imwanted properties. 

The invention of the germanium transistor in 1947 [I, 2] marked the birth of modem microelectronics, a revolution that has profoundly influenced our current way of life. This early device was actually a bipolar transistor, a structure that is mainly used nowadays in amplifiers. However, logical circuits, and particularly microprocessors, preferentially use field-effect transistors (FETs), the concept of which was first proposed by Lilicnficld in 1930 [3], but was not used as a practical application until 1960 [4]. In a FET, the current flowing between two electrodes is controlled by the voltage applied to a third electrode. This operating mode recalls that of the vacuum triode, which was the building block of earlier radio and TV sets, and of the first electronic computers. 

Silicon is also alloyed with germanium (Si Gei.x) for applications such as heterojunction bipolar transistors and optical emitters... 

Integrated circuits (IC s) are circuits in which bipolar transistors, field-effect transistors (FET), resistors, capacitors, and their required connections are combined on a single chip of semiconductor material which is usually made of single-crystal silicon. 

---