Collector, transistor

The bipolar junction transistor (BIT) consists of tliree layers doped n-p-n or p-n-p tliat constitute tire emitter, base and collector, respectively. This stmcture can be considered as two back-to-back p-n junctions. Under nonnal operation, tire emitter-base junction is forward biased to inject minority carriers into tire base region. For example, tire n type emitter injects electrons into a p type base. The electrons in tire base, now minority carriers, diffuse tlirough tire base layer. The base-collector junction is reverse biased and its electric field sweeps tire carriers diffusing tlirough tlie base into tlie collector. The BIT operates by transport of minority carriers, but botli electrons and holes contribute to tlie overall current. 

Eigure 11 shows a schematic and collector characteristics for a common emitter n—p—n transistor circuit. The load line crossing these characteristics shows the allowed operation of the transistor with a supply voltage, = 12 V a load resistor, 7 = 2 and a bias resistor, 7 g = 20 kQ. The load line corresponds to the equation = 7 7 -H. Plotting the load line on the collector characteristics defines BJT behavior 0.6 V is required... 

A high gain transistor requires a nearly equal to 1. In the absence of collector junction breakdown, a is the product of the base transport factor and emitter efficiency. The base transport factor, a, is the fraction of the minority current (electrons for an n—p—n transistor) that reaches the collector. 

To achieve the lowest possible delay a bipolar switching transistor developed by IBM minimizes parasitic resistances and capacitances. It consists of self-aligned emitter and base contacts, a thin intrinsic base with an optimized collector doping profile, and deep-trench isolation (36). Devices must be isolated from each other to prevent unwanted interactions in integrated circuits. While p—n junctions can be used for isolation, IBM s approach etches deep trenches in the siUcon wafer which are filled with Si02 to provide electrical insulation. 

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... 

A transistor, or n-p-n junction, is built up of two n-type regions of Si separated by a thin layer of weakly p-type (Fig. e). When the emitter is biased by a small voltage in the forward direction and the collector by a larger voltage in the reverse direction, this device acts as a triode amplifier. The relevant energy level diagram is shown schematically in Fig. f... 

One p-type area is used as the emitter (hole-conduction) while the other is used as the collector (of current) when the input voltage is applied between points B E at the transistor. This results hi a current, 1b. from the base as well as a current, Ic from the collector. Thus two local circuits are at work, one through the emitter-base and the other through the base-collector. The important part to notice is that although the current, 1b, is in micro-amps, the current, 1, , is in milliamps, an... 

The schematic of the temperature sensor on the bulk chip is shown in Fig. 5.3. The bulk chip temperature is measured via the voltage difference between a pair of diode-connected pnp-transistors (parasitic transistors as available in the CMOS process, collectors tied to substrate) working at different current densities. Transistor Qi is biased with a current of 40 pA, and transistor Q2 is biased with a current of 10 pA. 

In this section we will investigate how the DC current gain (Hfe) of a bipolar junction transistor varies with DC bias collector current Icq, DC bias collector-emitter voltage Vceq, and temperature. We will use the basic circuit shown below for all simulations ... 

EXERCISE 141 For the MJE3055T NPN power transistor, find the collector current where Hfe is maximum. Specify the maximum value of Hfe and the collector current where it occurs. Let Vce be constant at 5 V. 

Since this is a power transistor, it can handle much higher collector currents. Sweep IB from 100 pA to 100 mA ... 

EXEHC15E 143 The circuit below is a current mirror and is a constant current source of about 50 pA. Display how the collector current of Q2 varies with temperature. Let resistors have a linear temperature coefficient of 200 ppm. The part name of both transistors is LM3046-Q. 

In amplifier design it is important to know how your bias will change with device tolerances. In this section we will find the minimum and maximum collector current of a BJT when we include variations in the transistor current gain, 0F, and resistor tolerances. The circuit above was previously simulated in the Transient Analysis and AC Sweep parts. We will use the same resistor values as before, but we will change the resistor models to include tolerance. The BJT is also changed to the model QBf. This model allows 0F to have a uniform distribution between 50 and 350. 

As Figure 10 shows, the n—p—n bipolar junction transistor (BJT) may be regarded as two back-to-back p—n junctions separated by a thin base region (26,32,33). If external voltages are applied so that the base-emitter (BE) junction is forward biased and the base-collector (BC) junction is reverse biased, electrons injected into the base from the emitter can travel to the base-collector junction within their lifetime. If the time for minority carrier electrons to... 

A high gain transistor requires a nearly equal to 1. In the absence of collector junction breakdown, a is the product of the base transport factor and emitter efficiency. The base transport factor, aT, is the fraction of the minority current (electrons for an n-p—n transistor) that reaches the collector. ocT 1 — W2 /2L, where W is the base width, is the distance between emitter and collector junctions and Lg is the minority carrier diffusion length in the base. High gain transistors require a thin base as well as a long minority carrier lifetime for a large Lg. Because aT is >0.995 in modem transistors, there is little room for improvement. The emitter efficiency, the fraction of emitter current due to minority carriers injected into the base instead of the emitter,... 

In an HBT the charge carriers from an emitter layer are transported across a thin base layer and collected by a third layer called the collector. A small base current is present which includes the carriers that did not successfully cross the base layer from the emitter to the collector. The FET is a unipolar device making use of a single charge carrier in each device, either electrons or holes. The HBT is a bipolar device, using both electrons and holes in each device. The emitter and collector layers are doped the same polarity (n- or/>-type), with the base being the opposite polarity (p- or n-type). An HBT with a -type emitter is referred to as a n—p—n device ap—n—p device has ap-type emitter. The n—p—n transistors are typically faster and have been the focus of more research. For the sake of simplicity, the following discussion will focus on n—p—n transistors. 

The simulation results do not correlate well to the hardware. A possible cause is the ESR of a Mallory TDC106K505WSG 10 /U.F capacitor, C2. The feedback loop is originated at the collector of the PNP transistor to avoid sensitivity to the output capacitor s ESR. However, investigation into the poor correlation indicates that the circuit is sensitive to the ESR of capacitor C2. The ESR was measured using an HP 3577A network analyzer. The results are shown in Fig. 4.25. 

One problem, or should we say challenge, in designing this type of circuit is to ensure that the Darlington pair transistors are not overstressed by causing them to dissipate too much power. The maximum power the transistor can dissipate decreases with increasing collector-emitter voltage. 

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