Page 142 - Rashid, Power Electronics Handbook

130 B. M. Wilamowski

9.4 Bipolar Mode Operation of SI devices I
DS
(BSIT) [mA]

The bipolar mode of operation of SIT was ®rst reported in
1976 by Nishizawa and Wilamowski [8, 9]. Several complex 100
theories for the bipolar mode of operation were developed [2,
5, 6, 10, 23, 24], but actually the simple Eq. (9.5) works well =50m A
not only for the typical mode of the SIToperation, but also for 40m A 30m A 20m A 10m A 5m A 2m A =0
I G
the bipolar mode of the SIT operation. Furthermore, the same I G
formula works very well for classical bipolar transistors.
Typical characteristics of the SI transistor operating in both 50
normal and bipolar modes are shown in Figs. 9.7 and Fig. 9.8.
A potential barrier controls the current in the SIT and it is
given by

qD N S
n
n
J ¼ „ x 2 ð9:13Þ
expðÿjðxÞ=V Þdx 2 4 4 V [V]
x 1 T DS
FIGURE 9.8 Small-sized SIT transistor characteristic, operating in
where jðxÞ is the pro®le of the potential barrier along the both normal and bipolar modes, I D ¼ f ðV DS Þ with I G as a parameter.
channel.
For example, in the case of npn bipolar transistors the After inserting Eq. (9.14) into (9.13) one can obtain the well-
potential distribution across the base in reference to emitter known equation for electron current injected into the base
potential at the reference impurity level N ¼ N is described
E S
by: qD n 2 V BE
n i
J ¼ „ exp ð9:15Þ
n x 2
N ðxÞdx V T
B
x 1

N ðxÞN S V BE
B
jðxÞ¼ V ln 2 exp ÿ ð9:14Þ If Eq. (9.13) is valid for SIT and BJT then one may assume
T
n V
i T
that it is also valid for the bipolar mode of operation of the
SIT transistor. This is a well-known equation for the collector
current in the bipolar transistor, but this time it was derived
I using the concept of current ¯ow through a potential barrier.
DS V =0.75V
[mA] GS
V =0.7V 9.5 Emitters for Static Induction Devices
GS
V =0.65V
GS One of the disadvantages of the SIT is the relatively ¯at shape
of the potential barrier (Fig. 9.9a). This leads to slow, diffu-
100
sion-based transport of carriers in the vicinity of the potential
0.6V barrier. The carrier transit time can be estimated using the
formula:
0.5V 0.4V
2
50 0.3V 0V t transit ¼ l eff ð9:16Þ
-0.5V -1V D
-2V where l eff is the effective length of the channel and D ¼ mV is
T
-3V the diffusion constant. In the case of a traditional SIT
transistor this channel length is 2 mm, while in the case of
SIT transistors with sharper barriers (Fig. 9.9b) the channel
length is reduced to about 0:2 mm. The corresponding
2 4 V [V]
DS transient times are 2 ns and 20 ps respectively.
FIGURE 9.7 Small-sized SIT transistor characteristic, operating in The potential distributions shown in Fig. 9.3 are valid for
both normal and bipolar modes, I D ¼ f ðV DS Þ with V GS as parameter. SIT with an emitter made of a traditional p-n junction. A