184 CHAPTER 4. JUNCTIONS IN SEMICONDUCTORS:P-NDIODES
DIODE
PROPERTIES
CURRENT
SOURCENON-LINEAR I-V
AND C-VEMISSION
AND
ABSORPTIONSWITCH- SOLAR CELL
- PHOTODETECTOR
- TRANSISTOR
- MULTIPLIERS
- POWER DETECTORS
- TUNING CAPACITORS
- LEDs
- LASERs
- MODULATORS
- RECTIFIERS
- POWER CONDITIONING
- LEVEL SHIFTERS
Table 4.1: Some important applications of semiconductor diodes in electronics and optoelec-
tronics.
We now go through these applications briefly to help explain how these properties are har-
nessed. The goal is not to provide full details but to elucidate methodology. The diode has many
uses when employed as a current source. The diode when operated under reverse bias has the
properties of a current source (infinite output resistance or equivalently a constant current with
voltage). Consider figure 4.24. If a current source is available which can be controlled then it can
form the basis of several critical and valuable applications. If large changes in the current source
can be effected by a small change in input voltage,ΔVin) then the resultant change in output
voltage,ΔVout, could be large if the current is delivered to a large load resistance. The resultant
voltage gain ,ΔVout/ΔVin,formsthebasisoftransistoroperation and explains why the output
of a transistor is always represented by a current source. If the current source can be controlled
by incident photons, then the resultant current is basis of the operation of aphotodetector or a
solarcell. The transistor is described in detail in later chapters and we describe the solar cell and
photodetector below.
4.8.2 The Solar Cell and Photodetector .....................
Consider a reverse biased diode which is subjected to illumination with photons with energy
larger than the bandgap.
Generation Currents:p-nJunctions Illuminated With Light
For a reverse biased junction, equation 4.3.15 can be understood as follows. Any minority
carrier electrons generated within a diffusion length of the depletion edge can diffuse to the edge