BIPOLAR JUNCTION TRANSISTORS

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2 Νοε 2013 (πριν από 4 χρόνια και 6 μέρες)

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BIPOLAR JUNCTION TRA
NSISTORS

Operation of the bipolar junction transistor (BJT) depends on the flow of both minority and majority
carriers; hence the reference to “bipolar”. The BJT is one of two major classes of transistor in
operation; the other is the f
ield effect transistor (FET).

Classification of 3
-
terminal devices

BJT


Hetrojunction BJT (HBT)

FET


Junction FET (JFET)


Insulated Gate FET (IGFET)



Metal
-
Oxide
-
Semiconductor FET (MOSFET)


Metal
-
Semiconductor FET (MESFET)

High Electron Mobility Transisto
r (HEMT)


also known as Modulation
-
Doped FET
(MODFET)


Use of three
-
terminal devices (BJTs or FETs) allows for



amplification of a “small signal” applied to the controlling terminal



switching: we can have a low voltage, high current state and a high voltag
e, low current state.
This is the basis of binary logic.

Streetman, Figure 6
-
2


BJT FABRICATION

We look at the basic steps in BJT fabrication.

Figure 10.6

Notes

1. The emitter diffusion controls both the emitter doping density (which must be large) and

the base
width (which must be small).

2. The buried n
+

region (Fig. 10.6b) provides for a low resistance contact between base and collector.
If this were not present, series resistance would be a problem.

3. Isolation of devices (so that they do not talk

to each other) is provided by a channel stop (a reverse
-
biased pn junction).

4. Note the emitter and base contacts, which are metal
-
semiconductor tunnel ohmic contacts.

5. In a device used for amplification, the collector doping is particularly important
because it determines
the breakdown voltage of the B/C junction. For amplification, the B/C junction is in reverse bias, so a
lower collector doping provides a higher breakdown voltage.


Dopant Distribution:

Muller and Kamins,
Device Electronics for Integ
rated Circuits
, John Wiley
and Sons, Inc., 1986

The top figure shows a typical IC BJT. The bottom figure shows doping profiles as a function of depth
into a Si wafer. The profiles correspond to a line passing through the emitter/base/collector in the top

figure, or that of Fig. 10.6b in Pierret, for example.

As a function of distance from the silicon surface, we have:

Emitter

(0 to 0.75

m) At ~ 0.75

m, the emitter doping is equal to the base doping; this the
emitter/base junction.

Base

(0.75

m to 1

m
) At ~ 1

m, the base doping is equal to the collector doping. This is the
base/collector junction. The region forming the collector was initially an epitaxial layer of Si grown on
the substrate, which is at a distance of ~ 5.5

m and deeper.

Collector

(1

m to 2.75

m) The collector extends to about 2.75

m and has a constant doping
density of ~ 1.5 x 10
16

cm
-
3
.

Buried layer

(1.75

m to 5.5

m) This is the n
+

layer beneath the device. It provides a low resistance

path for electrons to travel to the c
ollector contact.


INTRODUCTORY OPERATI
ON CONSIDERATIONS

Current flow through a reverse
-
biased pn junction can be controlled via injection of carriers from a
nearby junction. By placing the two junctions in series, we arrive at “npn” and “pnp” BJTs.

Figu
res 10.8, 10.9

We will discuss a p
+
np BJT to be specific. The injection of minority carriers will be dominated by holes
in this case. Note the emitter, base, and collector regions of the device.

In the configuration we will examine (the “common emitter”),
a relevant parameter is the current gain,


= i
C
/i
B
. Note that for a given output current i
C
, the gain will be large if i
B

is small. The device will be
designed with a small base current in mind.

With reference to Fig. 10.9, we can identify the following
current components.

Emitter

There are two emitter current components:



Injection of holes into the base from the emitter: I
Ep
.



Injection of electrons into the emitter from the base: I
En
.

Base

There are three base current components:



Injection of electrons
into the emitter: I
En

= I
B1
.



Recombination of electrons with holes injected from the emitter: I
B2
.



Generation of electrons in reverse
-
biased base
-
collector junction: I
B3
. This component is small
enough to be ignored to first order.

Collector



Current flowin
g in from the emitter, via the base, arrives at the collector: I
Cp
. Due to recombination
in the base, not all the holes arrive here.



Generation of holes in reverse
-
biased base
-
collector junction: I
Cn
. This component is small enough
to be ignored to first
order.

PERFORMANCE PARAMETE
RS

Notation: We will use capital subscripts E, B, C to stand for the regions of the BJT. These subscripts
will be placed on current components to indicate where that current component is flowing, and on
dopant densities N, diffu
sion coefficients D, and diffusion lengths L

We wish to maximize current gain

. We consider the current components and ask what needs to be
done.