306 CHAPTER 7. TEMPORAL RESPONSE OF DIODES AND BIPOLAR TRANSISTORS
are usually not very appealing, especially for high-speed applications. This is one of that reasons
that diodes have been replaced by transistors and Schottky diodes (to be discussed later) in many
applications.
A homojunctionp-ndiode is a minority carrier device, i.e., it involves injection of electrons
into ap-type region and holes into ann−type region. In forward-bias conditions where the
diode is in a conducting state, the current is due to the minority charge injection. In figure 7.2a,
we show the minority charge (hole) distribution in then-side of a forward-biasedp-ndiode. If
this diode is to be switched, this excess charge must be removed. The device time response,
therefore, depends upon how fast one can alter the minority charge that has been injected. In
figure 7.2b we show how the minority charge can be extracted. As noted in this figure, one can
speed up the process either by introducing defects that speed up the recombination or by using
very narrow diodes. Both these approaches have problems. A high defect density causes non-
ideal diode behavior and increases reverse leakage and a narrow diode has a large reverse-bias
current.
For the reverse-biased case, where no minority charge injection occurs, the device speed can
be quite high and is dominated by the deviceRCtime constant. Let us examine the response of
thep−ndiode to large and small signals.
7.2.1 Small-Signal Equivalent Circuit of ap-nDiode..............
We will start by developing a model for the diode small-signal capacitance and resistance.
The diode capacitance arises from two distinct regions of charge: i) The junction capacitance
arises from the depletion region where there are regions of fixed positive and negative charge;
and ii) The diffusion capacitance is due to the region outside the depletion region where minority
carrier injection has introduced excess charges. The diffusion capacitance due to injected carriers
dominates under forward-bias conditions. While in the reverse bias case the junction capacitance
dominates. The small signal capacitance is in general defined by the relation
C=∣∣
∣∣dQ
dV∣∣
∣∣ (7.2.1)
It is important to note that by definition, capacitance is alossless energy storage element.
This implies that any charge which is stored in a capacitor must bereclaimable. Charge which
is lost during modulation (for example through electron-hole recombination) is not reclaimed
and therefore does not contribute to the capacitance defined in equation 7.2.1. We will see in this
section that inp-ndiodes, only a fraction of the stored charge is reclaimed during high frequency
operation. This impacts the diode small-signal response.
We will now use the equations derived in chapter 4 to calculate the capacitance. The junction
depletion width of thep-ndiode is
W=
[
2 (Vbi−V)
e(
Na+Nd
NaNd)] 1 / 2
(7.2.2)
The depletion region charge is
|Q|=eA WnNd=eA WpNa (7.2.3)